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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]

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]

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]

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]

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]

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]

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]

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]

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]

A key implication of the well known wet‐get‐wetter/dry‐get‐drier (WGW) scaling is that model biases in the representation of precipitation and evaporation in the present climate lead to spurious projected changes under global warming. Here we estimate the extent of such spurious changes in projections by 60 models participating in phases 5 and 6 of the Coupled Model Intercomparison Project. Utilizing known thermodynamic constraints on evaporation, we show that the WGW scaling can be applied to precipitation and evaporation separately (specific WGW scaling), which we use to correct for spurious projected changes in precipitation and evaporation over tropical oceans. The spurious changes in precipitation can be of comparable amplitude to projected changes, but are generally small for evaporation. The spurious changes may increase the uncertainty in projections of tropical precipitation and evaporation by up to 30% and 15% respectively. Plain Language Summary: It is well established that if winds are assumed unchanged, the response of the hydrological cycle to global warming will follow the simple rule that wet regions get wetter and dry regions get drier. A key implication of this rule is that climate model errors in the representation of precipitation and evaporation in the present climate lead to artificial simulated responses in a warming climate. Here we show that this simple rule can be applied separately to precipitation and evaporation over tropical oceans. We then use this rule to estimate the extent of the artificial response over tropical oceans to global warming in modern climate models, caused by errors in the representation of precipitation and evaporation. Overall, the artificial changes may increase the uncertainty in projections by up to 30% for precipitation and 15% for evaporation. Key Points: Biases in the representation of the present hydroclimate lead to spurious but correctable projected changes in precipitation and evaporationA wet‐get‐wetter/dry‐get‐drier scaling bias adjustment can be applied separately to precipitation and evaporationThe spurious changes increase uncertainty in precipitation and evaporation projections by up to 30% and 15% respectively [ABSTRACT FROM AUTHOR]

Low cloud feedback in global warming projections by climate models is characterized by its positive sign, the mechanism of which is not well understood. Here we propose that the positive sign is primarily caused by the increase in upward longwave radiation from the sea surface. We devise numerical experiments that enable separation of the feedback into components coming from physically distinct causes. Results of these experiments with a climate model indicate that increases in upward longwave radiation from the sea surface cause warming and absolute drying in the boundary layer, leading to the positive low cloud feedback. The absolute drying results from decrease in surface evaporation, and also from decrease in inversion strength which enhances vertical mixing of drier free tropospheric air into the boundary layer. This mechanism is different from previously proposed understanding that positive low cloud feedback is caused by increases in surface evaporation or vertical moisture contrast. Plain Language Summary: We project future climate change induced by atmospheric greenhouse gas increases by conducting numerical simulations using specialized computer codes, namely Global Climate Models. Results of such simulations are characterized by decreases in low cloud with warming at the Earth's surface, which amplifies the warming by reflecting less sunlight back to space and allowing more sunlight to be absorbed at the surface. This amplifying effect, called "positive low cloud feedback," is important because the amount of future warming affects our living and safety. However, the mechanism of the low cloud decreases with warming is not well understood. Here we propose that the low cloud decrease is primarily caused by increase in upward longwave radiation from the sea surface. We devise numerical simulations that enable the separation of the low cloud feedback into components coming from physically distinct causes. Results of the simulations indicate that increases in upward longwave radiation from the sea surface cause warming and drying near the Earth's surface, leading to the low cloud decrease. This mechanism is different from previously proposed understanding that the low cloud decrease is due to increases in sea surface evaporation or vertical moisture contrast. Key Points: The increase in longwave radiation from the sea surface is a leading order cause of the positive low cloud feedback in a climate modelThis increase in longwave radiation leads to warming and drying in the boundary layer, which contributes to the decrease in the low cloudThis mechanism is not associated with increases in surface evaporation or vertical moisture contrast [ABSTRACT FROM AUTHOR]

The evolution of tropical sea surface temperatures (SSTs) in response to greenhouse warming is of great societal and scientific interest. Most state‐of‐the‐art climate models predict a mean "El Niño‐like" warming pattern by century‐end, characterized by greater warming over the Pacific cold tongue compared to the western warm pool. However, it is unclear which proposed mechanism dominates in this response. Here, we present partially coupled abrupt CO2 doubling experiments in which surface wind stress and shortwave heating are overridden to values from a control simulation. Contrary to previous studies, we find that experiments with overriding of surface wind stress exhibit only 58% of the full reduction in east‐west SST contrast. When both surface wind stress and shortwave flux are overridden, only 34% of the full reduction remains, controlled by spatially‐varying evaporative cooling. These results underscore the importance of Bjerknes and shortwave feedbacks in the tropical Pacific SST response to global warming. Plain Language Summary: Sea surface temperature patterns over the tropical Pacific have an outsized impact on regional and global climates, as exemplified by the impact of present day El Niño events. As such, how tropical SST patterns will evolve under global warming has been a topic of keen scientific interest and scrutiny. While most state‐of‐the‐art climate models show a transition to a more "El Niño‐like" mean state by the end of the century, the precise reason why remains debated. Here, we present a method that estimates the contributions of changing surface winds and solar radiation to the tropical ocean warming pattern simulated by a state‐of‐the‐art climate model. While some previous studies have argued that changes in surface winds are unimportant, here we show that the weakening of surface easterly winds that maintain a colder eastern Pacific is a major factor leading to a reduced east‐west temperature contrast. Key Points: Climate models predict an eastern equatorial Pacific warming pattern in the tropical Pacific by the end of the 21st centuryWe use partially coupled climate simulations to assess contributions of wind stress, shortwave, and evaporative feedbacks to this patternWeakening of surface wind stress is found to be a major factor leading to a reduced zonal SST gradient over the tropical Pacific [ABSTRACT FROM AUTHOR]

Apparent hydrological sensitivity (ηa), the change in the global mean precipitation per degree K of global surface warming, is a key aspect of the climate system's response to increasing CO2 forcing. To determine whether ηa depends on the forcing amplitude we analyze idealized experiments over a broad range of abrupt CO2 forcing, from 2× to 8× preindustrial values, with two distinct climate models. We find little change in ηa between 2× and 4×CO2, and almost no change beyond 5×CO2. We validate this finding under transient CO2 forcing at 1%‐per‐year, up to 8×CO2. We further corroborate this result by analyzing the 1%‐per‐year output of more than 15 CMIP5/6 models. Lastly, we examine the 1,000‐year long LongrunMIP model output, and again find little change in ηa. This wealth of results demonstrates that ηa is a very weak function of CO2 forcing. Plain Language Summary: Hydrological sensitivity (HS) is defined as the change in globally‐averaged precipitation per degree K of surface temperature increase caused by increasing concentrations of greenhouse gasses, such as CO2. It is important to understand how HS changes with different levels of CO2 in the atmosphere. To do this we analyzed model experiments with varying increases of CO2. We find little change in HS between 2× to 4× the pre‐industrial levels of CO2, and almost no change beyond 5×CO2. Additionally, we analyze model experiments where CO2 concentrations increase by 1% per year and see similar results. Finally, we validate this finding with models with significantly longer run times. We thus conclude that HS is independent of the level of CO2 in the atmosphere. Key Points: We examine the dependence of apparent hydrological sensitivity (ηa, defined as ΔP/ΔT) on the magnitude of CO2 forcingWe find little change in ηa in abrupt 2× to 8×CO2 experiments and a transient 1%/year experiment up to 8×CO2Little change in ηa, notably at large CO2, is also found in most CMIP5, CMIP6, and Longrun‐MIP models [ABSTRACT FROM AUTHOR]

Despite multiple advances in the understanding of the water cycle intensification in a warmer climate, climate models still diverge in their hydrological projections. Here we constrain annual runoff projections over individual and aggregated Arctic river basins. For this purpose, we use two ensembles of global climate models and two statistical methods: a regression scheme assuming similar runoff sensitivities at interannual versus climate change timescales, and a Bayesian method where models are used to derive a posterior runoff response conditioned on historical observations. While both techniques are shown to narrow model uncertainties, more or less substantially depending on rivers, the Bayesian method is less sensitive to the choice of the model ensemble and is more skillful when tested with synthetic observations. It has also been applied over the whole Arctic watershed, showing so far a limited narrowing of the inter‐model spread, but its skill will further improve with increasing climate change. Plain Language Summary: Despite considerable progress in understanding the intensification of the water cycle in response to global warming, projections of changes in the various components of the continental water balance remain highly model‐dependent. This is particularly the case for the evolution of runoff, which remains very sensitive to the way atmospheric and continental processes are represented. Here we evaluate and compare two statistical methods to constrain climate projections of annual runoff at high latitudes via available or synthetic observations. Our Bayesian method, based on historical observations, is found to be more robust than a previous method based on observed interannual climate variability. It underlines the importance of having reliable flow reconstructions to constrain runoff projections and of using empirical observational constraints with caution, as they are likely to lead to over‐confident projections. Key Points: Runoff sensitivity to temperature depends on the considered timescaleObservational constraints based on interannual variability can thus lead to overconfident projectionsObservational constraints based on historical trends can be effective if such trends are driven by human emissions of greenhouse gasses [ABSTRACT FROM AUTHOR]

Previous research showed that radiative feedbacks are essential to the spontaneous development of convective aggregation (CSA) in idealized atmosphere models. We find that the contribution of radiative feedbacks decreases with warming and that, in warm climates, CSA occurs without radiative feedbacks. We perform 2D simulations in different climates using a cloud‐resolving model and use a layerwise moist static energy (LMSE) framework to quantify the contribution of radiative feedbacks to the increase of LMSE variance, which characterizes the development of CSA. The result shows that positive radiative contribution dominates the LMSE variance production when SST is below 300 K; when SST exceeds 300 K, adiabatic contribution shifts from negative to positive and dominates the variance production. Then we turn off radiative feedbacks by horizontally homogenizing radiative heating rates at all model levels. CSA still occurs in warmer climates (310–320 K). This result agrees with the LMSE diagnosis and additional 3D simulations. Plain Language Summary: Convective clouds often come together and combine to create larger storms. Previous research indicated that this aggregation is mainly driven by a feedback loop influenced by atmospheric radiation. However, our study reveals that this feedback mechanism becomes less effective as the climate warms. In particular, in a warmer climate where surface temperatures are around 10 or 20° higher than present, convective clouds can still aggregate even in the absence of this radiative feedback loop. These findings are derived from atmosphere simulations at high resolution and analyses of the variability of moist static energy at different altitudes. Key Points: Radiative feedbacks become less important to the development of convective self‐aggregation (CSA) with surface warmingIn warm climates, adiabatic production switches from negative to positive, becoming essential to the CSA developmentAt sea surface temperatures of 310 and 320 K, CSA can still occur without radiative feedbacks [ABSTRACT FROM AUTHOR]

Understanding and forecasting Tropical Pacific Decadal‐scale Variability (TPDV) strongly rely on climate model simulations. Using a Linear Inverse Modeling (LIM) diagnostic approach, we reveal Coupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the spatial structure and dominant mechanisms of TPDV. Specifically, while the models' ensemble mean pattern of TPDV resembles that of observations, the spread across models is very large and most models show significant differences from observations. In observations, removing the coupling between extratropics and tropics reduces TPDV by ∼60%–70%, and removing the tropical thermocline variability makes the central tropical Pacific a key center of action for TPDV and El Niño Southern Oscillation variability. These characteristics are only confirmed in a subset of models. Differences between observations and simulations are outside the range of natural internal TPDV noise and pose important questions regarding our ability to model the impacts of natural internal low‐frequency variability superimposed on long‐term climate change. Plain Language Summary: Tropical Pacific Decadal‐scale Variability (TPDV) has been shown to impact global‐scale climate fluctuations, weather regimes, and temperature trends such as the 1998–2012 global warming hiatus. Understanding and predicting TPDV's impacts strongly rely on climate model simulations and projections. However, the models show key deficiencies in reproducing the observed structure of TPDV. Although the arithmetic mean of TPDV's spatial pattern in each individual model is similar to observations, there is a large spread among models, and most models show significant differences from observations. Using an empirical dynamical model to decompose the mechanisms of the climate models reveals that more than 50% of the models fail to reproduce the roles of extratropics‐tropics coupling and the tropical thermocline variability play in TPDV. These differences show significant challenges exist across the models for modeling the decadal‐scale climate variability in the tropical Pacific. Improving the simulation of TPDV in climate models is vital for understanding the impacts of natural internal low‐frequency variability alongside long‐term climate change. Key Points: A Linear Inverse Model is used to decompose and compare tropical Pacific decadal variability dynamics in observations and climate modelsCoupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the observed dynamics and spatial pattern of tropical Pacific decadal variabilityMany models (>50%) cannot reproduce the roles of extratropics‐tropics coupling and thermocline variability in tropical Pacific variability [ABSTRACT FROM AUTHOR]

We show that the seasonal cycles of clouds over the mid‐latitude oceans in the Northern Hemisphere are predictors of the responses of clouds to increasing sea‐surface temperatures globally. These regions are therefore "natural laboratories" in which the processes responsible for low‐cloud feedbacks on global scales are observed as seasonal changes in local cloud properties. We use an ensemble of configurations of a global‐climate model to show that the sensitivities of cloud‐radiative anomalies to surface temperature and lower‐tropospheric stability in the "laboratory" regions predict the models' global cloud‐radiative feedbacks. Models with greater changes in low‐clouds between seasons are shown to have stronger negative feedbacks in the mid‐latitudes, and stronger positive feedbacks from the subtropical stratocumulus. The biases in the simulated seasonal cycles, compared to observations, imply that both feedbacks are too weak in the model. The consequences of this for configuring our model to have a lower climate sensitivity are discussed. Plain Language Summary: As the Earth's climate warms, feedbacks from changes in clouds are crucial for determining the rate of warming. In some climate models, positive cloud feedbacks are known to contribute to projected temperature increases which fall outside the range that is considered plausible given our current understanding of the Earth's climate. Methods are needed which directly relate present‐day model errors to projected cloud feedbacks so that we can understand how to improve models and simultaneously obtain plausible feedback strengths. We show that in the Met Office Hadley Centres' climate model, the seasonal cycles of clouds in a few key regions, specifically the poleward ocean basins in the Northern Hemisphere, can be used to predict cloud feedbacks, globally, across a wide range of configurations of the model. These regions, particularly the North‐west Pacific, are "natural laboratories" in which the same processes that are responsible for global, cloud‐feedbacks onto global‐warming rates, occur annually as part of the local seasonal cycle. Studying clouds in these regions and model biases in predictions of these clouds will allow us to directly improve estimates of global climate change. Key Points: Seasonal cycles of clouds constrain cloud feedbacksThe northern ocean basins are natural laboratories for low‐cloud feedbacksThe hot‐model problem in HadGEM3 will not easily be solved by tuning [ABSTRACT FROM AUTHOR]

Future projections indicate the Atlantic Meridional Overturning Circulation (AMOC) will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyze projected AMOC weakening in 27 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo‐Pacific, but does not later upwell in the Southern Ocean (SO), is particularly sensitive to warming, in part, because shallowing of the deep flow prevents it from entering the Indo‐Pacific via the SO. The present‐day strength of this Indo‐Pacific pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally based methods imply a wide range of AMOC weakening under the SSP5‐8.5 scenario of 29%–61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that move warm surface waters from the south to the north of the Atlantic Ocean where they cool, sink, and return southward at depth. Changes in the AMOC would have wide‐ranging impacts on our climate. It is predicted to weaken as the climate warms during the 21st century, but the extent of weakening varies among different climate models. We show that AMOC weakening is greatest in models that have a large exchange of water between the AMOC and the Indo‐Pacific Ocean along a specific pathway. The magnitude of this ocean pathway, inferred from four observation‐based estimates of the global overturning circulation, is uncertain. By using these estimates and analyzing the relationship between the aforementioned ocean pathway and AMOC weakening across many climate models, we can predict how the real‐world AMOC will change. Our findings indicate that by 2100, under a high greenhouse gas emission scenario, the AMOC will weaken by 29%–61%. This highlights the importance of reducing differences between observational estimates of the ocean's overturning pathways to reduce uncertainty in future AMOC weakening and to improve the representation of these pathways in climate models. Key Points: The magnitude of 21st century Atlantic Meridional Overturning Circulation (AMOC) weakening in CMIP6 models is highly correlated with an AMOC pathway into the Indo‐Pacific OceanThe real‐world "Indo‐Pacific diffusive" AMOC pathway inferred from observation‐based estimates is used to constrain future AMOC weakeningUnder high‐end greenhouse gas forcing, AMOC weakening based on this emergent constraint relationship ranges from 29% to 61% by 2100 [ABSTRACT FROM AUTHOR]

Effective climate sensitivity (EffCS), commonly estimated from model simulations with abrupt 4×CO2 for 150 years, has been shown to depend on the CO2 forcing level. To understand this dependency systematically, we performed a series of simulations with a range of abrupt CO2 forcing in two climate models. Our results indicate that normalized EffCS values in these simulations are a non‐monotonic function of the CO2 forcing, decreasing between 3× and 4×CO2 in CESM1‐LE (2× and 3×CO2 in GISS‐E2.1‐G) and increasing at higher CO2 levels. The minimum EffCS value, caused by anomalously negative radiative feedbacks, arises mainly from sea‐surface temperature (SST) relative cooling in the tropical and subtropical North Atlantic. This cooling is associated with the formation of the North Atlantic Warming Hole and Atlantic Meridional Overturning Circulation collapse under CO2 forcing. Our findings imply that understanding changes in North Atlantic SST patterns is important for constraining near‐future and equilibrium global warming. Plain Language Summary: Estimates of effective climate sensitivity (EffCS) are complicated by the nonlinear dependence of feedback on (a) global mean surface temperature and (b) the pattern of sea‐surface temperature. We find that EffCS and radiative feedbacks change non‐monotonically with CO2 concentrations due to a cooling sea‐surface temperature (SST) pattern over the North Atlantic associated with the formation of a "North Atlantic Warming Hole" (NAWH). While most previous studies focused on the impact of tropical Pacific SST patterns on EffCS, we here highlight an overlooked damping effect on EffCS from North Atlantic SST cooling across CO2 levels. Our results imply that understanding and constraining the NAWH under CO2 forcings is crucial for transient warming projections and EffCS constraints. Key Points: Effective climate sensitivity (EffCS) and radiative feedbacks change non‐monotonically with increasing CO2 concentrationsThe non‐monotonicity is associated with the formation of a sea‐surface temperature cooling pattern over the North AtlanticResults imply an overlooked radiative damping effect on global EffCS from the North Atlantic warming hole [ABSTRACT FROM AUTHOR]

Water vapor in the Arctic can be attributed to local evaporation and transport from mid‐latitudes. Here we investigate the response of Arctic water vapor to declining sea ice in a climate model, using an idealized warming scenario in which the Arctic becomes ice free year‐round. We find distinct monthly patterns in water vapor change with warming, with a strong influence of sea ice loss on changes to evaporation, except in summer when transports dominate. Before the Arctic becomes ice‐free, total column water increases at a rate slower than that indicated by the Clausius‐Clapeyron relationship; once ice‐free, total column water increases at a rate equal to Clausius‐Clapeyron. The exception is summer when the increase is initially faster then becomes slower when ice‐free. The boundary layer structure changes, under global warming, from stratified to well mixed in all seasons except summer, which remains largely unchanged throughout the ice‐free transition. Plain Language Summary: Surface evaporation from the Arctic Ocean is limited by the presence of sea ice, which creates a natural barrier between the ocean and atmosphere above. We show the loss of Arctic sea ice, in a climate model affects atmospheric water vapor and find that this varies throughout the year. The greatest increase in atmospheric water vapor is in winter, during the Arctic night, whilst there is very little change in summer. There are two reasons why sea ice loss does not impact water vapor in the summer: first, the summer air temperature barely changes between ice‐covered and ice‐free, whereas in winter the loss of ice cover can increase air temperatures by at least 20°C; second the summer Arctic water budget is dominated by transport of water vapor from midlatitudes rather than local evaporation. We also show changes in the vertical structure of the near‐surface atmosphere and cloud types, with a transition from stable to turbulent regimes in autumn and winter. Key Points: As Arctic sea ice declines with increased warming, Arctic evaporation and water vapor increase most in autumn and winterArctic sea ice limits water vapor availability, but once the Arctic is ice‐free, it increases with the Clausius‐Clapeyron relationshipArctic boundary layer changes from stably stratified to well mixed, during winter and autumn, but less change in spring and summer [ABSTRACT FROM AUTHOR]

Satellite measurements show that the Northern and Southern hemispheres reflect equal amounts of shortwave radiation ("albedo symmetry"), but no theory exists on if, how, and why the symmetry is established and maintained. Ambiguously, climate models are strongly biased in albedo symmetry but agree in the sign of the response to CO2. We find that mean‐state biases in albedo symmetry and hemispheric surface temperature asymmetry correlate negatively. Similarly, the response of albedo asymmetry to CO2 forcing correlates negatively with the magnitude of the asymmetry in surface warming. This is true across many and within single climate model simulations: a too warm or stronger warming hemisphere is darker or darkens more than its counterpart. In the 21 years of observations we find the same tendency and hypothesize (a) albedo symmetry is a function of the current climate state and (b) we will observe an evolution toward albedo asymmetry in coming decades. Plain Language Summary: Observations indicate that the Northern and Southern Hemispheres reflect equal amounts of shortwave radiation. It is not understood whether this "albedo symmetry" is by chance or physically necessary in the current or across different climate states. Planetary albedo changes substantially under global warming and acts as a radiative feedback. Thus, it seems paramount to understand its dynamics. We are doing so by using climate models. They are not tuned for albedo symmetry and indeed show huge biases since three generations of models. We flesh out our understanding where these biases come from and further, find a robust evolution of the albedo symmetry in response to CO2 forcing across model generations. Both, the models' bias and the forced change correlate well with the surface temperature difference between the hemispheres. Finally, we find this relation also present in the observed inter‐annual deviations from albedo symmetry and surface temperatures. This implies that we might observe an evolution toward an albedo asymmetry in the next decades and symmetry depends on the climate state. Key Points: Climate models persistently do not simulate the observed planetary albedo symmetryHowever, they do agree in a reduction of Northern minus Southern hemispheric reflectance under CO2 forcingModeled bias and forced change of albedo asymmetry, as well as observed deviations from symmetry might be governed by surface temperature [ABSTRACT FROM AUTHOR]

Previous studies have found inconsistent responses of the North Atlantic jet to Arctic sea‐ice loss. The response of wintertime atmospheric circulation and surface climate over the North Atlantic‐European region to future Arctic sea‐ice loss under 2°C global warming is analyzed, using model output from the Polar Amplification Model Intercomparison Project. The models agree that the North Atlantic jet shifts slightly southward in response to sea‐ice loss, but they disagree on the sign of the jet speed response. The jet response induces a dipole anomaly of precipitation and storm track activity over the North Atlantic‐European region. The changes in jet latitude and speed induce distinct regional surface climate responses, and together they strongly shape the North Atlantic‐European response to future Arctic sea‐ice loss. Constraining the North Atlantic jet response is important for reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Plain Language Summary: Variations in the North Atlantic jet affect temperature, precipitation, and storminess in Europe. It is not well understood how the jet and European climate will respond to future sea‐ice loss in the Arctic under human‐caused increases in global temperature. We study how atmospheric circulation and surface climate in winter over the North Atlantic‐European region would respond to future Arctic sea‐ice loss under 2°C global warming, using an unprecedented number of coordinated simulations from different climate models. In many models the North Atlantic jet stream in winter responds by shifting southward, but the response of its speed is less clear. These jet stream changes are found to strongly shape the responses in precipitation and storm track activity. More precise estimates of the jet stream response are key to reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Key Points: Projected Arctic sea‐ice loss causes a robust equatorward shift of the North Atlantic jet across models, but no robust change in jet speedChanges in jet position and speed strongly shape the European winter surface climate response to projected Arctic sea‐ice lossConstraining the jet response is important for reducing uncertainty in the European precipitation response to future Arctic sea‐ice loss [ABSTRACT FROM AUTHOR]

Daily maximum temperatures during the 2021 heatwave in the Pacific Northwest United States and Canada shattered century old records. Multiple causal factors, including anthropogenic climate change, contributed to these high temperatures, challenging traditional methods of attributing human influence. We demonstrate that the observed 2021 daily maximum temperatures are far above the bounds of Generalized Extreme Value distributions fitted from historical data. Hence, confidence in Granger causal inference statements about the human influence on this heatwave is low. Alternatively, we present a more conditional hindcast attribution study using two regional models. We performed ensembles of simulations of the heatwave to investigate how the event would have changed if it had occurred without anthropogenic climate change and with future warming. We found that global warming caused a ∼0.8°C–1°C increase in heatwave temperatures. Future warming would lead to a ∼5°C increase in heatwave temperature by the end of the 21st century. Plain Language Summary: While it is clear that global warming causes heatwaves to be warmer, the unique meteorological conditions behind the 2021 Pacific Northwest heatwave tax our ability to make quantitative estimates of the human contribution. We discuss why there is low confidence in traditional estimates of the human contribution to this heatwave's temperatures and present an alternative, albeit more highly constrained estimate that human activities caused a ∼0.8°C–1°C increase in the observed daily maximum temperatures. Additional future warming would lead to a ∼5°C increase in the heatwave by the end of the 21st century. Key Points: The Pacific Northwest (PNW) heatwave was unprecedented in the observational record over a large areaStatistical and global climate models fail to inform about human influence on the PNW heatwave, as it is a far outlierHindcast attribution methods can provide limited and conditional information about the human influence on the PNW heatwave [ABSTRACT FROM AUTHOR]

Retrospectively comparing future model projections to observations provides a robust and independent test of model skill. Here we analyze the performance of climate models published between 1970 and 2007 in projecting future global mean surface temperature (GMST) changes. Models are compared to observations based on both the change in GMST over time and the change in GMST over the change in external forcing. The latter approach accounts for mismatches in model forcings, a potential source of error in model projections independent of the accuracy of model physics. We find that climate models published over the past five decades were skillful in predicting subsequent GMST changes, with most models examined showing warming consistent with observations, particularly when mismatches between model‐projected and observationally estimated forcings were taken into account. Plain Language Summary: Climate models provide an important way to understand future changes in the Earth's climate. In this paper we undertake a thorough evaluation of the performance of various climate models published between the early 1970s and the late 2000s. Specifically, we look at how well models project global warming in the years after they were published by comparing them to observed temperature changes. Model projections rely on two things to accurately match observations: accurate modeling of climate physics and accurate assumptions around future emissions of CO2 and other factors affecting the climate. The best physics‐based model will still be inaccurate if it is driven by future changes in emissions that differ from reality. To account for this, we look at how the relationship between temperature and atmospheric CO2 (and other climate drivers) differs between models and observations. We find that climate models published over the past five decades were generally quite accurate in predicting global warming in the years after publication, particularly when accounting for differences between modeled and actual changes in atmospheric CO2 and other climate drivers. This research should help resolve public confusion around the performance of past climate modeling efforts and increases our confidence that models are accurately projecting global warming. Key Points: Evaluation of uninitialized multidecadal climate model future projection performance provides a concrete test of model skillThe quasi‐linear relationship between model/observed forcings and temperature change is used to control for errors in projected forcingModel simulations published between 1970 and 2007 were skillful in projecting future global mean surface warming [ABSTRACT FROM AUTHOR]

Defined as the difference between the daily maximum (Tmax) and minimum surface air temperature (Tmin), the diurnal temperature range (DTR) is critical to local climate, ecosystem, and socio‐economic functioning. This study investigates how DTR responds to the future global warming, using a novel, fine‐resolution convection‐permitting regional climate model. Results demonstrate the asymmetrical impact of global warming, that is, the more substantial warming in Tmin, and the lower in Tmax. This asymmetry results in the DTR reduction of 0.5°K in the mid‐latitude and 0.25°K in the low‐altitude area (under 2°K warming scenario). The increase in daytime cloudiness under warmer climates is likely responsible for the reduction of incoming short‐wave radiation reduction, consequently causing Tmax "underwarming." Study results also suggest that atmospheric instability could play a vital role in the difference in DTR response between the tropical and mid‐latitude areas. Plain Language Summary: Diurnal temperature range, which is defined as the difference between the daily maximum (Tmax) and daily minimum air temperature (Tmin), is a critical factor that modulates local climate, ecosystem, and socio‐economic functioning. This study investigates how global warming impacts diurnal temperature range (DTR) using a high resolution numerical model. Results demonstrate the asymmetrical response to global warming, that is, the more substantial warming is confirmed in Tmin, and the lower in Tmax. This asymmetry results in the DTR reduction of approximately 0.5°K in the mid‐latitude and 0.25°K in the low‐altitude area. A cause of the DTR reduction is the daytime‐cloudiness increase under warmer climates, which reduces short‐wave solar radiation reaching the surface, consequently causing the Tmax "underwarming." Also, the difference in atmospheric instability in the mid‐ and low‐latitude areas explains the difference in the DTR response, that is, more moderate DTR reduction is confirmed in low latitude areas. Key Points: This study demonstrates the reduction of the diurnal temperature range (DTR) under future warmer climatesThe reduction of DTR is primarily attributed to the increase in cloudiness which reducing daytime incoming‐shortwave radiationDifference in intrinsic atmospheric stability between tropical and mid‐latitude areas could explain the distinct responses of DTR [ABSTRACT FROM AUTHOR]

Africa's central Sahel region has experienced prolonged drought conditions in the past, while rainfall has recovered more recently. Global climate models project anything from no change to a strong wetting trend under unabated climate change; and they have difficulty reproducing the complex historical record. Here we show that when a period of dominant aerosol forcing is excluded, a consistent wetting response to greenhouse‐gas induced warming emerges in observed rainfall. Using the observed response coefficient estimate as a constraint, we find that Coupled Model Intercomparison Project Phase 6 climate models with a realistic past rainfall response show a smaller spread, and higher median, of projected future rainfall change, compared to the full ensemble. In particular, very small or negative rainfall trends are absent from the constrained ensemble. Our results provide further evidence for a robust Sahel rainfall increase in response to greenhouse‐gas forcing, consistent with recent observations, and including the possibility of a very strong increase. Plain Language Summary: Rainfall is a critical resource for a large population in the African Sahel region, but rainfall levels have strongly varied in the past. It is unclear what will happen in the next decades, because some climate models suggest little to no change in average rainfall, while other models project a strong increase due to climate change. Our study aims to narrow down this uncertainty. In the past, both greenhouse gases and aerosols influenced Sahel rainfall, to different degrees during different periods; and models may not capture both mechanisms equally well. However, future rainfall changes will likely be driven mainly by greenhouse gas levels. We identify models that closely match the rainfall change that was observed in a recent period with dominant greenhouse gas forcing. These models also tend to project stronger rainfall increases than other models. Indeed, their multi‐model mean projection is an increase by almost 50% by 2040; while none of them projects stable rainfall levels or even a reduction. Thus, it appears more likely now that rainfall in the Sahel will increase substantially over the next few decades. Key Points: We identify the response of Sahel rainfall to greenhouse gas‐induced global warming in the historical recordMany global climate models of the latest generation (Coupled Model Intercomparison Project Phase 6) do not reproduce the observed rainfall responseObservational constraint reduces model spread and implies stronger rainfall increase under further warming than in full ensemble [ABSTRACT FROM AUTHOR]

Observed surface temperature trends over recent decades are characterized by (a) intensified warming in the Indo‐Pacific Warm Pool and slight cooling in the eastern equatorial Pacific, consistent with Walker circulation strengthening, and (b) Southern Ocean cooling. In contrast, state‐of‐the‐art coupled climate models generally project enhanced warming in the eastern equatorial Pacific, Walker circulation weakening, and Southern Ocean warming. Here we investigate the ability of 16 climate model large ensembles to reproduce observed sea‐surface temperature and sea‐level pressure trends over 1979–2020 through a combination of externally forced climate change and internal variability. We find large‐scale differences between observed and modeled trends that are very unlikely (<5% probability) to occur due to internal variability as represented in models. Disparate trends in the ratio of Indo‐Pacific Warm Pool to tropical‐mean warming, which shows little multi‐decadal variability in models, hint that model biases in the response to historical forcing constitute part of the discrepancy. Plain Language Summary: Regional climate change depends not only on the magnitude of global warming, but also on the spatial pattern of warming. We show that the spatial pattern of observed surface temperature changes since 1979 is highly unusual, and many aspects of it cannot be reproduced in current climate models, even when accounting for the influence of natural variability. We find a particularly large discrepancy in the rate of warming within the western Pacific Ocean and eastern Indian Ocean, which suggests that models have systematic biases in the response of sea‐surface temperature patterns to anthropogenic forcing, because the contribution of natural variability to multi‐decadal trends is thought to be relatively small in this region. Our work raises the possibility that the recent trends toward more La‐Niña‐like conditions may be partly a response to anthropogenic forcing, even though most existing climate model and paleoclimate evidence suggests that trends will eventually reverse toward more El‐Niño‐like conditions, with an associated shift in regional climate trends. Key Points: The patterns of observed sea‐surface temperature and sea‐level pressure trends differ significantly from CMIP5/6 historical simulationsThe rate of Indo‐Pacific Warm Pool warming relative to tropical‐mean warming is particularly anomalous in observations compared to modelsPatterns of observed changes that climate models do not reproduce are isolated with a signal‐to‐noise maximizing pattern analysis [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) are frequent over the US Great Plains during spring. The link between large‐scale environments and spring MCS initiation were well established. Here, historical and future changes of spring large‐scale environments favorable for MCS initiation are investigated using an MCS tracking data set, ERA5 reanalysis, and 20 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The frequency of Great Plains low‐level jet (GPLLJ)‐related MCS environments is found to have increased by ∼41% from 1979 to 2019, consistent with the enhanced GPLLJ and more frequent MCSs. Comparing CMIP6 AMIP and historical experiments, we find that the observed GPLLJ strengthening and more frequent MCS environments are mainly due to the decadal sea‐surface temperature variations rather than external forcings. Under a high emission scenario, the frequency of GPLLJ‐related environments favorable for MCS initiation will increase by ～65% during 2015–2100, along with a stronger GPLLJ, suggesting more frequent MCSs over the US Great Plains in a warming world. Plain Language Summary: Convective storm systems that extend over several hundred kilometers are frequently observed and contribute to a majority of the warm season rainfall over the US Great Plains. It is a major challenge for climate models to simulate these storms because the processes governing their initiation and evolution are not well resolved. However, large‐scale atmospheric circulations such as the Great Plains low‐level jet (GPLLJ) that provide favorable environments for these storms in spring are well observed and simulated by climate models. Therefore, the GPLLJ may be used to understand past and future changes of these storms. Here, we found GPLLJ‐related environments become more frequent from 1979 to 2019, related to the observed strengthening of the GPLLJ. To determine whether these changes are due to global warming, we compare simulations driven by the observed sea‐surface temperature variation with simulations driven only by anthropogenic forcing. The results suggest that the observed GPLLJ strengthening is mainly due to the natural decadal variation of sea‐surface temperature. However, with continued warming under a high greenhouse gas emission scenario, the GPLLJ is greatly enhanced during 2015–2100. Accompanied by more frequent occurrence of the GPLLJ‐related environments, the frequency of large convective systems may also increase in the future. Key Points: Low‐level jet‐related mesoscale convective system environments in spring over the US Great Plains became more frequent from 1979 to 2019The more frequent convection environments are a response to decadal sea‐surface temperature variation rather than to external forcingAlong with the enhanced low‐level jet under a high emission scenario, the low‐level jet‐related storm environment will become more frequent [ABSTRACT FROM AUTHOR]

While the annual mean Arctic Amplification (AA) index varied between two and three during the 1970–2000 period, it reached values exceeding four during the first two decades of the 21st century. The AA did not change in a continuous fashion but rather in two sharp increases around 1986 and 1999. During those steps the mean global surface air temperature trend remained almost constant, while the Arctic trend increased. Although the "best" CMIP6 models reproduce the increasing trend of the AA in 1980s they do not capture the sharply increasing trend of the AA after 1999 including its rapid step‐like increase. We propose that the first sharp AA increase around 1986 is due to external forcing, while the second step close to 1999 is due to internal climate variability, which models cannot reproduce in the observed time. Plain Language Summary: The rate of Arctic warming varied in time, while the rate of global warming was nearly constant over the period 1960–2020. This led to a variable Arctic Amplification (AA) defined as a ratio of the Arctic warming trend to mean global warming trend. In addition, the Arctic experienced sudden changes in the rate of warming with step‐like changes in annual mean Arctic Amplification around the years 1986 and 1999. Climate models do not capture the second observed sharp increase of the AA. We suggest that the first AA increase around 1986 is primarily due to external forcing (increasing concentration of carbon dioxide) while the second sharp increase around 1999 is dominated by internal climate variability which current models cannot capture accurately within the time scale of the physical phenomena. Key Points: Annual mean Arctic Amplification (AA) within the period 1970–2020 changed in steep steps around 1986 and 1999. It reached values over 4.0Even those CMIP6 models best at reproducing the AA did not reproduce the second sudden increase near 1999We conjecture that the first AA sharp increase is due to external forcing, while the second is due to internal climate variability [ABSTRACT FROM AUTHOR]

Extreme extratropical storms are among the most impact‐relevant weather events in the midlatitudes. Under global warming, extreme storms are expected to intensify; however, little is known about the response of their spatial structure. Here, we show that with warming, extreme storms not only become stronger, but also grow larger. By employing multi‐model projections from the sixth Coupled Model Intercomparison Project and an idealized aquaplanet simulation, we demonstrate that global warming leads to a robust increase in extreme storm size that is relatively spatially uniform in the midlatitudes and consistent among the models. The physical mechanism for the size increase is the increase in the Rossby deformation radius due to the increase in low‐level dry static stability with warming. The storm expansion adds a substantial contribution (2.6%/K), along with the increase in precipitation intensity (3.2%/K), to the increase in storm total precipitation. The results improve our understanding and have significant implications for climate mitigation. Plain Language Summary: Extreme storms are responsible for most extratropical flooding. Due to the increased atmospheric moisture with global warming, extreme storms are projected to intensify. However, other aspects of extreme storms are poorly understood. Here we demonstrate that with warming, extreme storms not only become stronger, but also grow larger. The size increase is a robust feature consistent across climate models and is explained by a fundamental scaling of the Rossby radius of deformation. The size increase of extreme storms means larger impact regions, more storm‐total rainfall, and greater flood risks in a future warmer climate. Key Points: Global warming leads to a robust, spatially uniform expansion of extreme storm size in the midlatitudesThe expansion is due to the increase in the Rossby deformation radius associated with the increase in low‐level dry static stabilityThe storm size expansion has a substantial contribution (2.6%/K) to the increase in storm‐associated total precipitation (6.1%/K) [ABSTRACT FROM AUTHOR]

There is low confidence in projections of Antarctic sea ice area (SIA), due to deficiencies in climate model sea ice processes. Ensemble regression techniques can help to reduce this uncertainty. We investigate relationships between SIA climatology and 21st century change in the Coupled Model Intercomparison Project, phase 6 (CMIP6) multi‐model ensemble. In summer, under a strong forcing scenario, each model loses the majority of its sea ice. Therefore, models with greater historical SIA exhibit greater reductions, so the observed climatology of SIA strongly constrains projections. Ensemble spread in historical summer SIA is smaller than in CMIP phase 5 (CMIP5), and CMIP6 gives a more robust constraint on future SIA. In winter, by 2100 under a strong forcing scenario, 40% of SIA disappears on average, and ensemble spread in historical mean SIA explains approximately half the spread in projected change. A greater winter ice loss in CMIP6 than CMIP5 is explained by the higher climate sensitivities of some CMIP6 models. Plain Language Summary: Antarctic sea ice area is important for ecosystems, human activity, and for the dynamics of the atmosphere and ocean. However, major climate change assessments such as the Intergovernmental Panel on Climate Change Sixth Assessment Report have placed little confidence in projections of Antarctic sea ice area. This is because climate models struggle to simulate many aspects of sea ice area as observed by satellites. However, not all models should be treated equally. Models with relatively more sea ice area in the recent past project greater losses in the future, especially in summer. This suggests that models with historical sea ice area closest to what we have observed may be more reliable in their future simulations. We use this information in a simple statistical model to show that, firstly, the newest climate models largely show near‐total sea ice loss in summer by the end of the 21st century, which was not universally true for older climate models. Secondly, newer climate models lose more winter sea ice than their predecessors in the same time period, which is related to the known greater global warming response to given greenhouse gas concentrations in the newer models. Key Points: We compare Coupled Model Intercomparison Project (CMIP), phase 5 and the CMIP, phase 6 (CMIP6) projections of Antarctic sea ice loss, assessing drivers of uncertainty and the robustness of projectionsUnder strong forcing, CMIP6 models lose most Antarctic summer sea ice by 2100, so projections are constrained by historical climatologyIncreased winter sea ice loss in CMIP6 is related to increased equilibrium climate sensitivity [ABSTRACT FROM AUTHOR]

Convective aggregation refers to the clustering of convective events and occurs on a wide range of spatial scales. It has been suggested that the behavior of convective aggregation may change under global warming, with potential implications for future changes in precipitation extremes. Here, convective regions of the tropics are defined from a percentile threshold on gridded daily precipitation data and used to quantify large‐scale convective aggregation in an ensemble of global climate models. Applying three separate indices for aggregation, it is found that large‐scale convective aggregation increases in 17 of the 19 analyzed models under future warming. However, aggregation is found not to be correlated with tropical‐mean precipitation extremes, either climatologically or with respect to the sensitivity to warming. The large model spread in aggregation indices across the ensemble suggests the possible utility of large‐scale convective aggregation as a target for model evaluation. Plain Language Summary: Rainfall in the tropics is not evenly distributed, rather it occurs in clusters of clouds of various sizes and shapes, from a line of thunderstorms to "superclusters" spanning thousands of km. The size and spatial distribution of these clusters are hypothesized to influence the frequency and intensity of heavy rainfall in the tropics. In this study, we develop a method for quantifying the amount of clustering in the tropics, and we investigate whether it is projected to change in the future in a suite of state‐of‐the‐art climate models. Almost all the models project increases in clustering in the future, but, surprisingly, a model's projection of clustering does not seem to affect its projection of changes to heavy rainfall. The results suggest that effects other than clustering play a dominant role in determining the range of heavy rainfall outcomes projected by climate models. Key Points: According to three separate metrics, large‐scale convective aggregation increases with warming in 17 of 19 global climate models (GCMs) examinedThere is substantial spread in large‐scale convective aggregation behavior among the GCMs consideredNo statistically significant inter‐model relationship between degree of aggregation and tropical‐mean precipitation extremes was found [ABSTRACT FROM AUTHOR]

El Niños (EN) are known to decay more rapidly, while La Niñas (LN) tend to decay more slowly. Observational analyses and coupled model experiments are conducted to show that sea surface temperature (SST) anomalies over the subtropical northeastern Pacific (SNEP) and equatorial western Pacific (EWP) are key factors to determine the decay pace of the EN and LN and their asymmetry. In the present climate the LN produces larger cold SST anomalies over the regions than the warm SST anomalies produced by the EN. The magnitude difference over the SNEP and EWP helps to slow down the LN decay via subtropical footprinting and tropical thermocline variation mechanisms, respectively. Coupled Model Intercomparison Project Phase 6 models project the magnitude differences of SNEP SST anomalies between EN and LN to reduce in the future warming world, causing the asymmetric EN‐LN decay to weaken. Plain Language Summary: There are obvious asymmetric features in mature winter and decaying summer between the warm (El Niño [EN]) and cold phases (La Niña [LN]) of El Niño‐Southern Oscillation (ENSO). Cold sea surface temperature (SST) anomalies over the subtropical northeastern Pacific (SNEP) and equatorial western Pacific (EWP) in winter are larger in amplitude during LN than EN. This asymmetric feature in winter enables the LN to delay slower and last longer than the EN in the following summer, which is another asymmetric feature between these two phases of the ENSO. The large cold anomalies over the SNEP can spread to the equatorial Pacific to slow down the LN decay through subtropical ocean‐atmosphere coupling processes. The large cold anomalies over the EWP also contribute to the slow decay of LN but mainly through tropical ocean‐atmosphere coupling processes. Climate models project that the future global warming may reduce the magnitude differences of the SNEP SST anomalies between EN and LN and cause a rapid decline of LN in a warmer world. Key Points: The slow La Niña and rapid El Niño decays are caused by differences in their anomalies over the subtropical and tropical PacificThe subtropical (tropical) anomalies activate a seasonal footprinting (thermocline variation) mechanism to affect the decay paceThe asymmetric decay is projected to decrease in the future warming world due to the reduction of the subtropical anomaly difference [ABSTRACT FROM AUTHOR]

Previous studies have reported that enhanced eddy moisture convergence causes future increase of winter precipitation in the eastern North America in a warmer climate. This study investigates the mechanisms of the change in moisture convergence. Using ensemble simulations from climate models and composite analysis of moisture budget, we show that in the southeast United States, two thirds of the enhanced moisture convergence is caused by local thermodynamics due to increase of moisture, while one third is caused by stronger eddy wind convergence. In the north, it is primarily caused by the increase of moisture gradient associated with the spatial distribution of both the climatological temperature and the large amplitudes of the eddy temperature in the region of the storm tracks. Results help to better understand the physical mechanism of more winter precipitation in the eastern North America in a warmer climate. Plain Language Summary: It has been reported that winter precipitation will increase in the eastern North America under global warming, which is caused by the enhanced eddy moisture convergence. We perform composite analysis of moisture budget using large ensemble simulations of climate models, and find out that the enhanced eddy moisture convergence in the southeast United States can be explained by the combination of climatological eddy wind convergence and increase of moisture content. The intensity of eddy wind convergence will enhance and contribute about one third of the eddy moisture convergence increase in the southeast United States. In the north of eastern North America, the climatological eddy wind brings more moisture into the region due to enhanced moisture gradient. The contrast of different mechanisms in the south and north can be explained by the climatology and future change of the spatial pattern of water vapor and temperature. These results help us to better understand the physical mechanisms that are responsible for the future precipitation increase during winter in the eastern North America under global warming. Key Points: Winter precipitation increases with warming due to enhanced eddy moisture convergence in the eastern North AmericaLocal thermodynamics associated with increased moisture and stronger eddies induce more moisture convergence in the southeast United StatesEnhanced moisture gradient in regions of large eddy magnitudes causes more moisture convergence in the north of eastern North America [ABSTRACT FROM AUTHOR]

Previous studies documented possible connections between low frequency climate modes in the Northern Hemisphere ocean basins. We use observed sea surface temperatures and 270 large ensemble climate model simulations, which allows for improved methods of separating external and internal variability, such as removing the ensemble mean from each simulation. Detrending methods for observations have also improved since some of these previous studies were conducted. We also devise a modified statistical test using bootstrapping that is tuned specifically to this analysis. With these tools, we reexamine relationships among these modes. While previous studies have argued for the existence of an inter‐basin link, our results suggest that any internal connections between these modes are indistinguishable from random noise. Further, we show that external forcing affects each region in similar ways. This suggests that anthropogenic warming can cause an indirect link between the two basins, confounding the interpretation of a potential relationship. Plain Language Summary: We reexamine possible connections between climate patterns in the North Atlantic and North Pacific oceans. Improved climate model simulations and appropriate statistical methods allow us to build on previous research of these linkages. In contrast to previous studies, no natural connections are detected. However, global warming is shown to affect each region in similar ways, suggesting that climate change could cause an indirect link between the two basins. Key Points: A multi model large ensemble archive and modified statistical assumptions provide a framework for studying climatic relationshipsAn analysis of relationships between leading Northern Hemisphere climate modes does not show any internal connectionsExternal forcing such as global warming is shown to be a possible confounding factor in climate relationships [ABSTRACT FROM AUTHOR]

Precipitation clusters are spatially contiguous precipitating regions. Large clusters in the tropics are rare, extreme events that include organized precipitating systems. Changes to the probability distributions of tropical precipitation clusters under global warming are examined using models from the coupled model intercomparison project Phase 6 (CMIP6). Every analyzed model projects significant increases in frequencies of both very large‐sized clusters and clusters with very large area‐integrated precipitation (cluster power). The occurrence probability for the highest historical cluster power values increases by a factor between 4 and 15 among models in the end‐of‐century SSP5‐8.5 scenario. These changes primarily occur over the precipitating tropics: the western Pacific, Indian subcontinent, central and east Pacific convergence zones, and parts of South America. This spatial pattern is largely explained by Clausius‐Clapeyron scaling of current climate cluster power values. Societal impacts of cluster power increases could be acute in coastal regions of the Indian subcontinent and western Pacific islands. Plain Language Summary: Spatially continuous precipitating points are called precipitation clusters. In the tropics, the largest of these clusters represent systems like hurricanes or groups of thunderstorms. Large clusters also release greater rainfall and therefore hold more potential for destruction. In this study, we use state of the art climate models to examine how precipitation clusters with very large size and rain amounts will change under global warming. All the models we analyzed agree that there will be significant increases in the frequency of large and heavily precipitating clusters. The models project between a 4 and 15 times increase in the frequency of clusters bearing the largest rain amounts. These increases are projected to happen predominantly over tropical regions with warm surface waters. We also found a simple hypothesis that explains these changes. This hypothesis is based on the fact that warmer air can hold more moisture. Increases in the frequency of precipitation clusters with large rain amounts could be particularly devastating for the islands of the western Pacific and heavily populated coastal regions of the Indian subcontinent. Key Points: Probability distributions of precipitation clusters (contiguous precipitating regions) are reasonably reproduced in CMIP6 modelsUnder global warming, models project increases in frequencies of both clusters with large size and clusters with large precipitationThese increases primarily occur over tropical regions with a large incidence of heavily precipitating systems in current climate [ABSTRACT FROM AUTHOR]

With global temperatures approaching Paris climate agreement targets and a wide spread in modeled future warming, it is critical to rapidly identify any changes in warming rate. Here, I address the question of when we would be able to detect acceleration of warming in global‐mean temperature. Some standard techniques report significantly (p < 0.05) accelerated warming over 1980–2020 in the latest versions of the three main data sets used in the The Intergovernmental Panel on Climate Change 5th Assessment Report, with a mean acceleration of 49 m°C/decade2, which would add about 0.5°C additional warming over 50 years compared with linear warming. Accounting for El Nino, volcanism, and solar activity using a published method, the acceleration is 36 ± 30m°C/decade2. Using large ensemble output, I show that statistical uncertainties are underestimated by several common approaches. I recommend a better‐performing method and show that with these techniques 49m°C/decade2 acceleration would likely not be detectable with robust confidence until at least 2026. Plain Language Summary: The years since 2015 have all been hotter than any years recorded before. Some models project acceleration of global surface warming in recent decades and here I show that the available data make it look like warming has accelerated since 1980. However, such claims should be backed by rigorous statistical evidence and I show here that the statistical uncertainties are still likely too large to confidently detect accelerated global warming. The evidence for acceleration now is stronger than the mid‐2010s evidence was for a possible warming "slowdown" during 1998–2012. That earlier period attracted enormous public attention, which should not be repeated for any warming "surge" until the evidence is stronger. I estimate detection is improbable before at least 2026, and will likely depend on the presence of El Nino or La Nina conditions in the Pacific. Key Points: Several standard statistical techniques now report that global warming has acceleratedThis evidence is not as strong as it appears due to longer‐term internal climate variabilityClimate model large ensembles suggest that the currently estimated acceleration rate would only be detectable with confidence by 2026 [ABSTRACT FROM AUTHOR]

The global warming hiatus during the first decade of the 21st century has posed a challenge to the scientific community, though a leading explanation is that it was caused by internal variability overlaying a forced global warming trend. Here, we apply the Winton‐Held two‐layer model and show that the probability of the observed 2000–2012 hiatus period to arise from internal variability driven by white noise is larger if climate sensitivity is low. This is due to the delayed response of the oceans that cause the forced trend to increase faster with rising climate sensitivity than does natural variability, leading to a decreasing likelihood of observing the hiatus. The results are confirmed with the latest climate models participating in the Coupled Model Intercomparison Project (CMIP6). Plain Language Summary: The global warming during 2000–2012 was slower than the projections by global climate models. Several studies suggest that this was an expression of natural variability causing deviations from what is expected based on changes in greenhouse gases. In this study, we investigate whether the probability of such an event to occur depends on climate sensitivity, since a higher sensitivity is thought to result in more variability. However, we find that the forced trend increases faster than natural variability, leading to a decreasing likelihood of observing the hiatus. Key Points: In a two‐layer box model, the probability of obtaining the observed 2000–2012 global warming hiatus is higher if climate sensitivity is lowThis is because the forced temperature trend increases faster with increasing climate sensitivity than does global variabilityMemory of the deep oceans of greenhouse gas forcing and volcanic eruptions before the hiatus period is the underlying cause [ABSTRACT FROM AUTHOR]

The simultaneous occurrence of extremely wet winters at multiple locations in the same region can contribute to widespread flooding and associated socio‐economic losses. However, the spatial extent of precipitation extremes (i.e., the area in which nearby locations experience precipitation extremes simultaneously) and its future changes are largely overlooked in climate assessments. Employing new multi‐thousand‐year climate model simulations, we show that under both 2.0 °C and 1.5 °C warming scenarios, wintertime total precipitation extreme extents would increase over about 80%–90% of the Northern Hemisphere extratropics (i.e., of the latitude band 28°–78°N). Stabilizing at 1.5 °C rather than 2.0 °C would reduce the average magnitude of the increase by 1.7–2 times. According to the climate model, the increased extents are caused by increases in precipitation intensity rather than changes in the spatial organization of the events. Relatively small percentage increases in precipitation intensities (e.g., by 4%) can drive disproportionately larger, by 1–2 orders of magnitude, growth in the spatial extents (by 93%). Plain Language Summary: One of the most impact‐relevant and studied effects of global warming is the intensification of precipitation extremes. When extremely wet winters occur simultaneously at multiple locations within the same region, their cumulative impacts can be particularly high and enhanced as a result of limited resources available to cope with simultaneous damages. Despite the impacts caused by widespread extremes, climate change studies have typically disregarded the spatial extension of the extremes. Here, based on new multi‐thousand‐year climate model simulations, we show that—over most of the Northern Hemisphere extratropics, that is, most of the latitude band 28°–78°N—the spatial footprint of total wintertime precipitation extremes is projected to largely widen in the future. This widening results from a warmer, and therefore moister, atmosphere that will intensify precipitation. Holding global warming to 1.5 °C rather than 2.0 °C, in line with the Paris Agreement, would be beneficial to society as it could limit the average increase in the extension over the Northern Hemisphere extratropics by up to a factor of 2. To develop better preparedness for such extreme events, stakeholders should consider that a small increase in precipitation intensities (for example, by 4%) could result in large (by 93%) increases in spatial extent. Key Points: The spatial extent of wintertime total precipitation extremes is projected to increase over most of the Northern Hemisphere in the futureChanges in the spatial organization, that is, dependence, of precipitation extremes only marginally affect the spatial extentsIncreased precipitation magnitudes can cause a disproportionate (even 20–90 times larger) increase in the precipitation extreme extents [ABSTRACT FROM AUTHOR]

Climate model simulations project different regimes of summertime temperature distribution changes under a quadrupling of CO2 for dry land, moist land, and oceanic surfaces. The entire temperature distribution shifts over dry land surfaces, while moist land surfaces feature an elongated upper tail of the distribution, with extremes increasing more than the corresponding means by ∼20% of the global mean warming. Oceanic surfaces show weaker warming relative to land surfaces, with no significant elongation of the upper tail. Dry land surfaces show little change in turbulent sensible (SH) or latent (LH) fluxes, with new balance reached with compensating adjustments among downwelling and upwelling radiative fluxes. By contrast, moist land surfaces show enhanced partitioning of turbulent flux toward SH, while oceanic surfaces show enhanced partitioning toward LH. Amplified warming of extreme temperatures over moist land surfaces is attributed to suppressed evapotranspiration and larger Bowen ratios. Key Points: Dry land, moist land, and oceanic surfaces experience different changes in summertime mean and extreme temperaturesMoist land surfaces feature larger warming in extreme‐relative‐to‐mean temperatures, while dry surfaces warm more in the meanAmplified warming in extreme temperatures is attributed to suppressed evapotranspiration and larger Bowen ratios [ABSTRACT FROM AUTHOR]

The North Atlantic warming hole (NAWH) is referred to as a reduced warming, or even cooling, of the North Atlantic during an anthropogenic‐driven global warming. A NAWH is predicted by climate models during the 21st century, and its pattern is already emerging in observations. Despite the known key role of the North Atlantic surface temperatures in setting the Northern Hemisphere climate, the mechanisms behind the NAWH are still not fully understood. Using state‐of‐the‐art climate models, we show that anthropogenic aerosol forcing opposes the formation of the NAWH (by leading to a local warming) and delays its emergence by about 30 years. In agreement with previous studies, we also demonstrate that the relative warming of the North Atlantic under aerosol forcing is due to changes in ocean heat fluxes, rather than air‐sea fluxes. These results suggest that the predicted reduction in aerosol forcing during the 21st century may accelerate the formation of the NAWH. Plain Language Summary: Anthropogenic aerosols are particles suspended in the atmosphere, which were released due to anthropogenic activity. These particles have a general cooling effect on the Earth due to their interactions with radiation and with clouds. Here we show that the surface temperature in the North Atlantic Ocean is predicted to increase due to aerosol forcing (despite the global cooling). This trend is the opposite of the surface temperature trend predicted due to increase in greenhouse gases (global warming with a warming "hole" in the North Atlantic, trend known as the North Atlantic warming hole—NAWH). Using state‐of‐the‐art climate models, we show that aerosol forcing delays the formation of the NAWH by about 30 years. This trend could have important climatic impacts due to the key role of the North Atlantic surface temperatures in setting the Northern Hemisphere's climate and due to the predicted reduction in aerosol forcing in the next few decades. Key Points: Using CMIP6 and CESM‐LE simulations, we show that aerosol forcing generates a global cooling trend with a warming in the North AtlanticThis trend opposes the North Atlantic warming hole trend due to greenhouse gasesAerosol forcing delays the formation of the North Atlantic warming hole by about 30 years [ABSTRACT FROM AUTHOR]

To study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2‐doubling experiment using a coupled atmosphere‐ocean‐ice model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant. Plain Language Summary: The oceans take up the vast majority of the excess heat energy due to global warming. An important large‐scale ocean circulation is the Atlantic meridional overturning circulation (AMOC). Under global warming, it has been suggested that this circulation is important for storing excess heat energy in the deep ocean. Some evidence suggests that the AMOC has weakened since the mid‐twentieth century, and surface warming may intensify in response to the Atlantic storing less excess heat energy in the deep ocean. In order to examine this process more closely, we use a simple computer model of a world with no land masses and only two ocean basins: a small basin with a circulation similar to the AMOC, and a large basin without, similar to the Pacific. We approximate global warming by increasing the CO2 in the model atmosphere and find that the small basin stores heat at a faster rate than the large basin. The small basin's overturning circulation does weaken, but this weakening does not affect the small basin's heat storage rate. This is a surprising result and casts doubt on the idea that AMOC changes will significantly enhance surface warming as the Earth warms. Key Points: A deep meridional overturning circulation (MOC) connects the ocean surface to its interior, enhancing heat storage rate under global warmingThis circulation in the Atlantic (AMOC) may enable its enhanced heat storage rate relative to the Pacific in recent decadesMOC weakening does not necessarily impact ocean heat storage rate due to compensating physical processes [ABSTRACT FROM AUTHOR]

Most global climate models project a considerable stratospheric moistening during global warming. It is important to quantify how significantly the increase of stratospheric humidity affects the radiation budget and contributes to the surface warming. Here, we conduct a mechanism denial experiment to investigate the warming effect of the stratospheric water vapor (SWV) using a global climate model. By locking the SWV in a quadrupling CO2 experiment, we find that the surface warming effect of SWV is not as significant as previously thought, increasing the global mean surface warming by about 2%. This is due to compensating changes in other feedback, especially those of tropospheric temperature and cloud, affected by SWV. Plain Language Summary: How significantly does the increase of stratospheric humidity as projected by many global climate models contribute to the surface warming? Here, we conduct a mechanism denial experiment to address this question. By locking the SWV in a quadrupling CO2 experiment, we find that the warming effect of SWV is not as significant as previously thought, due to compensating changes in other feedback. Key Points: Locking experiment suggests a weaker stratospheric water vapor feedback than previously thoughtTropospheric feedback are coupled with the stratospheric water vapor feedbackThe coupling limits the surface warming effect of stratospheric water vapor [ABSTRACT FROM AUTHOR]

We examine CMIP6 simulations of Arctic sea‐ice area and volume. We find that CMIP6 models produce a wide spread of mean Arctic sea‐ice area, capturing the observational estimate within the multimodel ensemble spread. The CMIP6 multimodel ensemble mean provides a more realistic estimate of the sensitivity of September Arctic sea‐ice area to a given amount of anthropogenic CO2 emissions and to a given amount of global warming, compared with earlier CMIP experiments. Still, most CMIP6 models fail to simulate at the same time a plausible evolution of sea‐ice area and of global mean surface temperature. In the vast majority of the available CMIP6 simulations, the Arctic Ocean becomes practically sea‐ice free (sea‐ice area <1 × 106 km2) in September for the first time before the Year 2050 in each of the four emission scenarios SSP1‐1.9, SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5 examined here. Plain Language Summary: We examine simulations of Arctic sea ice from the latest generation of global climate models. We find that the observed evolution of Arctic sea‐ice area lies within the spread of model simulations. In particular, the latest generation of models performs better than models from previous generations at simulating the sea‐ice loss for a given amount of CO2 emissions and for a given amount of global warming. In most simulations, the Arctic Ocean becomes practically sea‐ice free (sea‐ice area <1 million km2) in September for the first time before the Year 2050. Key Points: CMIP6 model simulations of Arctic sea‐ice area capture the observational record in the multimodel ensemble spreadThe sensitivity of Arctic sea ice to changes in the forcing is better captured by CMIP6 models than by CMIP5 and CMIP3 modelsThe majority of available CMIP6 simulations lose most September sea ice for the first time before 2050 in all scenarios [ABSTRACT FROM AUTHOR]