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A fundamental divide exists between previous studies that conclude that polar amplification does not occur without sea ice and studies that find that polar amplification is an inherent feature of the atmosphere independent of sea ice. We hypothesize that a representation of climatological ocean heat transport is key for simulating polar amplification in ice-free climates. To investigate this, we run a suite of targeted experiments in the slab ocean aquaplanet configuration of CESM2-CAM6 with different profiles of prescribed ocean heat transport, which are invariant under CO2 quadrupling. In simulations without climatological ocean heat transport, polar amplification does not occur. In contrast, in simulations with climatological ocean heat transport, robust polar amplification occurs in all seasons. What is causing this dependence of polar amplification on ocean heat transport? Energy-balance model theory is incapable of explaining our results and in fact would predict that introducing ocean heat transport leads to less polar amplification. We instead demonstrate that shortwave cloud radiative feedbacks can explain the divergent polar climate responses simulated by CESM2-CAM6. Targeted cloud locking experiments in the zero ocean heat transport simulations are able to reproduce the polar amplification of the climatological ocean heat transport simulations, solely by prescribing high-latitude cloud radiative feedbacks. We conclude that polar amplification in ice-free climates is underpinned by ocean–atmosphere coupling, through a less negative high latitude shortwave cloud radiative feedback that facilitates enhanced polar warming. In addition to reconciling previous disparities, these results have important implications for interpreting past equable climates and climate projections under high-emissions scenarios. Significance Statement: Polar amplification is a robust feature of climate change in the modern-day climate. However, previous climate modeling studies fundamentally do not agree on whether polar amplification occurs in ice-free climates. In this study, we find in a state-of-the-art climate model that, if ocean heat transport is neglected, the response to an increase in CO2 is not polar amplified, whereas robust polar amplification occurs if ocean heat transport is included. Using targeted model experiments, we diagnose cloud radiative effects as the driver of this divergent behavior. We conclude that polar amplification is a robust feature of the atmosphere–ocean system. Our results have important implications for interpreting past warm climates and future projections under high-emissions scenarios. [ABSTRACT FROM AUTHOR]

Drought monitoring and early detection have improved greatly in recent decades through the development and refinement of numerous indices and indicators. However, a lack of guidance, based on user experience, exists as to which drought-monitoring tools are most appropriate in a given location. This review paper summarizes the results of targeted user engagement and the published literature to improve the understanding of drought across North America and to enhance the utility of drought-monitoring tools. Workshops and surveys were used to assess and make general conclusions about the perceived performance of drought indicators, indices and impact information used for monitoring drought in the five main Köppen climate types (Tropical, Temperate, Continental, Polar Tundra, Dry) found across Canada, Mexico, and the United States. In Tropical, humid Temperate, and southerly Continental climates, droughts are perceived to be more short-term (less than 6 months) in duration rather than long-term (more than 6 months). In Polar Tundra climates, Dry climates, Temperate climates with dry warm seasons, and northerly Continental climates, droughts are perceived to be more long-term than short-term. In general, agricultural and hydrological droughts were considered to be the most important drought types. Drought impacts related to agriculture, water supply, ecosystem, and human health were rated to be of greatest importance. Users identified the most effective indices and indicators for monitoring drought across North America to be the U.S. Drought Monitor (USDM) and Standardized Precipitation Index (SPI) (or another measure of precipitation anomaly), followed by the Normalized Difference Vegetation Index (NDVI) (or another satellite-observed vegetation index), temperature anomalies, crop status, soil moisture, streamflow, reservoir storage, water use (demand), and reported drought impacts. Users also noted the importance of indices that measure evapotranspiration, evaporative demand, and snow water content. Drought indices and indicators were generally thought to perform equally well across seasons in Tropical and colder Continental climates, but their performance was perceived to vary seasonally in Dry, Temperate, Polar Tundra, and warmer Continental climates, with improved performance during warm and wet times of the year. The drought indices and indicators, in general, were not perceived to perform equally well across geographies. This review paper provides guidance on when (time of year) and where (climate zone) the more popular drought indices and indicators should be used. The paper concludes by noting the importance of understanding how drought, its impacts, and its indicators are changing over time as the climate warms and by recommending ways to strengthen the use of indices and indicators in drought decision making. [ABSTRACT FROM AUTHOR]

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

Global warming in polar regions brings a risk of more frequent and long-lasting dry periods due to warmer and windier climate during polar summers. Mosses are well adapted to desiccation-rehydration events and they have evolved remarkable constitutive and inducible mechanisms of desiccation tolerance. In our study, Sanionia uncinata and Racomitrium lanuginosum were collected in Svalbard and used for laboratory-based repetitive 32-h-lasting rehydration cycles with continuous monitoring of restoration of their primary photosynthetic processes measured by chlorophyll fluorescence parameters. Immediately after the addition of water to dry thalli, potential quantum yield of PSII (FV/FM) was about 50% of its maximum reached after 32 h of rehydration. In a course of time of rehydration, both species showed an increase in FV/FM and effective quantum yield (ΦPSII) following a S-curve relationship. Non-photochemical quenching did not show clear trend with the rehydration time, It differed between the two species and showed both decrease and increase with the time of rehydration. Relative chlorophyll fluorescence decrease (RFd), which is considered a vitality indicator, increase with the time of rehydration showing similar trends in the first and the third cycle of rehydration. The results indicate that both Sanionia uncinata and Racomitrium lanuginosum are resistant to desiccation since FV/FM and ΦPSII recovered fully after 32 h of rehydration and there we only minor differences in the two parameters between the first and third rehydration cycle. [ABSTRACT FROM AUTHOR]

It is challenging to estimate how the regional climate will be shifted under future global warming. To reduce the potential risk of regional climate shift under future climates, examining the change in climate features over Asia is important, as approximately 60% of the world's population resides there. In this study, climate shifts are assessed over the Asian monsoon region under global mean temperature warming targets from 1.5°C to 5.0°C above preindustrial (PI) levels based on different shared socioeconomic pathway (SSP) scenarios. Global warming impacts the individual climate variables, and consequently, it impacts the regional climate features across the Asian region. Temperature change patterns are more dominant contributors to the spatial extent and magnitude of climate shifts than precipitation change patterns. Changes in regional climates show different behaviors according to the degree of global warming rather than the type of SSP scenario. Climate shifts are intensified under a higher level of global warming that is above the PI levels. The largest climate shifts in this region are shown under global warming of 5.0°C based on the SSP5‐8.5 scenario, especially in current polar climate zones. Future change patterns in individual climate zone can differ. Regions with tropical climates and arid climates are likely to be expanded, whereas some regions with warm temperate climates, cold climates, and polar climates are likely to shrink under global warming conditions. Therefore, this study supports the necessity of mitigating greenhouse gas (GHG) emissions and establishing an adaptation plan for future global warming conditions. Key Points: Potential global warming based on CMIP6 causes the significant changes in regional climates and consequently climate shifts over AsiaPace of climate zone shifts will become faster as the global warms, but the differences exist among the regional climate featuresCharacteristics of climate shifts are more sensitive to the degree of global mean temperature increase than the type of SSP scenarios [ABSTRACT FROM AUTHOR]

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

The global temperature has increased approximately 0.9°C over the past 50 years and is projected to continue to increase. Many efforts have been taken to investigate the evolutionary dynamics of climate classification zones in response to the rising temperature. However, the changing dynamics of the spatial climate patterns remain poorly understood. It is thus desired that the unobserved characteristics of the global climate patterns, such as aggregation and subdivision, are explored from the landscape perspective. Here, we demonstrate that the global climate zones have undergone a process of aggregation during the past 100 years and that this dynamic is expected to continue in the future. The climate aggregation effect is especially evident in the Arctic, Tibet, and East Africa. We find that the tropical, arid, temperate, and cold climates are aggregating while the polar climate zone exhibits subdivision. Furthermore, our analysis demonstrates a clear spatial aggregation process with a latitudinal feature. We should pay enough attention to the phenomenon of patch aggregation of global climate landscape pattern, which will undoubtedly help us to deepen our understanding of global climate change and help to further explore the feedback relationship between climate and biological systems. [ABSTRACT FROM AUTHOR]

This paper considers the results of complex glaciological and hydro meteorological observations in the basin of Chon Kyzyl Suu river for the period of 2013-2017; the analysis of flow formation from the Kara Batkak glacier was carried out. The main and minor factors of ice and snow ablation of Kara Batkak glacier are shown. The analysis of the main climatic parameters (air temperature and precipitation), which are changing in concordance with global warming, has been carried out. In the last 70 years the area of the Kara Batkak glacier has decreased by 21.8%, showing a pronounced trend of diminishing the area of this glacier, which is a consequence of global warming. It is established that the trend of increasing ablation over the entire observation period on the Kara Batkak glacier is associated with an increase in air temperatures over the last few decades. There is a good synchronization of annual trends of rise in air temperature at the three meteo stations for the period from 1961 to 2015 with a maximum of 7.0 °C at Kara-Batkak. This suggests that the same trend happens in all the regions of Teskey Ala Too, which in recent years, increase the trend of daily melting of ice. The mass balance in these basins over 60 years is negative due to rise in temperature and add to river runoff due to degradation and the reduction in the area of its glaciation confirmed the water-ice balance of mountain-glacial basins of the "continental" type glaciers. The close connection between the changes in climatic parameters, state of glaciations and glacial runoff in these river basin were established in this study. [ABSTRACT FROM AUTHOR]

The mechanism of polar amplification in the absence of surface albedo feedback is investigated using an atmospheric model coupled to an aquaplanet slab ocean forced by a CO2 doubling. In particular, we examine the sensitivity of polar surface warming response under different insolation conditions from equinox (EQN) to annual mean (ANN) to seasonally varying (SEA). Varying insolation greatly affects the climatological static stability. The equinox condition, with the largest polar static stability, exhibits a bottom-heavy vertical profile of polar warming response that leads to the strongest polar amplification. In contrast, the polar warming response in ANN and SEA exhibits a maximum in the midtroposphere, which leads to only weak polar amplification. The midtropospheric warming maximum, which results from an increased poleward atmospheric energy transport in response to the tropics-to-pole energy imbalance, contributes to polar surface warming via downward clear-sky longwave radiation. However, it is cancelled by negative cloud radiative feedbacks locally. Furthermore, the polar lapse rate feedback, calculated from radiative kernels, is negative due to the midtropospheric warming maximum, and hence is not able to promote the polar surface warming. On the other hand, the polar lapse rate feedback in EQN is positive due to the bottom-heavy warming response, contributing to the strong polar surface warming. This contrast suggests that locally induced positive radiative feedbacks are necessary for strong polar amplification. Our results demonstrate how interactions among climate feedbacks determine the strength of polar amplification. [ABSTRACT FROM AUTHOR]

The dramatic warming of the Arctic over the last three decades has reduced both the thickness and extent of sea ice, opening opportunities for business in diverse sectors and increasing human exposure to meteorological hazards in the Arctic. It has been suggested that these changes in environmental conditions have led to an increase in extreme cyclones in the region, therefore increasing this hazard. In this study, we investigate the response of Arctic synoptic scale cyclones to climate change in a large initial value ensemble of future climate projections with the CESM1-CAM5 climate model (CESM-LE). We find that the response of Arctic cyclones in these simulations varies with season, with significant reductions in cyclone dynamic intensity across the Arctic basin in winter, but with contrasting increases in summer intensity within the region known as the Arctic Ocean cyclone maximum. There is also a significant reduction in winter cyclogenesis events within the Greenland-Iceland-Norwegian sea region. We conclude that these differences in the response of cyclone intensity and cyclogenesis, with season, appear to be closely linked to changes in surface temperature gradients in the high latitudes, with Arctic poleward temperature gradients increasing in summer, but decreasing in winter. [ABSTRACT FROM AUTHOR]

Numerical water tracers implemented in a global climate model are used to study how polar hydroclimate responds to CO2-induced warming from a source-receptor perspective. Although remote moisture sources contribute substantially more to polar precipitation year-round in the mean state, an increase in locally sourced moisture is crucial to the winter season polar precipitation response to greenhouse gas forcing. In general, the polar hydroclimate response to CO2-induced warming is strongly seasonal: over both the Arctic and Antarctic, locally sourced moisture constitutes a larger fraction of the precipitation in winter, while remote sources become even more dominant in summer. Increased local evaporation in fall and winter is coincident with sea ice retreat, which greatly augments local moisture sources in these seasons. In summer, however, larger contributions from more remote moisture source regions are consistent with an increase in moisture residence times and a longer moisture transport length scale, which produces a robust hydrologic cycle response to CO2-induced warming globally. The critical role of locally sourced moisture in the hydrologic cycle response of both the Arctic and Antarctic is distinct from controlling factors elsewhere on the globe; for this reason, great care should be taken in interpreting polar isotopic proxy records from climate states unlike the present. [ABSTRACT FROM AUTHOR]

The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases. [ABSTRACT FROM AUTHOR]

• The Arctic is greatly affected by climate change. • It is unclear how Arctic meiofauna respond to ice melting. • We used metabarcoding to assess the seasonal shifting of meiofauna in the Arctic. • Meiofauna communities significantly shift across seasons and habitats. • Meiofauna can be used as a valuable indicator of global warming. Establishing robust estimates of polar marine biodiversity is important for interpreting future changes in the Arctic; however, despite a recent increase in scientific expeditions, this region remains relatively underexplored. Particularly overlooked in biodiversity assessments are small species, such as protists, fungi, and many small invertebrates that are collectively known as meiofauna. These species contribute to the foundation of food webs and are crucial for the survival of larger species that are economically and socially important. The application of high-throughput sequencing methodologies has proven effective for biomonitoring small metazoan species but has sparingly been applied in the Arctic. We used a metabarcoding approach to assess the diversity of sea ice and sediment-associated metazoans from Utqiaġvik (Barrow), Alaska. Sea ice and sediment samples were collected six times over eight months (January through August) encompassing three seasons (winter, spring, and summer) from polar night to ice-out in August. Biodiversity was assessed as both richness and community composition by incorporating incidence data and phylogenetic distance. Environmental conditions associated with ice, sediment, water, and snow were measured and tested for possible correlations with biodiversity estimates. We found a high number of taxa distributed locally, suggesting that metabarcoding can be effectively applied to Arctic biomonitoring programs. In addition, these results show that season and habitat are significant predictors of meiofaunal biodiversity, supporting hypotheses that meiofauna can be used as a valuable indicator of climate change. [ABSTRACT FROM AUTHOR]

We investigate how the ocean response to CO2 forcing affects hemispheric asymmetries in polar climate sensitivity. Intermodel comparison of Phase 5 of the Coupled Model Intercomparison Project CO2 quadrupling experiments shows that even in models where hemispheric ocean heat uptake differences are small, Arctic warming still exceeds Antarctic warming. The polar climate impact of this evolving ocean response to CO2 forcing is then isolated using slab ocean experiments in a state‐of‐the‐art climate model. Overall, feedbacks over the Southern Hemisphere more effectively dissipate top‐of‐atmosphere anomalies than those over the Northern Hemisphere. Furthermore, a poleward shift in ocean heat convergence in both hemispheres amplifies destabilizing ice albedo and lapse rate feedbacks over the Arctic much more so than over the Antarctic. These results suggest that the Arctic is intrinsically more sensitive to both CO2 and oceanic forcings than the Antarctic and that ocean‐driven climate sensitivity asymmetry arises from feedback destabilization over the Arctic rather than feedback stabilization over the Antarctic. Plain Language Summary: Anthropogenic greenhouse gas emissions impact climate globally, but nowhere more so than over the Arctic, a phenomenon known as polar amplification. Surprisingly, the climate response over the Antarctic is much more muted than the climate response over the Arctic, which has been attributed to the large uptake of heat over the Southern Ocean which cools the Southern Hemisphere. Here we show that a weaker climate response over the Antarctic is due, in part, to weaker intrinsic sensitivity to both greenhouse gas forcing and the state of the ocean. Even climate models with similar amounts of heat uptake into the deep ocean in both hemispheres warm more over the Arctic than the Antarctic. Furthermore, similar increases in winter season heat transport into the polar oceans in both hemispheres trigger more destabilizing climate feedbacks over the Arctic than the Antarctic. Therefore, greater climate change over the Arctic than the Antarctic can be expected even if ocean heat uptake or ocean heat transport changes are similar in both hemispheres. Key Points: The Arctic has greater intrinsic sensitivity to ocean state changes than the AntarcticThe ocean response triggers more destabilizing radiative feedbacks over the Arctic than the AntarcticPolar radiative feedbacks respond more to increased high‐latitude ocean heat convergence than to subpolar ocean heat uptake [ABSTRACT FROM AUTHOR]