Search

Understanding the impact of various climate forcing agents, such as aerosols, on extreme precipitation is socially and scientifically vital. While anthropogenic absorbing aerosols influence Earth's energy balance and atmospheric convection, their role in extreme events remains unclear. This paper uses convective‐resolving radiative‐convective‐equilibrium simulations, with fixed solar radiation, to investigate the influence of absorbing aerosols on extreme precipitation comprehensively. Our findings reveal an underappreciated mechanism through which absorbing aerosols can, under certain conditions, strongly intensify extreme precipitation. Notably, we demonstrate that a mechanism previously reported for much warmer (hothouse) climates, where intense rainfall alternates with multi‐day dry spells, can operate under current realistic conditions due to absorbing aerosol influence. This mechanism operates when an aerosol perturbation shifts the lower tropospheric radiative heating rate to positive values, generating a strong inhibition layer. Our work highlights an additional potential effect of absorbing aerosols, with implications for climate change mitigation and disaster risk management. Plain Language Summary: Aerosols, particles suspended in the atmosphere, can interact with the incoming solar radiation by scattering or absorbing it. Aerosol species that absorb solar radiation generate local warming of the atmosphere. This local warming changes the vertical profile of temperature and by that affects cloud and precipitation development. In this paper we use idealized computer simulations to investigate the effect of absorbing aerosols on precipitation, and specifically on extreme precipitation events in the tropics. We demonstrate that under certain conditions, absorbing aerosols can strongly enhance extreme precipitation even despite reducing the mean. We show that this trend can be explained by a mechanism previously reported for much warmer climate conditions than currently found on Earth, involving heating by radiation of the lower part of the troposphere. These results have implications for climate change mitigation and disaster risk management. Key Points: The effect of absorbing aerosol on extreme precipitation is examined in idealized convective‐resolving radiative‐convective‐equilibrium simulationsAerosol perturbation that shifts the lower tropospheric radiative heating rate to positive values strongly enhances extreme precipitationThis trend is explained by a mechanism reported before for hothouse climate conditions involving a shift into an "episodic deluge" regime [ABSTRACT FROM AUTHOR]

Scafetta (2022, https://doi.org/10.1029/2022gl097716) purports to test Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models through a comparison of temperature changes over three decades. Unfortunately, the paper contains numerous conceptual and statistical errors that undermine all of the conclusions. First, no uncertainty is given for the observational temperature difference, making it impossible to assess compatibility with any model result. Second, the CMIP6 data are the ensemble means for each model, but the metric being tested is sensitive to the internal variability and so the full ensemble for each model must be used. When this is corrected, the conclusion that "all models with ECS > 3.0°C overestimate the observed global surface warming" is not sustained. Third, the statistical test in Section 2 would reject all models even in a perfect model setup given sufficient ensemble members, thus the second conclusion "that spatial t‐statistics rejects the data‐model agreement" is also not sustainable. Plain Language Summary: Comparisons of models and observations need to account from multiple sources of uncertainty in both the observations and due to the chaotic dynamics of the weather. The analyses in Scafetta (2022, https://doi.org/10.1029/2022gl097716) do not take either of these issues into account and thus the conclusions in that paper are not supportable. Key Points: Scafetta (2022) contains errors in both of the statistical tests used that make the conclusions unsupportable [ABSTRACT FROM AUTHOR]

The cloud feedback may result in amplification or damping of Arctic warming. Two common techniques used to diagnose the top‐of‐the‐atmosphere cloud feedback are the Adjusted Cloud Radiative Effect (AdjCRE) method and the Cloud Radiative Kernel (CRK) method. We apply both to CMIP5 and CMIP6 model data, finding that the AdjCRE calculated Arctic shortwave cloud feedback is twice as correlated with sea ice loss in CMIP5, and four times in CMIP6, as the CRK method. We find that the CRK method produces Arctic all‐sky residual percentages exceeding 20% in 15 of 18 models. We use the CRK method to decompose the feedback in CMIP5 and CMIP6 finding that its median value changed from negative to positive driven by a less‐negative cloud optical depth feedback. Despite its lack of closure, we conclude that the CRK method is better suited for Arctic SW feedbacks as it is less impacted by surface albedo changes. Plain Language Summary: The cloud feedback is the process by which cloud property changes in a warming climate can either further enhance warming or damp it. The Arctic is warming faster than the rest of the globe, and one of the largest sources of uncertainty in its climate projections is the cloud feedback. There are two popular methods to calculate the cloud feedback: the Adjusted Cloud Radiative Effect technique, and the Cloud Radiative Kernel technique. In this paper we compare the two methods in a suite of climate models by considering the extent to which changes in Arctic sea ice impact the cloud feedbacks. From this analysis we conclude that the Cloud Radiative Kernel method is less affected by sea ice loss. We then apply the Cloud Radiative Kernel technique to data from the two most recent generations of global climate models to investigate how polar day Arctic cloud feedbacks have changed between these generations. We find that the median value of these Arctic feedbacks is slightly positive in the newest generation of models, a change from slightly negative in the previous generation that is largely fueled by a weakening of the feedback associated with changes in cloud optical depth. Key Points: The Cloud Radiative Kernel method is less sensitive to surface albedo changes than the Adjusted Cloud Radiative Effect techniqueThe Cloud Radiative Kernel method provides poor radiative closure in a suite of global climate modelsThe median shortwave Arctic cloud feedback in recent climate models is slightly positive due to a weakened cloud optical depth feedback [ABSTRACT FROM AUTHOR]

In response to global warming, ozone is predicted to increase aloft due to stratospheric cooling but decrease in the tropical lower stratosphere. The ozone reductions have been primarily attributed to a strengthening Brewer‐Dobson circulation, which upwells ozone‐poor air. Yet, this paper finds that strengthening upwelling only explains part of the reduction. The reduction is also driven by tropospheric expansion under global warming, which erodes the ozone layer from below, the low ozone anomalies from which are advected upwards. Strengthening upwelling and tropospheric expansion are correlated under global warming, making it challenging to disentangle their relative contributions. Therefore, chemistry‐climate model output is used to validate an idealized model of ozone photochemistry and transport with a tropopause lower boundary condition. In our idealized decomposition, strengthening upwelling and tropospheric expansion both contribute at leading order to reducing tropical ozone. Tropospheric expansion drives bottom‐heavy reductions in ozone, which decay in magnitude into the mid‐stratosphere. Plain Language Summary: The ozone layer absorbs ultraviolet light otherwise harmful to life. Due to compliance with the Montreal Protocol, the ozone layer is generally recovering from depletion. But, at the same time, global warming is predicted to impact ozone, increasing ozone in the upper stratosphere and decreasing ozone in the tropical lower stratosphere. These decreases are typically argued to result from a strengthening of tropical stratospheric upwelling under global warming. Yet, this paper shows that in addition to contributions from strengthening upwelling, much of the ozone loss arises from a deepening of the troposphere under global warming. The deepening of the troposphere erodes the ozone layer from below, with the low ozone anomalies in the eroded region subsequently transported upwards by the background upwelling. Deepening of the troposphere therefore helps to explain the predicted ozone reductions throughout the tropical lower stratosphere. Key Points: Global warming reduces ozone in the tropical lower stratosphere, an effect typically attributed to strengthening stratospheric upwellingYet, global warming also deepens the troposphere, which erodes the ozone layer and reduces transport of ozone into the lower stratosphereAlong with strengthening upwelling, tropospheric expansion contributes at leading order to reductions in tropical lower stratospheric ozone [ABSTRACT FROM AUTHOR]

Blooms of various types of seaweeds have been reported worldwide, with recent expansions in surface waters. While most of the expansions have been attributed to eutrophication due mainly to human activities, any potential role of climate change is unclear. Here we show that, in the East China Sea and Yellow Sea, increased biomass of Sargassum horneri (S. horneri, brown seaweed) from 2000 to 2021 appears to be caused primarily by ocean warming, as S. horneri prefers a certain temperature range to grow. In contrast, while increases of Ulva prolifera (U. prolifera, green seaweed) in the same regions might also be related to ocean warming, during the same period, human activities such as coastal aquaculture or seaweed mitigation may muddle such effects. With the projected ocean warming in the next decades, we hypothesize that S. horneri blooms may occur earlier during the year and may continue to expand in the future. Plain Language Summary: Seaweed macroalgae blooms have been reported around the world in the last two decades, yet the exact reasons are often unclear. This paper attempts to solve this puzzle for the Sargassum and Ulva seaweeds in the East China Sea and Yellow Sea using long‐term observations and laboratory‐based seaweed physiology measurements. The paper shows not only how seaweeds have expanded in the two marginal seas in the past four decades, but also how a warming ocean (due to climate variability) and nutrient enrichment (due mainly to human activities) controlled such expansions. Among 63 large marine ecosystems (LMEs) in the world, the East China Sea (where seaweeds expanded) is one of the three LMEs that experienced "super‐fast" warming. Given the IPCC‐projected seawater temperature rise of 1.6°C by 2050, will these seaweeds continue to expand? Key Points: Multi‐sensor satellite data show significant and near‐synchronized expansions of Sargassum horneri and Ulva prolifera in the past 38 yearsOcean warming appears to be a main factor behind the recent expansions of S. horneri because of its physiological requirementOcean warming may also play a role in U. prolifera expansions, but the effect appears confounded by human mitigation efforts [ABSTRACT FROM AUTHOR]

Cirrus clouds provide a significant radiative forcing on the Earth's climate system. This paper looks at the connection in space and time between monthly mean lightning activity observed from the Lightning Imaging Sensor on board the International Space Station, and the global monthly mean cirrus cloud cover obtained from the MERRA‐2 reanalysis product. The correlation coefficient between the global monthly mean cloud optical thickness of the cirrus clouds (clouds at altitudes above the 400 hPa pressure levels) with the monthly mean lightning flash counts is 0.84, implying that monthly mean lightning can explain 70% of monthly variability of the global high cloud optical thickness. In addition, lightning amount explains nearly 60% of the monthly mean global area coverage of cirrus clouds. Given these statistically significant connections between lightning and cirrus clouds, we propose using global lightning data as an additional tool for monitoring monthly variability of cirrus clouds. Plain Language Summary: Cirrus clouds are one of the most important components maintaining the Earth's radiation budget. They reflect shortwave radiation from the Sun while absorbing the longwave radiation from the Earth. The net cloud radiative forcing for cirrus clouds results a warming of the climate. More/less cirrus clouds result in more/less warming of the planet. The moisture for the formation of cirrus clouds in the upper atmosphere is transported there in large part via deep convective storms, many associated with lightning activity and hence defined as thunderstorms. An increasing in cirrus clouds in a warmer atmosphere will amplify the initial warming. We explored in this study the relationship between global cirrus cloud coverage and global lightning activity. The results of the research show that 60%–70% of the monthly variability in global cirrus clouds can be explained by mean global lightning activity. Key Points: Cirrus clouds are one of the essential components in the atmosphere, with many important feedbacks on the radiation balance of EarthMonthly mean lightning can explain 70% of the monthly variability of the global high cloud optical thicknessLightning explains nearly 60% of the monthly mean global area coverage of cirrus clouds [ABSTRACT FROM AUTHOR]

Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

The magnitude and distribution of the ocean's biological pump (the downward flux of organic carbon (OC) from the ocean surface) influences the pCO2 of the atmosphere and the O2 content of the deep sea, but has not been well quantified. We determine this flux in the ocean's five subtropical gyres using upper‐ocean oxygen mass balance and measurements of T, S, and pO2 by autonomous profiling floats. Our results suggest that the biological OC pump is not globally uniform among the subtropical gyres: values in the North Pacific and Atlantic indicate distinct autotrophy (1–2 mol C m−2 yr−1) while near zero values in the S. Indian Ocean suggest the possibility of net heterotrophy. There is a correlation between the surface water iron/nitrate ratio and the magnitude of the biological pump suggesting an important role for nitrogen fixation in controlling the global distribution. Plain Language Summary: The ocean's biological pump is the most important natural flux controlling ocean and atmosphere carbon concentrations on decadal to millennial time scales, with a value that is fortuitously about the same as the present anthropogenic flux of carbon to the atmosphere. Understanding this flux is key to determining the marine carbon cycle feedback to global warming, but presently even the sign of this flux is debated in some subtropical ocean regions. This paper demonstrates how it is possible to determine the global value of the biological pump by remote measurements on autonomous profiling floats. Our findings suggest that the ocean's biological pump is very different in the subtropical northern and southern hemispheres probably because of the geographic variability of the flux of iron to the surface ocean. Key Points: The geographic distribution of the downward flux of biological organic carbon from the upper ocean (the ocean's biological pump) is uncertain and difficult to determine from observationsT, S, and pO2 measurements from 21 profiling floats in the world's subtropical oceans are interpreted in terms of the net biological oxygen production in the upper oceanThe measured net biological oxygen flux is higher in the northern hemisphere subtropics than in the southern subtropics and correlated with surface‐water iron concentration [ABSTRACT FROM AUTHOR]

Under the context of global warming, we identified a significant increase in lightning activity in the last two decades over the Tibetan Plateau (TP), the highest plateau on earth, based on two data sets. The largest rise of lightning activity was found over the region with high lightning density represented by the eastern TP at a rate of 0.072 ± 0.069 fl km−2 yr−1 during 1996–2013 from Optical Transient Detector/Lightning Imaging Sensor. World Wide Lightning Location Network data also showed a remarkable increase in lightning strokes during 2010–2019 in the maximum lightning density region, while the change was insignificant in the low lightning density region. Further investigations revealed that most of the TP region experienced a significant increase in thunderstorm frequency, which was responsible for the increase in lightning activity, rather than an increase in thunderstorm intensity. Increased lightning activity implies a potential risk to human beings and the fragile ecosystem over the TP in a warmer climate. Plain Language Summary: Lightning is a natural source of nitrogen oxides and wildfires, and thus has important effects on atmospheric chemistry and ecosystem. Lightning is also a serious threat to human life and property. Using satellite‐based and ground‐based data, the trend of lightning activity over the Tibetan Plateau (TP) in the past two decades has been investigated in this paper. In the global warming context, TP experienced an increase in lightning activity. The lightning increase was significant in the high lightning density region, such as the central and eastern TP, and insignificant in the lower lightning density region, such as the western TP and the Brahmaputra Valley. The thunderstorm frequency also experienced an increase over most parts of the TP, which caused the rising trend in lightning activity. However, the thunderstorm intensity shows weak and insignificant changes. Since the TP is getting warmer much faster than that observed over the globe, the increased lightning activity over the TP may exert enhanced potential threat to human beings and impacts on the fragile ecosystem over the Plateau. Key Points: Lightning activity over the Tibetan Plateau increased significantly in the past two decades based on the Optical Transient Detector/Lightning Imaging Sensor and World Wide Lightning Location Network datasetsLightning activity in the high‐flash‐density region increased faster than that in the low‐flash‐density regionThe apparent rise in lightning activity is due to increase in thunderstorm rate rather than change in thunderstorm intensity [ABSTRACT FROM AUTHOR]

In this paper, we investigate whether the relationship between the El Niño‐Southern Oscillation and tropical cyclone (TC) rain rate over the western North Pacific (WNP) changes between the global warming slowdown (2001–2012) and resumption (2013–2020) periods. Using high‐resolution satellite rainfall data, we show that the average TC rain rate increases by 9% but decreases by 14% during the recent El Niño and La Niña years (since 2013), respectively. During the El Niño years, the increase is related to higher TC track density at lower latitudes, where more water vapor and stronger convergence are present. On the other hand, the decrease in TC rain rate during the recent La Niña years is mainly related to an increase in track density at higher latitudes and a decrease in TC intensity. These changes in TC track density and intensity are both associated with the strengthening of the subtropical high since 2013. Plain Language Summary: The impacts on various weather phenomena associated with the global surface warming slowdown (or "hiatus": 1998–2012) and its subsequent resumption since 2013 have been extensively investigated. An unresolved issue is whether the tropical cyclone (TC) rain rate over the western North Pacific (WNP) basin has also changed, especially during the El Niño (or La Niña) years. Considering the availability of high‐resolution satellite rainfall data, we mainly focus on the period of 2001–2020 (slowdown period: 2001–2012; resumption period: 2013–2020). Our results show that, consistent with the increase in atmospheric water vapor after the global warming resumption, the average TC rain rate has increased by ∼2%. However, TC rain rate increases in El Niño years but decreases in La Niña years during the resumption period. This opposite change in TC rain rate is mainly influenced by the strengthened WNP subtropical high. Our findings suggest that it is necessary to properly consider the influence of the subtropical high on the projections in TC migration, intensity, and rain rate under global warming. Key Points: Compared with El Niño‐Southern Oscillation (ENSO) years in the global warming slowdown period, tropical cyclone (TC) rain rate increases and decreases in recent El Niño and La Niña yearsThis opposite change in TC rain rate in recent ENSO years is mainly affected by the changes in TC track density and TC intensityChanges in TC track density and intensity are both influenced by the extremely strengthened subtropical high since 2013 [ABSTRACT FROM AUTHOR]

Using four reanalysis data sets and ground‐based observations, this paper uncovers that on average, 30% of the time, Northern Hemisphere snow cover experiences unfrozen bottom soil. It is demonstrated that the probability of occurrence of unfrozen soils is correlated with the snow types and is maximum over the ephemeral followed by the maritime and prairie snow. The results based on reanalysis data unveil that the seasonal evolution of the unfrozen soil areas is not synchronous with the snow cover extent and exhibits sub‐annual bi‐modality with two annual maxima in April and October. Interannual trend analyses indicate that shrinkage of spring snow in the past few decades has been accompanied by an increase in the proportion of unfrozen bottom soils, more significantly over polar climate regimes dominated by the tundra and taiga snow. The findings imply that the snowpack basal melting could have increased due to global warming. Plain Language Summary: Unfrozen bottom soils below snow layers play an important role in the persistence and stability of snowpack, yet less knowledge is known about its spatial variability and seasonal evolution on a global scale. This study uncovers that, on average, around 30% of annual Northern Hemisphere's snow cover extent is over unfrozen soils with a spatial variability that is highly correlated with snow types. Specifically, unfrozen soils appear more frequently below the ephemeral followed by the maritime and prairie snow. In addition, it is demonstrated that the unfrozen soil areas exhibit different seasonal evolution from snow cover extent with two annual peaks in April and October. The results show that the areas of unfrozen soils are expanding in spring as the snow cover extent is shrinking. Key Points: On average 30% of the year the soil below global snowpack is unfrozenSeasonal evolution of the areas of unfrozen soil below snowpack is not synchronous with snow cover extentBasal snowpack melting could have increased in a warming climate [ABSTRACT FROM AUTHOR]

Orbital Earth Radiation Budget measurement comparisons to models, are critical for climate prediction confidence. Satellite systems must reduce calibration drifts for this purpose. NASA Clouds and the Earth's Radiant Energy System (CERES) measures Earth albedo reductions that if correct, would increase solar forcing and suggest greater sunlight absorption is driving much of recent temperature increases. Such results are presented, alongside those from the Moon and Earth Radiation Budget Experiment (MERBE). MERBE uses constant lunar reflectivity for tracking and compensation of instrument telescope degradation, undetectable by CERES. MERBE finds Earth albedo constant compared to that of the Moon, because Arctic solar warming effects are balanced by solar cooling elsewhere, likely due to negative feedbacks. Contrary to NASA, this shows the Sun is not increasing warming and that CERES results are not as stable as claimed and assumed. Furthermore, MERBE can actually resolve Cloud Radiative Forcing (CRF) signals from the existing record, rather than in decades with official observations. Plain Language Summary: In contrast to NASA Clouds and the Earth's Radiant Energy System results, this paper shows that the Earth reflectivity has not changed since the year 2000, by comparing the sunlight bouncing off it to that from the Moon. Disagreeing with NASA, this provides physical evidence that global warming is not being increased by the Sun. It also immediately brings data to the standard requested by the climate community for cloud forcing/feedback signal detection. The Moon and Earth Radiation Budget Experiment data is being staged for free on the FAIR compliant website Pangaea for climate scientists. Key Points: The Sun is not causing climate change and Cloud Radiative Forcing (CRF) signals can now be detected immediatelyArctic heating is being balanced by reflectivity increases elsewhereThe calibration accuracy of data is sufficient to detect uncertain cloud feedback signals immediately, rather than in decades [ABSTRACT FROM AUTHOR]

Quantifying changes in oceanic aerobic respiration is essential for understanding marine deoxygenation. Here we use an Earth system model to investigate if and to what extent oxygen utilization rate (OUR) can be used to track the temporal change of true respiration (Rtrue). Rtrue results from the degradation of particulate and dissolved organic matter in the model ocean, acting as ground truth to evaluate the accuracy of OUR. Results show that in thermocline and intermediate waters of the North Atlantic Subtropical Gyre (200–1,000 m), vertically integrated OUR and Rtrue both decrease by 0.2 molO2/m2/yr from 1850 to 2100 under global warming. However, in the mesopelagic Tropical South Atlantic, integrated OUR increases by 0.2 molO2/m2/yr, while the Rtrue integral decreases by 0.3 molO2/m2/yr. A possible reason for the diverging OUR and Rtrue is ocean mixing, which affects water mass composition and maps remote respiration changes to the study region. Plain Language Summary: The ocean is losing oxygen due to an imbalance in oxygen supply and aerobic respiration. Therefore, monitoring temporal changes in the aerobic respiration rate contributes to understanding marine deoxygenation. Based on simulations of an Earth system model, we investigate an indirect diagnostic measure of the respiration rate (oxygen utilization rate, OUR), calculated as the slope of the least square regression of the apparent oxygen utilization (AOU, saturated oxygen concentration minus local oxygen concentration) and seawater age that can be computed from transient abiotic tracers. As the reference to OUR, true respiration (Rtrue) is the oxygen consumption rate resulting from the degradation of organic matter in the model ocean. Results show that in the North Atlantic Subtropical Gyre intermediate water (200–1000 m), both vertically integrated OUR and Rtrue decrease by 0.2 molO2/m2/yr from 1850 to 2100. However, in the Tropical South Atlantic intermediate water, the OUR integral increases by 0.2 molO2/m2/yr and the Rtrue integral decreases by 0.3 molO2/m2/yr. We hypothesize that changes in ocean mixing over time, which can affect water mass composition and map remote respiration changes to the study region, explain the discrepancy of OUR and Rtrue tendencies. Key Points: Our model study confirms earlier findings that oxygen utilization rate (OUR) underestimates true respiration (Rtrue) in mesopelagic oceanDespite OUR underestimate Rtrue, OUR can adequately estimate long‐term changes in Rtrue in the mesopelagic North Atlantic subtropical gyreOUR cannot adequately estimate climate‐driven changes in Rtrue in the mesopelagic tropical South Atlantic where different water masses mix [ABSTRACT FROM AUTHOR]

This paper presents a multivariate classification of the global wave climate into types driven by atmospheric circulation patterns. The primary source of the net long-term variability is evaluated based on historical wave simulations. Results show that the monsoon, extratropical, subtropical, and polar wave climate types of the Pacific and North Atlantic Oceans are dominated by natural variability, whereas the extratropical and subtropical wave climate types in the Indian Ocean, and the tropical wave climate types of the Atlantic and Pacific Oceans exhibit a global warming signal. In the Pacific sector of the Southern Ocean, strong natural variability may mask a global warming signal that is yet to emerge as being statistically significant. In addition, wave climate teleconnections were found across the world that can provide a framework for joint strategies to achieve the goals of climate adaption for resilient coastal communities and environments. Plain Language Summary Near-surface wind systems drive global ocean wave conditions that are usually studied at regional or local scale. However, analyzing these relationships at the global scale is important to understand how natural climate oscillations, such as El Niño-Southern Oscillation, and global warming, impact wave climate. Here we identify the large wave climates in homology to the climate regions (e.g., Tropical, Temperate, Polar) and are originated by the planetary wind systems. The results show that global warming signals have already emerged in the Indian Ocean and the tropical regions of the Pacific and Atlantic basins. In contrast, in the other ocean basins, natural variability is still the dominant signal and could mask global warming signals. [ABSTRACT FROM AUTHOR]

Shallow cloud fields over the subtropical ocean exhibit many spatial patterns. The frequency of occurrence of these patterns can change under global warming. Hence, they may influence subtropical marine clouds' climate feedback. While numerous metrics have been proposed to quantify cloud patterns, a systematic, widely accepted description is still missing. Therefore, this study suggests one. We compute 21 metrics for 5,000 satellite scenes of shallow clouds over the subtropical Atlantic Ocean and translate the resulting data set to its principal components (PCs). This yields a unimodal, continuous distribution without distinct classes, whose first four PCs explain 82% of all 21 metrics' variance. The PCs correspond to four interpretable dimensions: Characteristic length, void size, directional alignment, and horizontal cloud top height variance. These dimensions span a space in which an effective pattern description can be given, which may be used to better understand the patterns' underlying physics and feedback on climate. Plain Language Summary: Satellite images show that clouds which develop in the lowest 5 km of the atmosphere organize into many visually distinct patterns. Because different patterns reflect sunlight differently, a change in the relative occurrence of a pattern may influence Earth's response to warming. To study this effect, one typically first encapsulates various aspects of the complex patterns in a few, simple "metrics." In this paper, we compute 21 commonly used metrics for 5,000 cloud fields observed by satellite over the Atlantic Ocean east of Barbados. We show that these 21 metrics contain a large amount of redundant information: To effectively describe and interpret the cloud patters, one only needs four derived metrics, which capture a cloud field's typical cloud size, the size of connected clear sky patches, the clouds' degree of directional alignment and spatial variance in cloud top height. Combinations of these four metrics do not identify the existence of multiple, typical cloud patterns, as previously suggested. However, the four metrics form a new, effective and interpretable pattern description, which can be used to better understand how cloud patterns develop and how this impacts the wider climate system. Key Points: Shallow cloud patterns in satellite observations are quantified by 21 metrics and follow a unimodal, continuous distributionMost existing metrics are redundant; four principal components capture 82% of the variance of 21 metricsCharacteristic length, void size, directional alignment, and cloud top height variance combine to effectively describe the patterns [ABSTRACT FROM AUTHOR]

Based on the Coupled Model Intercomparison Project Phase 5 multimodel simulations, this paper investigates the future changes in extreme wet‐bulb temperatures (TWs) and extreme dry‐bulb temperatures (Ts) globally under both low‐emission and high‐emission scenarios (Representative Concentration Pathways of 4.5 [RCP4.5] and 8.5 [RCP8.5]). Under a warming climate, T and TW exhibit a higher absolute increase in their mean values over the mid–high latitudes while they show higher relative increases in their mean values and variability over tropics. Humid and dry heat events which consist of consecutive extreme TWs and Ts show higher occurrences over tropics, governed by the joint increases in mean values and variability of TW and T therein. Humid heat events show intensifications to dry heat events with higher frequency, duration, and intensity, driven by the lower variability of TW than T. In the mid–high latitudes, heat events will start earlier and last longer in warm season under RCP8.5 than RCP4.5, motivating the need for global warming mitigation. Plain Language Summary: The adverse impacts of high temperatures and high humidity on health and economics are worse than extreme temperatures alone. Wet‐bulb temperature (TW) combining temperature (T) and humidity is a useful index for humid heat. We investigate future changes in extreme TWs and Ts based on a set of simulations under low‐emission and high‐emission scenarios. In a warmer climate, both extreme TWs and Ts show apparent absolute increases over the mid–high latitudes but higher increases in mean values and variability in relative terms over tropics. Governed by the joint impacts of mean values and variability of TWs and Ts, humid and dry heat events featuring consecutive days of extreme TWs and Ts show higher occurrences in the tropics. Future humid heat events are more frequent, intense, and longer lasting than dry heat events as TW is more sensitive to global warming than T. Both humid and dry heat events start earlier and last longer under the high‐emission than low‐emission scenario. Key Points: Extreme high wet‐bulb temperature is more sensitive to the changing climate than extreme high dry‐bulb temperatureFuture heat events have relatively higher occurrences in the tropics, and humid heat events are substantially intensified than dry heatThe intensification of humid heat events is associated with the projected increases in specific humidity in the warmer climate [ABSTRACT FROM AUTHOR]

Global mean precipitation changes due to climate change were previously shown to be relatively small and well constrained by the energy budget. However, local precipitation changes can be much more significant. In this paper we propose that for large enough scales, for which the water budget is closed (precipitation [P] roughly equals evaporation [E]), changes in P approach the small global mean value. However, for smaller scales, for which P and E are not necessarily equal and convergence of water vapor still plays a role, changes in P could be much larger due to dynamical contributions. Using 40 years of two reanalysis data sets, 39 Coupled Model Intercomparison Project Phase 5 (CMIP5) models and additional numerical simulations, we identify the scale of transition in the importance of the different terms in the water budget to precipitation to be ~3,500–4,000 km and demonstrate its relation to the spatial scale of precipitation changes under climate change. Plain Language Summary: Predicting precipitation changes due to climate change is of great importance for society. We propose that the present‐day characteristic scale of the hydrological cycle (for which precipitation roughly equals evaporation) predicts the spatial scale of future precipitation changes under global warming. For smaller scales than the characteristic scale of the hydrological cycle, changes in precipitation could be much larger than the global mean change due to water vapor convergence contributions. However, above this scale the precipitation changes approach the relatively small global mean change. Using reanalysis data sets, Coupled Model Intercomparison Project Phase 5 (CMIP5) models, and additional numerical simulations, we identify the characteristic scale of the hydrological cycle to be ~3,500–4,000 km and demonstrate its relation to the spatial scale of precipitation changes under climate change. These results suggest that changes in precipitation on the regional‐continental scale could be much larger than the global mean change. Key Points: The present‐day characteristic scale of the hydrological cycle predicts the spatial scale of precipitation changes under climate changeUsing reanalysis data sets and CMIP5 models, we identify the characteristic scale of the hydrological cycle to be ~3,500–4,000 kmThese results suggest that changes in precipitation on the continental scale could be much larger than the global mean change [ABSTRACT FROM AUTHOR]

Both historical observations and recent modeling studies reveal a faster warming in the Arctic compared to the Antarctic. To understand the role of the Antarctic Circumpolar Circulation (ACC) in this warming asymmetry, we simulate the climate mean state and climate response to doubled CO2 under different climate mean ACC states by closing or opening the Drake Passage (DP) with the Community Earth System Model. From closed to open DP, a stronger climate mean ACC leads to a stronger climate mean Atlantic Meridional Overturning Circulation (AMOC), as well as a colder Antarctic but a warmer Arctic in the climate mean state. The less climate mean sea ice coverage in a warmer Arctic implies less extensive sea ice melting under global warming. This causes a reduced asymmetry in warming between the two poles in response to the doubled CO2. Plain Language Summary: To understand the role of the Antarctic Circumpolar Circulation (ACC) in the asymmetric Arctic and Antarctic warming, we conduct two numerical model experiments. We consider two different climate states by closing or opening the Drake Passage (DP). The former yields a weak ACC, while the latter results in a strong ACC. A stronger climate mean ACC leads to a colder Antarctic but a warmer Arctic, due primarily to a concurrent stronger Atlantic Meridional Overturning Circulation (AMOC). The diminished coverage of sea ice in the warmer Arctic signifies a reduction in sea ice melting under global warming. Consequently, this leads to a reduction in the asymmetry in warming between the two poles in response to doubled CO2. These findings highlight a significant impact of the ACC on the global climate mean state and polar warming asymmetry, and shed light on the uncertainties in the coordinated interaction between major ocean circulation systems (e.g., AMOC and ACC) and on those in inter‐hemisphere interactions. Key Points: A stronger climate mean Antarctic Circumpolar Circulation (ACC) is accompanied by a stronger mean Atlantic Meridional Oceanic Circulation (AMOC)A stronger climate mean AMOC results in a warmer Arctic with less climate mean sea ice coverageThe less asymmetric polar warming in the presence of stronger climate mean ACC and AMOC is largely due to less sea ice melting in Arctic [ABSTRACT FROM AUTHOR]

Near‐term projections of drought in the southwestern United States (SWUS) are uncertain. The observed decrease in SWUS precipitation since the 1980s and heightened drought conditions since the 2000s have been linked to a cooling sea surface temperature (SST) trend in the Equatorial Pacific. Notably, climate models fail to reproduce these observed SST trends, and they may continue doing so in the future. Here, we assess the sensitivity of SWUS precipitation projections to future SST trends using a Green's function approach. Our findings reveal that a slight redistribution of SST leads to a wetting or drying of the SWUS. A reversal of the observed cooling trend in the Central and East Pacific over the next few decades would lead to a period of wetting in the SWUS. It is critical to consider the impact of possible SST pattern trends on SWUS precipitation trends until we fully trust SST evolution in climate models. Plain Language Summary: Precipitation trends in the southwestern United States (SWUS) are sensitive to the pattern of sea surface temperature (SST) trends in the Tropical Pacific. Since the turn of the century, a decrease in SWUS precipitation has been linked to a cooling of the Central and Eastern Pacific (1990–2020). Notably, climate models are unable to simulate this observed cooling SST trend. In this study, we answer how SWUS precipitation projections may be impacted by potential error in the simulation of future SST trends by climate models. We first demonstrate that slight changes in the pattern of SST trends leads to either a wetting or drying of the SWUS. Second, if the current 30‐year cooling trend in the Central and East Pacific switches to a warming trend, the SWUS could experience a near‐term increase in precipitation. While climate models are the main tool to predict the global response to anthropogenic climate change, we must consider and account for their error in projections of global warming. Key Points: The observed Equatorial Pacific cooling trend, unpredicted by climate models, may have led to decreased precipitation in the southwestern USWith a sea surface temperature‐precipitation Green's function, we find that small changes in sea surface temperature can either wet or dry the southwestern USA reversal of the cooling trend in the Equatorial Pacific could lead to a wetting trend in the southwestern US [ABSTRACT FROM AUTHOR]

Accurate Arctic sea‐ice forecasting for the melt season is still a major challenge because of the lack of reliable pan‐Arctic summer sea‐ice thickness (SIT) data. A new summer CryoSat‐2 SIT observation data set based on an artificial intelligence algorithm may alleviate this situation. We assess the impact of this new data set on the initialization of sea‐ice forecasts in the melt seasons of 2015 and 2016 in a coupled sea ice‐ocean model with data assimilation. We find that the assimilation of the summer CryoSat‐2 SIT observations can reduce the summer ice‐edge forecast error. Further, adding SIT observations to an established forecast system with sea‐ice concentration assimilation leads to more realistic short‐term summer ice‐edge forecasts in the Arctic Pacific sector. The long‐term Arctic‐wide SIT prediction is also improved. In spite of remaining uncertainties, summer CryoSat‐2 SIT observations have the potential to improve Arctic sea‐ice forecast on multiple time scales. Plain Language Summary: Arctic sea ice is rapidly declining due to global warming, especially in summer. Accurate sea‐ice forecasting is important to understand the potential influence of these changes and devise effective responses. The performance of sea‐ice forecasts highly depends on the accuracy of the initial sea‐ice states. So refining the initial conditions of sea‐ice forecasts with satellite observations is a common way to reduce forecast errors. However, obtaining reliable summer pan‐Arctic satellite sea‐ice thickness (SIT) data is challenging due to complex ice‐surface conditions in summer. A new artificial‐intelligence‐based summer SIT satellite data product may improve initial SIT states. We integrate this data set into a sea‐ice forecast system to evaluate its impact on forecast skill. We find that the new summer satellite SIT data can reduce short‐term ice‐edge location forecast errors and benefit long‐term SIT forecasts. Key Points: Assimilating summer CryoSat‐2 sea‐ice thickness (SIT) observations makes more skillful Arctic ice‐edge forecasts on multiple time scalesThe long‐term SIT forecasts improve with the assimilation of summer CryoSat‐2 SIT observationsFurther refinement is needed for summer CryoSat‐2 SIT observations [ABSTRACT FROM AUTHOR]

Observed El Niño‐Southern Oscillation (ENSO) varies between decades with high ENSO amplitude and more extreme Eastern Pacific (EP) El Niño events and decades with low ENSO amplitude and mainly weak El Niño events. Based on experiments with the CESM1 model, ENSO may lock‐in into an extreme EP El Niño‐dominated state in a +3.7 K warmer climate, while in a −4.0 K cooler climate ENSO may lock‐in into a weak El Niño‐dominated state. The state shift of ENSO with global warming can be explained by the location and amplitude of the strongest warming over the eastern equatorial Pacific, which amplifies the Bjerknes feedback and allows a southward migration of the Intertropical Convergence Zone onto the equator, a prerequisite of extreme EP El Niños. In light of these results, we discuss to what extent the state of ENSO may be a tipping element in the climate system. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) is the dominant mode of climate variability with far‐reaching impacts. El Niños, the warm events, occur in different flavors. In particular extreme Eastern Pacific (EP) El Niño events are associated with heavy precipitation events and extreme droughts, thus cause large socio‐economic impacts in the Pacific region and beyond. In the observational record, they are quite rare so far. Here we present experiments of one climate model, which suggests that in a warmer climate ENSO may lock‐in in a different state, in which nearly each El Niño is an extreme EP El Niño. On the other hand in a colder climate ENSO may lock‐in in a state with nearly no extreme EP El Niños. In both climates this would have huge consequences for the socio‐economic impacts of ENSO. Against this background we raise the discussion if ENSO may be a tipping element in the climate system. Key Points: El Niño‐Southern Oscillation (ENSO) may be a tipping element in the climate system as its characteristics can be very different under colder or warmer mean climatesIn a warmer climate, ENSO can lock‐in into a state with high variability and nearly each El Niño is an extreme Eastern Pacific (EP) El NiñoMore extreme EP El Niño events would strongly increase the socio‐economic impacts of ENSO [ABSTRACT FROM AUTHOR]

The East China Sea Kuroshio (ECS‐Kuroshio) and the Ryukyu Current are the major poleward heat carriers in the North Pacific. Anomalous changes of ECS‐Kuroshio and Ryukyu Current could exert substantial influence on the climate in mid‐latitude regions. However, owing to limited observations and coarse resolution of climate models, how they might change under anthropogenic warming remains unknown. Here, we find an accelerating ECS‐Kuroshio (1.5 Sv) and a decelerating (−2.2 Sv) Ryukyu Current using in‐situ observation during 1958–2022, equivalent to 7% strengthening and 20% weakening in the 65 years. The trend is also simulated by four high‐resolution climate models, with multi‐model ensemble‐mean acceleration (deceleration) of the ECS‐Kuroshio (Ryukyu Current) of 1.2 ± 0.6 Sv (−6.2 ± 2.5 Sv) over 1950–2050. The weakening subtropical wind field reduces their summed transport o. Enhanced stratification, which induces uplift of current system and weaker topography‐flow interaction, leads to the intensifying ECS‐Kuroshio and disappearing Ryukyu Current. Plain Language Summary: The East China Sea Kuroshio (ECS‐Kuroshio) and the Ryukyu Current are the major components of western boundary current (WBC) system in the North Pacific. They transport large amount of heat poleward, maintaining the mid‐latitude region warm and habitable. In this study, we find that in both observation and high‐resolution numerical models, the ECS‐Kuroshio is intensifying and the Ryukyu Current is rapidly declining. Particularly, in climate models, the Ryukyu Current is predicted to experience a 45% weakening during 1950–2050. Weakening wind field in the North Pacific tends to reduce the total transport of WBC system. The rapid weakening of the Ryukyu Current is credit to the enhanced stratification under global warming, which induces uplift of the Kuroshio east of Taiwan. Hence, less water will be blocked by bottom topography (Ilan Ridge) and bifurcates into the Ryukyu Current. Our finding highlights the needs for comprehensive studies on the local and climate effects of the evolution of the WBC system in the North Pacific. Key Points: The East China Sea Kuroshio is intensifying while the Ryukyu Current is rapidly declining in a warming climateThe change is mainly caused by enhanced stratification which induces uplift of current system and weaker topography‐flow interaction [ABSTRACT FROM AUTHOR]

Probabilistic projections from the UK Climate Projections 2018 are presented for four global warming levels (GWLs) at 1.5, 2, 3, and 4°C above the 1850–1900 baseline. Our results show how uncertainties associated with climate models and four representative concentration pathways (RCP) emission scenarios translate to UK regional scale changes in maximum temperature and precipitation, with data also available for minimum and mean temperatures, humidity and surface net downward shortwave radiation flux. We compare weighting the likelihood of RCPs based on (hypothetical) policy decisions, against our baseline assumption that each RCP is equally likely. Differences between weighted and unweighted GWL distributions are small, particularly in relation to the full breadth of uncertainties that are incorporated into the probabilistic projections. Finally we quantify the relative importance of scenario, model and internal variability on regional projected GWLs and show that uncertainty associated with an uncertain climate response to forcings dominates at all GWLs. Plain Language Summary: Global warming levels define the global averaged increase in temperature since the pre‐industrial period. This study looks at what the regional climate in the UK could be like in the future. We estimate how temperature and precipitation in the UK might change under four different global warming levels (GWLs) (1.5, 2, 3, and 4°C above pre‐industrial times). We account for a wide range of outcomes from both climate models and future emissions scenarios. Assuming that the range of future emissions scenarios are equally likely, the center of the ranges of years that we could reach GWLs of 1.5, 2, 3, and 4°C are 2032, 2050, 2072, and 2078 respectively. The largest influence of future climate on these range estimates comes from our understanding of how the Earth actually responds to changes in greenhouse gases. Even if we knew the exact quantity of greenhouse gas emissions we would emit in the future, we still wouldn't be able to predict the exact changes in UK climate with perfect accuracy because our understanding of the regional climate response to these emissions requires further investigation. Key Points: Probabilistic climate projections from the UK are presented for four global warming levels (GWLs) across four representative concentration pathways (RCP)We show how uncertainty associated with climate models and future emission scenarios translate to UK regional scale changesUncertainty associated with an uncertain climate response to forcings dominates at all GWLs [ABSTRACT FROM AUTHOR]

The last glacial inception (LGI) marks the transition from the interglacial warm climate to the glacial period with extensive Northern Hemisphere ice sheets and colder climate. This transition is initiated by decreasing boreal summer insolation but requires positive feedbacks to stimulate the appearance of perennial snow. We perform simulations of LGI with climate model AWI‐ESM‐2.1, forced by the astronomical and greenhouse gas forcing of 115,000 years before present. To compare with the preindustrial (PI) simulation, we use a consistent definition of the seasons during the LGI and the PI and evaluate model output on an angular astronomical calendar. Our study reveals a prominent role of the sea‐ice albedo feedback to amplify the delayed climate signal at polar latitudes. Through a radiative budget analysis, we examine that the ice‐albedo feedback exceeds the shortwave radiative forcing, contributing to the cooling and high latitude snow built‐up during LGI. Plain Language Summary: The onset of the last ice age marks the transition from the interglacial warm climate to the ice age with extensive Northern Hemisphere (NH) ice sheets and colder climate. This transition is initiated by decreasing boreal summer irradiance and reinforced by positive feedbacks. We perform climate simulations under the astronomical and greenhouse gas forcing, and use a consistent definition of seasons. Our study shows that a delayed ice‐albedo feedback plays an important role in generating a cold NH climate. Key Points: Climate simulations reveal year‐round cooling in the Northern Hemisphere high latitudes during the last glacial inceptionA refined angular calendar provides a tool to analyze the seasonal dynamicsWe identify the crucial role of the albedo feedback on the high‐latitude radiative budget [ABSTRACT FROM AUTHOR]

Surface, aircraft, and satellite measurements indicate pervasive early cold season (Augut–September) CO2 emissions across Arctic regions, consistent with increased ecosystem metabolism in plants and soils. A key remaining question is whether cold season sources will become large enough to permanently shift the Arctic into a net carbon source. Polar orbiting GHG satellites provide robust estimation of regional carbon budgets but lack sufficient spatial coverage and repeat frequency to track sink‐to‐source transitions in the early cold season. Mission concepts such as the Arctic Observing Mission (AOM) advocate for flying imaging spectrometers in highly elliptical orbits (HEO) over the Arctic to address sampling limitations. We perform retrieval and flux inversion simulation experiments using the AURORA mission concept, leveraging a Panchromatic imaging Fourier Transform Spectrometer (PanFTS) in HEO. Our simulations demonstrate the potential benefits of increased CO2 sampling for detecting emissions during the early cold season. Plain Language Summary: The Arctic is warming rapidly, leading to an acceleration of carbon exchange across tundra and boreal ecosystems. These changing flows of carbon can be difficult to detect using traditional observing systems. We propose that measurement systems which observe Arctic ecosystems continuously from a quasi‐GEO orbit can more accurately detect these important changes, such that unexpected and dangerous feedbacks to climate warming can be monitored and accounted for. Key Points: The Arctic Fourier Transform Spectrometer Investigation (AURORA) mission concept addresses GHG sampling limitations in the ArcticAURORA uses a highly elliptical orbit (HEO) to increase repeat frequency, and a panchromatic iFTS for SWIR O2 and CO2 bands (0.7–2.5 μm)Simulation experiments show potential benefits of increased sampling frequency for grid scale detection of early cold season CO2 emissions [ABSTRACT FROM AUTHOR]

The role of internal variability in generating an apparent link between autumn Barents‐Kara sea (BKS) ice and the winter North Atlantic Oscillation (NAO) has been intensely debated. In particular, the robustness and causality of the link has been questioned by showing that BKS‐NAO correlations exhibit nonstationarity in both reanalysis and climate model simulations. We show that the lack of ice observations means nonstationarity cannot be confidently assessed using reanalysis prior to 1961. Model simulations are used to corroborate an argument that forced nonstationarity could result from ice edge changes due to global warming. Consequently, the observed change in BKS‐NAO correlations since 1960 might not be purely a result of internal variability and may also reflect that the ice edge has moved. The change could also reflect the availability of more accurate ice observations. We discuss potential implications for analysis based on coupled climate models, which exhibit large ice edge biases. Plain Language Summary: Does the amount of ice in the Barents‐Kara Sea influence European air pressure or are the patterns we see caused by random changes in the weather? In climate models and in estimates of the atmosphere's history these patterns change depending on which years we look at. This has been interpreted as evidence that the patterns are random. However, there are very few measurements of ice in this region before 1961, so we argue that looking at these years is not helpful. Since 1961, where we have more measurements, the winter sea ice edge has been moving Northwards because of global warming. When the ice in a particular region disappears, it changes the expected relationship with the atmosphere because heat can now quickly leave the ocean. We therefore hypothesize that some of the changes seen in the patterns may therefore not be random, but a result of ice edge changes. We also observe that different climate models put the ice edge in different places, and the same hypothesis therefore implies that models might not get potential ice‐air pressure relationships in specific regions correct. Key Points: A lack of observations means that ice‐NAO links cannot be confidently assessed with reanalysis prior to 1961Nonstationarity since 1961 may reflect forced changes to the ice edge, due to the dependence of heatflux anomalies on ice edge variabilityThe magnitude of this potential forced nonstationarity in the real world is currently unclear [ABSTRACT FROM AUTHOR]

The Arctic sea ice cover has decreased rapidly over the last few decades both in extent and thickness. Here we present multi‐year (2013–2022) observations of sea ice thickness in the northwestern Barents Sea based on Upward Looking Sonar measurements and show that the winter sea ice has become thicker over the last decade. Sea ice thickness from the Pan‐Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) reproduces both the observed variability and recent 10‐year trend and shows that this thickening (0.24 m decade−1) has not been seen since the 1990s. Using PIOMAS we find that the recent increase in sea ice thickness can be explained by increased sea ice freezing as a result of lower temperatures in the ocean and in the atmosphere. The recent thickening is set in the context of a long‐term thinning trend, with PIOMAS showing much thinner ice now than in the 1980s. Plain Language Summary: The Arctic sea ice cover is becoming smaller and thinner due to global warming. We have measured sea ice thickness in the Barents Sea since 2013, and find that the ice thickness has increased since the measurements were initiated, contrary to what we would expect in a warming world. The Arctic sea ice cover can, however, increase for periods typically up to a decade due to natural climate variability. We find that increased sea ice formation due to lower ocean and air temperatures has caused the recent thickening of the ice cover. We also find that the Barents Sea ice thickness has decreased since the 1980s, and despite the recent thickening, the ice cover is much thinner now than it used to be. Key Points: Upward Looking Sonar measurements in the Barents Sea show increased sea ice thickness during the last decadeIn an ice‐ocean reanalysis (PIOMAS) the recent thickening is due to increased ice freezing, associated with lower ocean and air temperaturesThe long‐term thickness trend is negative, meaning that despite recent thickening, ice is now much thinner than in the 1980s [ABSTRACT FROM AUTHOR]

Previous studies have suggested that global ocean circulation would be significantly changed under global warming, while the change of North Equatorial Countercurrent (NECC) and its mechanisms are still unclear. Here, we investigate the location and intensity changes of NECC under global warming based on CESM1 high‐resolution long‐term simulations from the perspective of the Inter‐Tropical Convergence Zone (ITCZ), considering the close connection between NECC and ITCZ and well‐established changes of ITCZ. It is found that the annual‐mean NECC shifts equatorward of 0.39° and weakens by 21.71% in the RCP8.5 scenario at the end of 21st century. The NECC change is seasonally dependent, with maximum shift and weakening during spring, consistent with the changes of ITCZ. Both the location and intensity changes of ITCZ are important in the NECC changes, especially for spring. The weakening and equatorward shift of ITCZ contributes almost equally to the strongest decrease of spring NECC (47.63%). Plain Language Summary: Global ocean circulation is suggested to exhibit significant changes under global warming, but whether and why the North Equatorial Countercurrent (NECC) would change is still unclear. It is well known that the NECC and Inter‐Tropical Convergence Zone (ITCZ) are closely coupled through air‐sea interaction. Under global warming, previous studies found that the ITCZ would move closer to the equator with maximum shift in spring and the annual‐mean NECC would be weakened in response to the enhanced equatorial warming. Hence, it is important to investigate whether the NECC would change following the ITCZ change. Here, based on long‐term simulations from a high‐resolution climate model, it is found that the NECC will indeed move closer to the equator under global warming following the equatorward shift of ITCZ. The NECC also gets weakened under global warming following the weakened ITCZ convergence, although the ITCZ precipitation gets stronger due to more moisture under warming. In spring, the largest equatorward shift of ITCZ has a comparable contribution to the largest decrease of NECC. Key Points: The North Equatorial Countercurrent (NECC) would be southward shift and weakened under global warmingThe future changes of NECC are seasonally dependent, with maximum southward shift and weakening in springThe NECC changes are closely related to the location and intensity changes of Inter‐Tropical Convergence Zone, especially in spring [ABSTRACT FROM AUTHOR]

Global warming has intensified the intensity of compound drought‐hot extremes (CDHEs), posing more severe impacts on human societies and ecosystems than individual extremes. The vulnerability of global terrestrial ecosystems under CDHEs, along with its key influencing factors, remains poorly understood. Based on multiple remote sensing data, we construct a Vine Copula model to appraise vegetation vulnerability under CDHEs, and attribute it to climatic and biotic factors for five different vegetation types. High vulnerability is detected in central and southern regions of North America, eastern and southern regions of South America, Southern Africa, northern and western Europe, and northern and eastern Australia. The drier the climate, the higher will be the vulnerability. Furthermore, biodiversity and biomass are key biotic factors influencing the vulnerability of various vegetation types, such that ecosystems with richer biodiversity and higher biomass have lower vulnerability to CDHEs. The findings deepen understanding of terrestrial ecosystem response to CDHEs. Plain Language Summary: Drought and hot‐related extremes often coincide or follow one another, known as compound drought‐hot extremes (CDHEs), adversely affecting various vegetation processes. Thus, investigating the response relationship between vegetation dynamics and CDHEs is critical for maintaining the sustainable development of ecosystems under the background of climate warming. High ecosystem vulnerability is detected in central and southern regions of North America, eastern and southern regions of South America, Southern Africa, northern and western Europe, and northern and eastern Australia. Climatic factors (precipitation, arid index, temperature, and radiation) dominate the vulnerability of all vegetation types. The drier the climate, the higher will be the vulnerability of vegetation to CDHEs. Furthermore, biodiversity and biomass are key biotic factors influencing the vulnerability. The results provide us with essential insights for managing and adapting terrestrial ecosystems against climate change and are of interest to a wide range of audiences, including but not limited to ecologists, hydrologists, and climatologists. Key Points: A framework is developed to estimate the vegetation vulnerability in response to compound drought‐hot extremes and attribute it to various factorsVegetation vulnerability is higher for grasses and shrubs than for evergreen and deciduous forestsVegetation vulnerability is both affected by climate and biotic factors, but climate factors are more dominant [ABSTRACT FROM AUTHOR]

Existing studies have shown changes in the impact of atmospheric teleconnections on Indian Summer Monsoon (ISM) at interannual time scales due to the changing background state. However, the exploration of potential changes at subseasonal time scales remains limited. In this study, we use a causal discovery method to find the tropical atmospheric drivers of ISM subseasonal anomalies, and quantify the causal effects of the drivers on ISM, as well as the changes in causal effects over the past 40 years. We find the impact of tropical western Pacific on ISM subseasonal anomalies has strengthened, while the ISM self‐feedback has weakened. Wavenumber‐frequency analysis shows that the Rossby wave energy over 15°N–30°N at periods of 10–20 days has increased over the past four decades, which offers a plausible explanation for the enhanced tropical atmospheric teleconnection between western Pacific and ISM at subseasonal time scales. Plain Language Summary: The subseasonal variability of Indian Summer Monsoon (ISM) is vital to agriculture and economy. Therefore, it is important to understand the drivers of ISM subseasonal anomalies, how strong an impact of a driver is, as well as potential changes in the strength of such an impact. In this study, we explore the possible drivers for ISM subseasonal anomalies by using a causal discovery method, and taking into account the tropical geopotential height anomalies together with the ISM. Two tropical atmospheric drivers of ISM subseasonal anomalies are detected: one is the geopotential height anomalies over the tropical western Pacific, and the other is the ISM itself. Over the past four decades, the tropical western Pacific has exerted an enhanced influence on ISM subseasonal anomalies, while the self‐feedback of ISM has weakened. Under global warming, more anomalous convection signals can emanate over the western Pacific, and propagate westward to influence the ISM region, which might explain the physical mechanism behind the enhanced impact of the western Pacific on ISM subseasonal anomalies. Key Points: Causal analysis reveals the causal impact of the tropical western Pacific on Indian Summer Monsoon (ISM) and of ISM on itself at subseasonal time scalesThe causal impact of the western Pacific on ISM has strengthened, while the ISM self‐feedback has weakened over the past 40 yearsThe impact of the tropical western Pacific on ISM subseasonal anomalies is strengthening due to amplified Rossby wave energy over 15°N–30°N [ABSTRACT FROM AUTHOR]

The Fixed Anvil Temperature (FAT) hypothesis proposes that upper tropospheric cloud fraction peaks at a special isotherm that is independent of surface temperature. It has been argued that a FAT should result from simple ingredients: Clausius‐Clapeyron, longwave emission from water vapor, and tropospheric energy and mass balance. Here the first cloud‐resolving simulations of radiative‐convective equilibrium designed to contain only these basic ingredients are presented. This setup does not produce a FAT: the anvil temperature varies by about 40% of the surface temperature range. However, the tropopause temperature varies by only 4% of the surface temperature range, which supports the existence of a Fixed Tropopause Temperature (FiTT). In full‐complexity radiative‐convective equilibrium simulations, the spread in anvil temperature is smaller by about a factor of 2, but the tropopause temperature remains more invariant than the anvil temperature by an order of magnitude. In other words, our simulations have a FiTT, not a FAT. Plain Language Summary: Tropical anvil clouds play a large role in Earth's radiation balance, and their effect on anthropogenic global warming has been a point of contention. One popular school of thought is the Fixed Anvil Temperature (FAT) hypothesis, which argues that no matter how much the Earth warms, anvil clouds would continue to radiate to space at the same temperature. This behavior would amplify the warming caused by the addition of greenhouse gases to the atmosphere. In this paper, we show that the chain of logic underlying the FAT hypothesis—which attributes anvil cloud formation to a rapid decline of radiative cooling at the top of the troposphere—contains many weak and unsupported links. These weak links undermine the FAT hypothesis, which does not hold empirically in our simulations. However, the radiative tropopause does remain at a fixed temperature as the surface warms in our simulations. Therefore, we find evidence for a Fixed Tropopause Temperature (FiTT) rather than a FAT. There is currently no accepted theory for the tropopause temperature or why it appears to be independent of the surface temperature. Key Points: A Fixed Anvil Temperature (FAT) does not result from temperature‐dependent longwave radiative emission from water vaporSimulations of radiative‐convective equilibrium without a FAT still have a Fixed Tropopause Temperature (FiTT)A FiTT does not imply a FAT because anvil clouds are not produced by enhanced detrainment below the tropopause [ABSTRACT FROM AUTHOR]

Models struggle to accurately simulate observed sea ice thickness changes, which could be partially due to inadequate representation of thermodynamic processes. We analyzed co‐located winter observations of the Arctic sea ice from the Multidisciplinary Drifting Observatory for the Study of the Arctic Climate for evaluating and improving thermodynamic processes in sea ice models, aiming to enable more accurate predictions of the warming climate system. We model the sea ice and snow heat conduction for observed transects forced by realistic boundary conditions to understand the impact of the non‐resolved meter‐scale snow and sea ice thickness heterogeneity on horizontal heat conduction. Neglecting horizontal processes causes underestimating the conductive heat flux of 10% or more. Furthermore, comparing model results to independent temperature observations reveals a ∼5 K surface temperature overestimation over ice thinner than 1 m, attributed to shortcomings in parameterizing surface turbulent and radiative fluxes rather than the conduction. Assessing the model deficiencies and parameterizing these unresolved processes is required for improved sea ice representation. Plain Language Summary: Numerical sea ice models rely on conceptual simplifications of the sea ice and snow conditions observed in the Arctic and Southern Ocean. In particular, we cannot account for the variations in sea ice thickness and snow depth at the meter scale because we have limited computing capabilities and lack a detailed understanding of the processes defining how the sea ice system evolves. Furthermore, when designing sea ice models, we assume that the heat can flow only vertically, while in reality, this exchange is also horizontal and depends on the local topography. Thanks to observations collected during the Multidisciplinary Drifting Observatory for the Study of Arctic Climate expedition, we can better quantify model errors when using thermodynamic approximations. Our findings suggest that slightly more heat flows through the sea ice system than we can simulate with a simplified sea ice model and that the surface temperature of the model is too warm for thin ice and snow conditions. Learning the nature of these errors is useful because we could formulate corrections for our models, investigate the occurrence of climate feedback mechanisms, and possibly provide more reliable predictions about the current state of the sea ice and its future evolution, which is currently heavily impacted by global warming. Key Points: Multidisciplinary Drifting Observatory for the Study of Arctic Climate observations enable improved thermodynamic process understanding and representation in sea ice modelsResolved meter‐scale ice and snow thickness heterogeneity and horizontal heat conduction enhance conductive heat flux by up to 10% in modelsA positive surface temperature model bias up to 5 K suggests a misrepresentation of the surface energy balance over thin ice [ABSTRACT FROM AUTHOR]

We show that steric sea‐level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6‐year cycle do not always correspond to the equilibrium nodal tide. Plain Language Summary: The orbital position of the moon and the gravity pull it exerts on the earth varies with a period of 18.6 years. This cycle is called the lunar nodal cycle and it results in small variations of yearly averaged sea level (∼1–2 cm). Understanding this variability is important because it allows, for example, to quickly detect an acceleration in local sea‐level rise due to global warming. Here we show that the lunar nodal cycle also has an influence on the temperature and salinity in the surface 400m of the ocean. As a result, the ocean density changes and amplifies sea level variations along the western European coast. We make the hypothesis that since the lunar nodal cycle also influences the amplitude of the semidiurnal tides, and since those tides are known to be responsible for a large part of ocean mixing, a change in ocean mixing could be the cause of the ocean density variability that we observe. Key Points: Steric sea level changes are influenced by the 18.6‐year lunar nodal cycle along the western European coastThis influence could result from the modulation of semidiurnal tides by the lunar nodal cycle and the associated change in ocean mixingThis finding is a step toward resolving the long‐standing discrepancy between the theoretical long‐period nodal tide and observed signal [ABSTRACT FROM AUTHOR]

Humid heatwaves, characterized by high temperature and humidity combinations, challenge tropical societies. Extreme wet‐bulb temperatures (TW) over tropical land are coupled to the warmest sea surface temperatures by atmospheric convection and wave dynamics. Here, we harness this coupling for seasonal forecasts of the annual maximum of daily maximum TW (TWmax). We develop a multiple linear regression model that explains 80% of variance in tropical mean TWmax and significant regional TWmax variances. The model considers warming trends and El Niño and Southern Oscillation indices. Looking ahead, the strong‐to‐very‐strong El Niño at the end of 2023, with an Oceanic Niño Index of ∼2.0, suggests a 2024 tropical land mean TWmax of 26.2°C (25.9–26.4°C), and a 68% chance (24%–94%) of breaking existing records. This method also predicts regional TWmax in specific areas. Plain Language Summary: The heat and humidity in the tropics can be particularly challenging for people to stay comfortable and healthy. This combination of heat and moisture is described using a measure called the wet‐bulb temperature (TW). We found that these extremely humid and hot conditions on land can be predicted about 5 months in advance using a physics‐based statistical model. The forecast is possible because the peak of El Niño comes before the peak in the warmest sea surface temperatures, which affects the maximum TW on land. This prediction can help tropical societies to better prepare for extreme heat. Key Points: Tropical wet‐bulb temperatures (TW) peak around 5 months after El Niño wintersA multiple linear regression model considering the El Niño‐Southern Oscillation index and the long‐term warming trend effectively explains TWmax variabilityOur model quantifies the likelihood of strong El Niño and human‐induced warming pushing TWmax to record‐breaking levels [ABSTRACT FROM AUTHOR]

The Arctic is notable as a region where the greatest rate of increase in precipitation associated with global warming is anticipated. The Arctic precipitation simulated by the Coupled Model Intercomparison Project Phase 6 models showed a strong increasing trend since the 1980s. We found that the forcing factor of the trend is a combination of the continued strengthening of greenhouse gas forcing and the leveling off of aerosol forcing dominated in earlier periods. From an energetic perspective, we found that the increased atmospheric radiative cooling and reduced sensible heat transport from lower latitudes contributed equally to the recent increase in Arctic precipitation. The combination of these two energetic factors suggests a doubling of the Arctic amplification factor for precipitation relative to that for temperature. Future Arctic precipitation will change in proportion to the temperature change, and the fractional contributions of the energetic factors will remain stable across various scenarios. Plain Language Summary: The Arctic region is inherently a low‐precipitation area. However, because of global warming, precipitation is expected to increase substantially in the Arctic region compared with the global average when viewed as a percentage change from the original precipitation. This severely affects climate change in the Arctic environment. The latest climate model simulations show that there has been a rapid increase in precipitation in the Arctic region in recent decades. The driving factors behind the rapid increase are the effects of the accelerating growth of greenhouse gas concentrations, which were previously suppressed by the increasing anthropogenic aerosol emissions before the 1980s. Based on the heat budget of the atmosphere, we identified important factors contributing to these precipitation changes. These include enhanced radiative cooling (responding locally to increased air temperature) and reduced heat transport from lower latitudes due to greater temperature increases at higher latitudes. Future precipitation will change in proportion to the temperature change while maintaining consistent fractional contributions across different scenarios. Key Points: Trends in Arctic precipitation in the recent and future decades are examined from multimodel simulationsThe recent rapid increase is driven by accelerating greenhouse gas concentrations and plateauing growth in anthropogenic aerosol emissionsIncreased radiative cooling and reduced poleward sensible heat transport equally contributed to the Arctic precipitation changes [ABSTRACT FROM AUTHOR]

Interannual variations provide insight into the sensitivity of convective processes. Thus, CloudSat and ERA5 are used to explore the relationship among convective cores, outflows and environmental conditions during El Niño‐Southern Oscillation (ENSO) cycles. Results reveal greater upper‐tropospheric stability during El Niño, resulting in a lower level of neutral buoyancy compared to La Niña. However, outflow levels remain relatively consistent across ENSO cycles. This suggests that, despite less favorable conditions for deep convection during El Niño, stronger convective intensity is required to achieve outflow levels comparable to those in La Niña. Indeed, our results suggest that convection observed during El Niño tends to have broader cores and lower entrainment rates, translating to greater intensity compared to La Niña. These findings emphasize the importance of considering both large‐scale and convective‐scale processes, providing an update to the fixed anvil temperature (FAT) and the proportionately higher anvil temperature (PHAT) hypotheses as originally proposed. Plain Language Summary: Examining year‐to‐year variations provides unique insights into understanding how storms may change in a warmer climate. We use CloudSat and ERA5 reanalysis to examine variations in convective outflow (i.e., an airflow pushed out of storms), environmental factors, and cloud properties during ENSO cycles. Comparing El Niño to La Niña, we find that the atmosphere higher up in the troposphere tends to be more stable, which usually slows down the development of storms, during El Niño. However, the heights where convective outflow occurs do not change much during El Niño and La Niña events. This happens because during El Niño, the upward movement of air in storms is more powerful. This stronger upward movement offsets the stabilizing effect of the upper troposphere, so the overall outflow from the storms stays about the same. Our study aligns with the main idea of the FAT hypothesis, which postulates that in a warming climate, the temperature of the anvils (i.e., wide, flat clouds crawling out of the storm top) stays relatively constant primarily due to a thermodynamic constraint. However, our results show that convective‐scale processes with dynamic control play a key role as well, providing an update to the FAT as originally proposed. Key Points: LNB is lower during El Niño due to a greater UT stability, but convective outflow levels remain consistent across ENSO cyclesThe intensity of convective cores during El Niño has to be stronger than that during La NiñaOur finding necessitates a modification to FAT and the proportionately higher anvil temperature (PHAT) [ABSTRACT FROM AUTHOR]

Headwater catchments play a vital role in regional water supply and ecohydrology, and a quantitative understanding of the hydrological partitioning in these catchments is critically needed, particularly under a changing climate. Recent studies have highlighted the importance of subsurface critical zone (CZ) structure in modulating the partitioning of precipitation in mountainous catchments; however, few existing studies have explicitly taken into account the 3D subsurface CZ structure. In this study, we designed realistic synthetic catchment models based on seismic velocity‐estimated 3D subsurface CZ structures. Integrated hydrologic modeling is then used to study the effects of the shape of the weathered bedrock and the associated storage capacity on various hydrologic fluxes and storages in mountainous headwater catchments. Numerical results show that the weathered bedrock affects not only the magnitude but also the peak time of both streamflow and subsurface dynamic storage. Plain Language Summary: In mountainous terrains, precipitation falling as snow and rain not only flows into streams but also infiltrates into the subsurface, replenishing the subsurface water storage. This stored water in the subsurface (including both soil moisture and rock moisture) supports various ecohydrological processes such as groundwater recharge and plant water use. In this study, we use computational simulation to study how the below‐ground heterogeneity affects the partitioning of precipitation into streamflow, subsurface water storage, plant water use, and deep groundwater recharge. Here, below‐ground heterogeneity refers to the depth extent of weathered rock at the hillslope‐ or catchment‐scale. Simulation results show that both the annual magnitude and weekly variations of the streamflow and subsurface storage change noticeably as the below‐ground weathering pattern varies, in particular under a snow‐dominated weather scenario. The outcome of this study will potentially help stakeholders develop more informed water management practices in the face of a warming climate and more extreme weather events. Key Points: Integrated hydrologic modeling of catchments with different subsurface critical zone structures was performedSimulation results show that the shape of the weathered bedrock and associated storage capacity affect various hydrologic processes in the catchmentThe effect of weathered bedrock on hydrological partitioning is more significant under weather conditions featuring large snow fractions [ABSTRACT FROM AUTHOR]

The Maritime Continent (MC), a critical region for inter‐basin climate interaction, harbors the world's highest marine biodiversity. Ocean warming in the MC, although with notable impacts on regional climate and marine ecosystems, remains poorly constrained by observations. By applying a volume‐correction algorithm to existing gridded observational data sets, this study provides an estimate for the ocean heat content (OHC) change of the MC. The results suggest a substantial OHC increase of 2.65 ± 0.46 Zettajoules during 1990–2015 (1.08 ± 0.17 W m−2) and limited changes before and after. This increase primarily arose from the enhanced Pacific Walker circulation, which drove a convergence of upper‐layer warm water toward the MC. A potential heat storage "hotspot" with enhanced warming below 500 m emerges within the Sulu Sea, which is supported by analysis of profile data collected in boreal winter but not in other seasons. Plain Language Summary: The Maritime Continent (MC) between the Pacific and Indian Oceans is a tropical region of numerous islands separating small sea basins. This region possesses the highest marine biodiversity of the world, which has been affected by the warming climate. How much the MC has warmed awaits assessment. In this work, we applied a volume‐correction method to existing observational data sets to improve the estimate of ocean heat content (OHC) change. We found that the OHC of the MC increased abruptly by 2.65 ± 0.46 Zettajoules during 1990–2015, equivalent to 1.08 ± 0.17 W m−2, and showed limited changes before and after this period. This increase was mainly caused by stronger than normal Pacific trade winds. Notably, the Sulu Sea showed stronger warming below 500 m than other seas of the MC, which is particularly robust in boreal winter. Key Points: The volume‐corrected 0–2,000 m ocean heat content of the Maritime Continent (MC) showed a substantial increase of 2.65 ± 0.46 ZJ during 1990–2015The enhanced Pacific Walker circulation during 1990–2015 drove a convergence of upper‐layer warm water toward the MCThere is a potential heat storage "hotspot" in the Sulu Sea with enhanced warming below 500 m [ABSTRACT FROM AUTHOR]

Drought has affected much of the western United States since about the year 2000, including the Upper Colorado River Basin (UCRB). Using a time series of UCRB streamflow derived from a tree‐ring based reconstruction of UCRB streamflow for the years 1 CE through 1905 CE, together with naturalized UCRB streamflow values for 1906 CE through 2021 CE, we identify 51 drought events, including the 2000–2021 drought. Although the recent 2000–2021 drought has been relatively severe, it is not the most severe of the UCRB drought events we identified. Results also indicate that natural variability combined with projected climate warming can result in UCRB drought events that are more severe than any drought since 1 CE. Plain Language Summary: A long and severe drought has affected much of the western United States since about the year 2000 CE, including the Upper Colorado River Basin (UCRB). Comparing this drought to past UCRB droughts (during 1 CE through 2021 CE), we find that the 2000–2021 drought is not the most severe UCRB drought. The results also suggest that natural variability combined with projected climate warming could result in UCRB drought events that are more severe than any drought since 1 CE. Key Points: We identify 51 drought events for the years 1 CE through 2021 CE for the Upper Colorado River Basin (UCRB)Twelve of the 51 UCRB droughts we identify were more severe than the 2000–2021 droughtNatural variability and/or a 1°C temperature increase can trigger UCRB droughts that are more severe than the 2000–2021 drought [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) that undergo rapid intensification (RI) before landfall are notoriously difficult to predict and have caused tremendous damage to coastal regions in the United States. Using downscaled synthetic TCs and physics‐based models for storm tide and rain, we investigate the hazards posed by TCs that rapidly intensify before landfall under both historical and future mid‐emissions climate scenarios. In the downscaled synthetic data, the percentage of TCs experiencing RI is estimated to rise across a significant portion of the North Atlantic basin. Notably, future climate warming causes large increases in the probability of RI within 24 hr of landfall. Also, our analysis shows that RI events induce notably higher rainfall hazard levels than non‐RI events with equivalent TC intensities. As a result, RI events dominate increases in 100‐year rainfall and storm tide levels under climate change for most of the US coastline. Plain Language Summary: Tropical cyclones (TCs) that rapidly intensify (RI) before hitting land are typically hard to predict and cause immense destruction. We used synthetic TCs downscaled from global climate models and physics‐based hazard models to examine the dangers posed by these RI storms in historical and future climates. The TC simulation shows that, as the climate warms, the number of TCs undergoing rapid intensification could rise substantially in the North Atlantic region. Additionally, the likelihood of rapid intensification within 24 hr of landfall significantly increases. These TCs are much riskier, particularly in terms of heavy rainfall, even when compared to equally strong TCs that did not rapidly intensify. Consequently, 100‐year rainfall and storm tide levels will greatly increase under climate change, largely due to the increase of RI events in the future. Key Points: Future climate warming causes large increases in the probability of rapid intensification (RI) within 24 hr of landfallRI events show notably higher rainfall hazard levels than non‐RI events even with equivalent TC intensityChanges in RI events dominate future increases in 100‐year rainfall and storm tide levels for most of the US coastline [ABSTRACT FROM AUTHOR]

Subseasonal droughts including flash droughts have occurred frequently in recent years, which are accompanied by heatwaves or wildfires that raise a wide concern on environmental risk. However, the changing characteristics of subseasonal drought propagation, and the possible climate and environmental drivers remain unknown. This study quantifies the propagation characteristics from meteorological drought to soil drought using a Copula‐based Bayesian framework, and shows that higher propagation risks mainly occur in more humid regions with denser vegetation cover. Trends in drought propagation risk vary regionally, with a global increase of 2%/decade (p < 0.01) during 1980–2022. Vegetation greening and climate warming are the key drivers over >71% of the global vegetated lands, with mean contribution rates of 39.5% and 36.5% respectively. Other climatic factors including vapor pressure deficit and precipitation also paly critical roles, which closely correlate with temperature and vegetation. These findings highlight the importance of vegetation greening on subseasonal drought propagation dynamics. Plain Language Summary: Changing characteristics of seasonal‐interannual drought propagation have been well studied at watershed scales, and they were found to be principally ruled by climate divers and moderately affected by vegetation factors. In a changing environment, subseasonal droughts have occurred frequently worldwide, showing different characteristics and environmental responses with long‐term droughts. However, how climate and environment factors affect the subseasonal drought propagation characteristics remains unknown. This study investigates the propagation risk from meteorological drought to soil drought at subseasonal timescales based on a conditional probability method. We found that higher subseasonal drought propagation risks were mainly located in more humid vegetated regions, and the propagation risk showed an overall increasing trend (p < 0.01) from a global perspective but with significant spatial variations during 1980–2022. Climatic and vegetation factors jointly affect the trends in subseasonal drought propagation risks. Among them, vegetation greening and climate warming were the most important drivers over >71% of global vegetated regions. Key Points: Higher propagation risk from subseasonal meteorological to soil drought mainly occurred in humid regions with denser vegetation coverTrends in subseasonal drought propagation risk varied regionally, with a global mean increase of 2%/decade during 1980–2022Greening and warming were the key drivers of the trends, with global mean contribution rates of 39.5% and 36.5% respectively [ABSTRACT FROM AUTHOR]

Preparing for climate change requires an understanding of the degree to which global warming has regional implications. Here we document a strong relationship between the magnitude and extent of warming and explain its origin using a simple model based on binomial statistics. Applied to HadCRUT5 instrumental observations, the model shows that 96% of interannual variability in the proportion of regions experiencing anomalous warmth over the last century can be explained on the basis of the magnitude of global mean surface temperature (GMST) anomalies. The model performs similarly well when applied to a variety of unforced and forced model simulations and represents a general thermodynamic link between global and local warming on annual timescales. Our model predicts that, independent of the baseline that is chosen, 95% of the globe is expected to experience above‐average annual temperatures at 0.7°C of GMST warming, and 99% at 1.0°C of warming. Plain Language Summary: Whereas many studies focus on global mean surface temperature (GMST), it is also critical to understand the spatial patterns of this warming in preparing for a warmer world. Using a simple statistical model, we describe a strong relationship between GMST and the spatial extent of above‐average temperature anomalies each year. This relationship is shown to hold across various model simulations and historical temperature data, capturing both annual and decadal trends. We can use our model alongside projections to gain insight into the extent of warming variations. For instance, 1°C GMST warming from a given baseline climate generally corresponds to a world in which 99% of places across the globe will be anomalously warm every year. Key Points: A simple statistical model relates the proportion of Earth's area that is anomalously warm to global mean surface temperature (GMST)The model predicts 96% of the interannual variance in the proportion of anomalous warmth between 1900 and 2021 on the basis of GMST1.0°C of GMST warming corresponds to 99% of the globe being anomalously warm, independent of baseline [ABSTRACT FROM AUTHOR]

This study presents a methodological advancement in the field of clumped‐isotope (∆47) thermometry, specifically tailored for application to freshwater ostracods. The novel ostracod clumped isotope approach enables quantitative temperature and hydrological reconstruction in lacustrine records. The relationship between ∆47 and the temperature at which ostracod shell mineralized is determined by measuring ∆47 on different species grown under controlled temperatures, ranging from 4 ± 0.8 to 23 ± 0.5ºC. The excellent agreement between the presented ∆47 ostracod data and the monitored temperatures confirms that ∆47 can be applied to ostracod shells and that a vital effect is absent outside the uncertainty of measurements. Results are consistent with the carbonate clumped‐isotope unified calibration (Anderson et al., 2021, https://doi.org/10.1029/2020gl092069), therefore, an ostracod‐specific calibration is not needed. The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies all around the world, as lakes and ostracods are found in all climatic belts. Plain Language Summary: In the framework of global warming, the reconstruction of past climatic conditions is important to understand the future evolution of climate and its impact. Lake sediments can be used as archives to quantify these effects. This study presents a novel paleo‐thermometer based on the application of clumped‐isotope technique (i.e., measurement of the number of 13C–18O bonds in carbonate minerals that depends on the temperature of carbonate precipitation) on carbonatic microcrustacea, named ostracods that commonly live in lakes. By using ostracods that formed their shells at known temperatures, we demonstrate that they can be easily used to reconstruct water temperature and hydrological conditions (precipitation/evaporation). The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies around the world, as lakes and ostracods are located in all climatic belts. Key Points: ∆47—ostracod signal accurately records the shell calcification temperature∆47—ostracod signal is not affected by the so called "vital effect"The unified calibration of Anderson et al. (2021) can be used to convert the ∆47—ostracod signal into accurate temperatures [ABSTRACT FROM AUTHOR]

Accurately representing the ocean carbon cycle in Earth System Models (ESMs) is essential to understanding the oceanic CO2 sink evolution under CO2 emissions and global warming. A key uncertainty arises from the ESM's inability to explicitly represent mesoscale eddies. To address this limitation, we conduct eddy‐resolving experiments of CO2 uptake under global warming in an idealized mid‐latitude ocean model. In comparison with similar experiments at coarser resolution, we show that the CO2 sink is 34% larger in the eddy‐resolving experiments. 80% of the increase stems from a more efficient anthropogenic CO2 uptake due to a stronger Meridional Overturning circulation (MOC). The remainder results from a weaker reduction in CO2 uptake associated to a weaker MOC decline under global warming. Although being only a fraction of the overall response to climate change, these results emphasize the importance of an accurate representation of small‐scale ocean processes to better constrain the CO2 sink. Plain Language Summary: Today, the ocean absorbs ∼25% of the CO2 emissions caused by human activities. This CO2 sink is primarily driven by the increase of CO2 in the atmosphere, but it is also influenced by physical changes in the ocean's properties. Earth System Models are used to project the future of the ocean CO2 sink. Due to limited computational capacity, ESMs need to parameterize flows occurring at scales smaller than ∼100 km, their typical horizontal grid resolution. To overcome the computational limitations, we use an ocean biogeochemical model representing an idealized North Atlantic ocean of reduced dimensions. We conduct simulations of global warming using increasingly finer horizontal resolutions (from ∼100 km to ∼4 km). Our findings demonstrate that the ocean CO2 uptake is highly influenced by resolution. This sensitivity primarily stems from how the overturning circulation's mean state depends on resolution, as well as how it responds to global warming. Although our results capture only a fraction of the overall oceanic response to climate change, they emphasize the significance of accurately representing the role of small‐scale ocean processes to better constrain the future evolution of ocean carbon uptake. Key Points: We conducted idealized ocean simulations under global warming and rising atmospheric CO2 at coarse and eddy‐resolving resolutionsCO2 sink is larger by 34% at eddy resolution, due to larger anthropogenic CO2 uptake combined with weaker climate feedbackThis ensues from the model's overturning circulation sensitivity to resolution in both historical and future state [ABSTRACT FROM AUTHOR]

Detecting the response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic warming can only be made with fingerprints indirectly because of the lack of sufficiently long direct measurements. However, whether the relationship between the AMOC and its fingerprints is stationary is rarely examined. This study uses coupled and ocean‐alone model simulations to investigate the sensitivity of two typical AMOC fingerprints under future anthropogenic warming. We found a lower sensitivity of the North Atlantic warming hole fingerprint in future warming scenarios associated with the differing vulnerability of deep‐water origins to external forcing and climate feedback. In contrast, the remote South Atlantic salinity pile‐up fingerprint is relatively insensitive to variations in AMOC sources, and its sensitivity to the AMOC is slightly enhanced by an intensified hydrological cycle. Our study implies that fingerprints outside the northern deep convection region may become more suitable in representing the response of AMOC to future warming. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is an important component of the Earth's climate system. Due to the lack of sufficiently long direct measurements, the response of the AMOC to global warming has to be inferred from the AMOC fingerprints. Here we show that the sensitivity of AMOC fingerprints can be nonstationary under continuous warming scenario. It raises concerns when reconstructing AMOC change using these fingerprints in the context of climate change. Possible reasons for variations in sensitivity are further investigated. For the subpolar SST fingerprint, its sensitivity to AMOC change will be reduced as a result of differing responses of multiple deep‐water sources to global warming. The fingerprint outside the subpolar North Atlantic is less influenced by the variation in AMOC sources and can more faithfully represent the AMOC change in a warming climate. Key Points: Relationship between Atlantic meridional overturning circulation (AMOC) fingerprints and AMOC can be nonstationary in a changing climateWarming hole shows reduced sensitivity to AMOC in a warmer climate while the sensitivity of salinity pile‐up is slightly strengthenedDiffering vulnerability of AMOC sources and changed hydrological cycle influence the sensitivity of the two fingerprints, respectively [ABSTRACT FROM AUTHOR]

The influence of climate feedbacks on regional hydrological changes under warming is poorly understood. Here, a moist energy balance model (MEBM) with a Hadley Cell parameterization is used to isolate the influence of climate feedbacks on changes in zonal‐mean precipitation‐minus‐evaporation (P − E) under greenhouse‐gas forcing. It is shown that cloud feedbacks act to narrow bands of tropical P − E and increase P − E in the deep tropics. The surface‐albedo feedback shifts the location of maximum tropical P − E and increases P − E in the polar regions. The intermodel spread in the P − E changes associated with feedbacks arises mainly from cloud feedbacks, with the lapse‐rate and surface‐albedo feedbacks playing important roles in the polar regions. The P − E change associated with cloud feedback locking in the MEBM is similar to that of a climate model with inactive cloud feedbacks. This work highlights the unique role that climate feedbacks play in causing deviations from the "wet‐gets‐wetter, dry‐gets‐drier" paradigm. Plain Language Summary: Climate feedbacks, which act to amplify or dampen global warming, play an important role in shaping how the climate system responds to changes in greenhouse‐gas concentrations. Here, we use an idealized climate model, which makes a simplified assumption about how energy is transported in the atmosphere, to examine how climate feedbacks influence the patterns of precipitation and evaporation change under global warming. We find that the cloud feedback acts to narrow the band of rainfall on the equator known as the Intertropical Convergence Zone and that the surface‐albedo feedback acts to shift the location of maximum rainfall. We also find that the cloud feedback accounts for most of the uncertainty associated with feedbacks in regional hydrological change under warming. The idealized model with a locked cloud feedback also simulates a change in precipitation and evaporation that is similar to a comprehensive climate model with an inactive cloud feedback. Key Points: A moist energy balance model (MEBM) is used to investigate the influence of climate feedbacks on regional hydrological changes under warmingCloud feedbacks act to narrow and increase tropical P − E and are the dominant source of feedback uncertainty in regional hydrological changesThe MEBM with a locked cloud feedback largely replicates a climate model with an inactive cloud feedback [ABSTRACT FROM AUTHOR]

Future climate projections suggest a poleward shift of the maximum intensity of tropical cyclones (TCs) over the western North Pacific. However, the global nature of the latitudinal change in TC genesis under global warming remains poorly understood. We show, using large‐ensemble high‐resolution atmospheric model simulations (d4PDF) with four warming scenarios, that the poleward shift is a robust change over the globe, attributable to the weakening of the Hadley circulation. The weakened ascent driven by the upper‐tropospheric warming suppresses the TC genesis within 5°–20° latitudes, whereas the weakened descent enhances the TC genesis in the poleward latitudes. We further estimate the poleward shift of TC genesis to emerge at the 2 K global warming over the Arabian Sea, South Atlantic and Pacific Oceans and at the 4 K warming over the North Pacific. The present results underscore the potential for increasing social and economic risks associated with TCs at higher latitudes. Plain Language Summary: Climate models have projected a decrease in TC genesis frequency in future warming. However, the global nature of the latitudinal change in TC genesis under global warming remains uncertain partly due to insufficient resolution as well as the ensemble size of climate model simulations. We show a global feature of the robust poleward shift of the TC genesis during the active seasons of both hemispheres scaled with the global warming level, which can be attributed to the weakening of the Hadley circulation. The weakened ascending branch of the Hadley circulation, driven by the increased upper tropospheric warming, potentially hinders TC genesis within 5°–20° latitudes. Conversely, the weakened descending branch of the Hadley circulation enhances the likelihood of TC genesis within 20°–35° latitudes. We further estimate that the signal of TC genesis is expected to emerge over high latitudes of the Arabian Sea, South Atlantic and South Pacific Oceans at the 2 K warming and at the 4 K warming over the North Pacific. The present analyses have significant implications not only for assessing the reliability of future TC‐related changes in climate models but also for estimating the increased TC‐related hazards at higher latitudes under global warming. Key Points: We project a global feature of the robust poleward shift of tropical cyclone (TC) genesis during active seasons of both hemispheresMore TC genesis at high latitudes can be attributed to the weakening of the Hadley circulationPoleward shift of TC genesis emerges at 2 K warming over Arabian Sea, South Atlantic and Pacific Oceans and at 4 K warming over North Pacific [ABSTRACT FROM AUTHOR]

Based on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current. Plain Language Summary: As an important component of the Asian monsoon system, the Southeast Asian summer monsoon (SEASM) is crucial for the livelihoods of billions of people in East Asia, and is closely connected to a climate phenomenon called El Niño‐Southern Oscillation (ENSO). Understanding how the relationship between SEASM and ENSO will change in the future is important for enhancing our knowledge of climate change in East Asia. Outputs of 28 climate models from the Phase 6 of the Coupled Model Intercomparison Project show that ENSO will exert enhanced impacts on the SEASM in a warmer climate. The enhanced influences of ENSO on the monsoon will exacerbate the reduction of rainfall over the western North Pacific during the post‐El Niño summer. We find that such projected changes are mainly caused by weakened warm sea surface temperature (SST) anomalies (SSTAs) in the western equatorial Pacific (WEP). Further analyses indicate that the change in WEP SSTAs can be linked to the El Niño‐like change in climatological SSTs in the tropical Pacific. This study depicts detailed physical processes responsible for the projected changes in ENSO's impacts on the SEASM. Key Points: The effects of the El Niño‐Southern Oscillation on the Southeast Asian summer monsoon will strengthen under global warmingThe enhanced El Niño's impacts result from the weakened warm sea‐surface temperature (SST) anomalies in the western equatorial Pacific (WEP)The weakened WEP SST anomalies are related to the eastward shift of anomalous Walker circulation and the slackened mean zonal ocean currents [ABSTRACT FROM AUTHOR]

Global warming is expected to have far‐reaching impacts on the frequency and intensity of extreme events, but the effects of anthropogenic warming on the emergence seasons of year‐maximum near‐surface wind speed (NSWS) remain poorly understood. We provide a comprehensive map of the changing emergence seasons of year‐maximum NSWS using Coupled Model Intercomparison Project Phase 6 projections. Our analysis reveals a rapid response of synoptic‐scale extreme NSWS to global warming, with consistent spatial patterns observed across various periods and warming scenarios. The most significant increase (∼16%) in the emergence season is projected to occur in December‐January‐February (DJF) over Mid‐high‐latitude Asia by the end of the 21st century. The study also anticipates changes in the emergence seasons of year‐maximum NSWS at a regional scale. These results deepen our understanding of the complex and interconnected nature of global climate change and underscore the need for concerted efforts in addressing this pressing challenge. Plain Language Summary: Global warming is indisputably triggering changes in the world's weather systems, leading to more frequent and intense extreme weather events. However, it is unclear how anthropogenic warming affects the timing of the annual strongest near‐surface wind speed (NSWS). In this study, we used state‐of‐the‐art global climate models to create a comprehensive map illustrating these NSWS patterns of response to global warming. We discovered that these changes are consistent across various time periods (near to long term) and warming scenarios (low to high warming), revealing a robust relationship between extreme NSWS and global warming. The most significant change is observed during December‐January‐February in Mid‐high‐latitude Asia, with an increase of about 16% by the end of the 21st century. Our findings suggest that we can expect more year‐maximum NSWS occurs in different regions during specific seasons: December‐January‐February in North America and Asia, March‐April‐May in Africa, June‐July‐August in Asia and West Africa, and September‐October‐November in South America and Australia. These results offer valuable insights for guiding adaptation efforts even if ambitious climate actions manage to limit global warming at a lower level. Key Points: Changing emergence seasons of the land year‐maximum near‐surface wind speed (NSWS) map is createdThere is a rapid response of emergence seasons of year‐maximum NSWS to anthropogenic warmingThe strongest increase (16%) in emergence season is projected to occur in December‐January‐February over Mid‐high‐latitude Asia [ABSTRACT FROM AUTHOR]

High‐resolution regional climate model (RCM) simulations of global warming consistently predict larger percentage increases in precipitation in the lee of midlatitude mountain ranges than on their windward slopes, indicating a weakening of the orographic rain shadow. This redistribution of precipitation could have profound consequences for water resources and ecosystems, but its underlying mechanisms are unknown. Here we show that rain‐shadow weakening is just one manifestation of a more general decrease in the influence of orography on precipitation under global warming. We introduce a simple model of precipitation change based on this principle, and find that it agrees well with an ensemble of high‐resolution simulations performed over the western United States. We argue that diminished orographic influence can be explained by the unique vertical structure of orographically forced ascent, which tends to maximize in the lower atmosphere where condensation is thermodynamically less sensitive to warming. Plain Language Summary: Mountains supply about half of the fresh water used by humans globally and often regulate the seasonal cycle of stream flows by accumulating snowpack during the wet season and releasing it over the dry season. Regional simulations tend to predict large future changes in the spatial pattern of precipitation in mountainous regions, but the reasons for these changes are not well understood. Here we show that the predicted patterns of precipitation change in the western US can be explained by a simple mechanism that causes the influence of mountains to weaken with increasing temperatures. We introduce a simple model of precipitation change based on this effect, and show that its predictions closely match those of far more sophisticated regional simulations of future warming over the western US. Key Points: Orographic influence on precipitation is reduced in simulations of global warmingFractional precipitation change scaled by warming is reproducible with a simple model [ABSTRACT FROM AUTHOR]