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Surface anticyclones connected to the ridge of an upper‐tropospheric Rossby wave are the main dynamical drivers of mid‐latitude summer heatwaves. It is, however, unclear to what extent an anomalously low zonal phase speed of the wave in the upper troposphere is necessary for persistent temperature extremes at the surface. Here, we use spectral decomposition to separate fast and slow synoptic‐scale waves. A composite analysis of ERA5 reanalysis data reveals that, while in some regions heatwaves become more frequent during episodes of weak or no phase propagation, temperature extremes in other regions are commonly associated with more rapidly eastward propagating Rossby waves. Reflected in the mean heatwave duration as well, this relationship is possibly linked to a longitudinal phase preference of slow and fast waves or a meridional storm track shift. These findings open up new questions about the influence of mid‐latitude dynamics on temperature extremes. Plain Language Summary: High pressure systems tend to be associated with mid‐latitude summer heatwaves, defined as multiple consecutive hot days. The persistence of heatwaves raises the question whether the associated high pressure system has to be anomalously persistent as well to cause a heatwave. Using a combination of observational data and atmospheric model output that is commonly regarded as ground truth for prediction purposes we find that this is not the case. By re‐distributing the location of on‐average high air pressure, some regions experience an increased heatwave frequency even if the atmospheric circulation is less persistent than usual. Understanding the link between persistent surface temperature extremes and the atmospheric circulation is important for the prediction and projection of extreme events in a warming climate. Key Points: Phase speed of synoptic‐scale waves is crucial for where, but less important for whether heatwaves occurA longitudinal phase preference of slow and fast waves is reflected in the mean heatwave durationChanges in phase speed are linked to a meridional storm track shift [ABSTRACT FROM AUTHOR]

Solar radiation‐topography interaction plays an important role in surface energy balance over the Tibetan Plateau (TP). However, the impacts of such interaction over the TP on climate locally and in the Asian regions remain unclear. This study uses the Energy Exascale Earth System Model (E3SM) to evaluate the regional and teleconnected impacts of solar radiation‐topography interaction over the TP. Land‐atmosphere coupled experiments show that topography regulates the surface energy balance, snow processes, and surface climate over the TP across seasons. Accounting for solar radiation‐topography interaction improves E3SM simulation of surface climate. The winter cold bias in air temperature decreases from −4.57 to −3.79 K, and the wet bias in summer precipitation is mitigated in southern TP. The TP's solar radiation‐topography interaction further reduces the South and East Asian summer precipitation biases. Our results demonstrate the topographic roles in regional climate over the TP and highlight its teleconnected climate impacts. Plain Language Summary: The Tibetan Plateau (TP) is characterized by high elevation and complex topography. Interaction between solar radiation and the undulating topography has important impacts on the regional surface energy balance and hydrologic cycle. Here we use Earth System Model simulations to show the local and remote impacts of the TP's solar radiation‐topography interaction on the surface climate of the Asian regions. Such interaction overall increases the air temperature especially in winter over the TP and reduces the summer precipitation in southern TP. Teleconnectedly, the interaction further alters the precipitation patterns in South and East Asia, by altering the atmospheric circulation that influences moisture transport and clouds. Accounting for such interaction generally improves the model performance when benchmarked against observations. These findings underscore the important roles of the TP's solar radiation‐topography interaction in modulating the climate of the local and remote Asian regions. Key Points: Solar radiation‐topography interaction over the Tibetan Plateau (TP) increases annual average regional near‐surface air temperature by 0.26 KSolar radiation‐topography interaction over the TP also affects the precipitation patterns in South and East AsiaIncluding solar radiation‐topography interaction improves the simulation of surface climate over the TP and Asian regions [ABSTRACT FROM AUTHOR]

The dominant pattern of Northern Hemisphere extratropical climate variability is the Northern Annular Mode (NAM), which in wintertime represents the coupling of the stratospheric and tropospheric circulations. The internal variability associated with the NAM has been shown to give rise to substantial uncertainty in climate projections of regional surface air temperatures (SATs), but it's unclear how much of this variability arises from stratosphere‐troposphere coupling processes. Here, using three large‐ensemble coupled climate model simulations from 1850 to 2100 with varying fidelity of stratosphere‐troposphere coupling processes, we demonstrate that regional SAT variability across timescales is amplified when the tropospheric NAM is coupled to the stratospheric NAM. Moreover, models that lack adequate simulation of stratosphere‐troposphere coupling processes may not capture the magnitude of uncertainty in near‐term SAT projections associated with this coupling. Plain Language Summary: Large‐scale atmospheric circulation variability drives substantial variation in regional surface air temperatures (SATs), so that even on 30–40 year timescales local cooling trends are possible even as global‐mean temperatures increase. We provide evidence that when the stratospheric mean flow is coupled to the tropospheric mean flow, the decadal variability of SATs is amplified in some regions. Some models do not represent well the observed coupling of the stratosphere to the troposphere, and these models may underestimate decadal variability associated with this coupling. Key Points: The Northern Annular Mode (NAM) exhibits decadal variability that can accelerate or slow the rate of anthropogenic regional surface warmingAmplified decadal variability of extratropical surface temperatures occurs when the NAM is coupled from the stratosphere to the troposphereModels that fail to adequately capture vertical coupling may underestimate the magnitude of decadal variations in surface air temperatures [ABSTRACT FROM AUTHOR]

North Atlantic Oscillation (NAO) has a significant impact on surrounding winter weather and climate. However, the causes of its occasional reversal between early and late winter remain unclear. This study proposes a mechanism for the mid‐winter reversal of NAO from the perspective of local midlatitude air‐sea interaction. Strong sea surface temperature (SST) tripole events, which are defined by empirical orthogonal function of winter‐mean interannual North Atlantic SST anomalies, are primarily induced by NAO in early winter and peak in January. In late winter, the persistent SST tripole exerts active feedback on atmosphere through diabatic heat and transient eddy forcing. The resulting atmospheric circulation anomalies exhibit an almost reversed NAO pattern in February, which forms a wavetrain originating above the Gulf Stream and propagating to the Middle East and weakens the former SST tripole. Consequently, significant reversals of air temperature anomalies occur in Europe and the Caspian Sea area between February and December. Plain Language Summary: The North Atlantic Oscillation (NAO) is a crucial atmospheric system that significantly impacts the weather and climate of the surrounding regions during winter. However, it often reverses between early and late winter, and the reasons for its mid‐winter reversal remain unclear. The NAO behavior can be influenced by multiple factors, such as atmospheric internal processes, underlying surface, and remote climate system, which makes the issue more complex. This study highlights the role of underlying sea surface temperature anomalies (SSTAs) in the mid‐winter reversal of NAO. The North Atlantic SSTAs are closely associated with the NAO and exhibit a tripole pattern. For strong sea surface temperature (SST) tripole events, the NAO primarily induces the early winter North Atlantic SST tripole. The SSTAs develop in early winter, peak in January, and feedback on the atmospheric circulation in late winter. The atmospheric circulation anomalies exhibit an almost reversed NAO pattern in February, forming a wavetrain that propagates above the Gulf Stream to the Middle East. As a result, during the warm phase of the SST tripole, Europe experiences colder temperatures, and the Caspian Sea experiences warmer temperatures in February than usual, as opposed to the positive NAO's control over warm Europe and cold Caspian Sea in December. Key Points: In early winter, North Atlantic Oscillation (NAO) forces a North Atlantic sea surface temperature (SST) tripole, which persists and tends to induce a nearly reversed NAO pattern in FebruarySST tripole induces a wavetrain over the Gulf Stream to the Middle East via diabatic heating and transient eddy forcing in FebruaryAs a result, there are remarkable reversals of air temperature anomalies in Europe and Caspian Sea area between February and December [ABSTRACT FROM AUTHOR]

Changes in ice‐sheet size impact atmospheric circulation, a phenomenon documented by models but constrained by few paleoclimate records. We present sub‐centennial‐scale records of summer temperature and summer precipitation hydrogen isotope ratios (δ2H) spanning 12–7 ka from a lake on Baffin Island. In a transient model simulation, winds in this region were controlled by the relative strength of the high‐pressure systems and associated anticyclonic circulation over the retreating Greenland and Laurentide ice sheets. The correlation between summer temperature and precipitation δ2H proxy records changed from negative to positive at 9.8 ka. This correlation structure indicates a shift from alternating local and remote moisture, governed by the two ice‐sheet high‐pressure systems, to only remote moisture after 9.8 ka, governed by the strong Greenland high‐pressure system after the Laurentide Ice Sheet retreated. Such rapid atmospheric circulation changes may also occur in response to future, gradual ice‐sheet retreat. Plain Language Summary: Continental ice sheets alter atmospheric circulation, influencing global heat and moisture distribution. Records of atmospheric circulation during previous periods of ice‐sheet retreat can provide insights into the changes that are possible in the future. This study examines summer atmospheric circulation in Baffin Bay from 12,000 to 7,000 years ago, a period of dramatic ice‐sheet retreat. Precipitation isotopes reflect moisture source, which responds to changes in air temperature and atmospheric circulation. This study uses records of temperature and precipitation isotopes from the same sediment archive to tease apart the influence of temperature from that of atmospheric circulation. The precipitation isotopes in this record distinct changes in moisture sources, which a climate model simulation suggests was caused by retreating ice sheets. Before about 10 ka, when the Laurentide Ice Sheet (LIS) covered eastern Canada, summer winds in Baffin Bay shifted regularly between south and north, carrying air with unique temperature and precipitation isotope signatures. As the LIS retreated, the Greenland Ice Sheet (GIS) remained relatively large and dominated atmospheric circulation, causing a rapid shift to southeasterly winds. As the GIS retreats in the future, atmospheric circulation may undergo similar rapid changes. Key Points: Summer precipitation isotopes in the Arctic reflect temperature and moisture transport historyPaired records of summer temperature and precipitation isotopes isolate the influence of temperature from that of transportMoisture delivery to the eastern Canadian Arctic changed dramatically due to Early Holocene Laurentide Ice Sheet retreat [ABSTRACT FROM AUTHOR]

The international Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative works to recover global climate history and build an accessible databank, with data from the past 250 years of terrestrial and marine surface. Argentina is part of that effort with ACRE Argentina recovering data from various sources that have been recorded throughout the country since the 19th century. In this paper, weather observations transcribed from the original records into digital form and taken every day during the years 1860–1879 at Bahía Blanca and from 1873–1886 at Corrientes are presented. The digitization was carried out through the project Meteorologum ad Extremum Terrae (MET) launched in the Zooniverse interface, which is currently working with near 900 citizen scientist volunteers per day. The present analysis corresponds to data retrieved from the collection 'Anales de la Oficina Meteorológica Argentina' and contains information on atmospheric pressure, air temperature, relative humidity, cloud cover, cloud types, wind direction, wind speed, rainfall and weather remarks. For the present analysis, only temperature and pressure values, measured in °C and mm of Hg, respectively, were considered, after a quality control of the digitization process was applied. Data values were tested and used to rebuild the time series of both places and correlation with SOI index and monthly pressure values for both places was tested using Spearman correlation. Results show that the influence of El Niño episode during 1877–1878 can be found in the pressure values at Corrientes. [ABSTRACT FROM AUTHOR]

River discharge has experienced diverse changes in the last decades due to modification of hydrological patterns, anthropogenic intervention, re-vegetation or annual and interannual climatic and atmospheric fluctuations. Assessing the recent changes in river discharge and understanding the main drivers of these changes is thus extremely important from theoretical and applied points of view. More specifically, here we want to draw attention toward the impacts of streamflow changes on reservoir storage and operation. We describe the hydrological dynamics of the Yesa reservoir draining catchment, located in the central Spanish Pyrenees, and characterize the reservoir operation modes over the last 60 years (1956–2020). We analyze concurrent climatic (precipitation, air temperature, drought index), atmospheric mechanisms, land cover (Normalized Different Vegetation Index) and discharge (inlet and outlet of Yesa reservoir) time-series. By using the wavelet transform methodology, we detect historical breakpoints in the hydrological dynamics at different time-scales. Distinctive periods are thus identified. More regular seasonal flows characterized the catchment's dynamics during the first decades of the study period, while the last decades were characterized by a high inter-annual variability. These changes are primarily attributed to the natural re-vegetation process that the catchment experienced. Furthermore, we related changes in atmospheric circulation with a decline of the longterm discharge temporal features. This research contributes to the understanding of long-term river discharge changes and helps to improve the reservoir management practices. [ABSTRACT FROM AUTHOR]

The response of the Euro‐Atlantic wintertime circulation to climate change is deeply uncertain. The Atlantic jet is caught in a "tug‐of‐war" between rapid warming trends in both the tropics and the Arctic leading to debate over the changing "waviness" of the jet, which is subject to strong non‐linearity and internal variability. From the complementary perspective of weather regimes, there is considerable uncertainty in how atmospheric blocking will alter under climate change. By applying the hybrid approach of geopotential‐jet regimes to 6th phase of the coupled model inter‐comparison project projections, we show that the centers of action of anticyclonic regimes hardly alter even under severe warming. Instead, regimes are expected to become less persistent, with zonal flow conditions becoming more prevalent, although models disagree on the details of regime changes. Finally, we show the regime response can be captured qualitatively in a simple Lorenz‐like model, emphasizing the conceptual link between observed regimes and those in basic mathematical systems. Plain Language Summary: The impact of climate change on European weather can be broken into two components: a thermodynamic part relating to increasing air temperature and humidity, and a dynamic part relating to changes in the atmospheric circulation such as the direction and strength of prevailing winds. While the thermodynamic part is relatively well understood, the dynamic part is very uncertain and this is a major problem in constraining European climate projections. Looking at the winter season, we study the dynamic response of CMIP6 models under climate change using so‐called "regimes," and show that the types of prevailing circulation are not predicted to change strongly. However the regimes are projected to be less long lived. We also show that these features can be well captured in a simple 5 equation model of regime dynamics, providing a potentially useful tool for understanding regime systems in more detail. Key Points: The spatial structure of anticyclonic circulations over Europe are projected to stay the same under climate changeThe persistence of anticyclonic circulations are in general expected to decrease, although there is considerable inter‐model variabilityWe show that these qualitative features of the atmospheric response can be reproduced in a simple forced regime model [ABSTRACT FROM AUTHOR]

In late June 2021, western North America, and in particular the Pacific Northwest experienced a heatwave with temperatures usually only encountered in hot desert climates. Using a blend of reanalysis data and Earth System Model (ESM) simulations, we disentangle the physical drivers underlying this exceptional event. Our analysis highlights the role of the anticyclonic circulation aloft, which converted previously gained potential energy—some of which by intense latent heating thousands of kilometers upwind over the North Pacific—back into sensible heat through subsidence. We demonstrate that this upwind latent heat release did not only result in a hot troposphere above the heatwave region, but also contributed directly to escalating near‐surface temperatures. Facilitated by the mountainous terrain and dry soils in the region, deep atmospheric boundary layers were established over the course of several days, connecting the air close to Earth's surface to a massive heat reservoir many kilometers above. Anomalous soil moisture acted to raise the heatwave temperatures by 3°C in a large region during the peak of the event, with local anomalies exceeding 5°C. Overall, we conclude that this heatwave was the outcome of an intricate interplay between dynamic and thermodynamic processes. ESM experiments suggest that the same large‐scale atmospheric circulation fueled by enhanced thermodynamic drivers, such as more available moisture for condensation upwind, could enable even more extreme near‐surface temperatures, in particular in a warmer climate. Plain Language Summary: In late June 2021, western North America, and in particular the Pacific Northwest, experienced temperatures normally encountered in hot deserts. Our analysis highlights the role of the anticyclonic circulation aloft, whose downward spiraling air masses converted previously gained potential energy back into sensible heat. We show that in addition to heating through sinking, the air was previously heated by condensation in ascending air streams thousands of kilometers upwind, over the North Pacific. Together, these processes fostered a massive heat reservoir above the heatwave region, which contributed to escalating near‐surface temperatures through strong vertical mixing. Dry soils in the region intensified surface heating, boosting maximum temperatures in excess of 5°C. Overall, we conclude that this heatwave was the outcome of an intricate interplay of the atmospheric flow and processes such as condensational and surface heating, further exacerbated by human‐induced background warming. Our experiments suggest that if fueled by more available moisture for condensation upwind, the same large‐scale atmospheric circulation could enable even more extreme near‐surface temperatures. Key Points: A strong "Omega Block" enabled the heatwave, yet near‐surface air temperatures were more extreme than suggested by the large‐scale flowSinking air aloft previously experienced strong latent heating over the North Pacific, contributing to the high near‐surface temperaturesDeep atmospheric boundary layers brought the extreme heat down to the surface, with local soil moisture effects in excess of 5°C [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a neurotoxic pollutant that can be transported globally by atmospheric circulation and poses risks to wildlife and humans. Strong atmospheric Hg seasonality has been observed in the temperate Northern Hemisphere and explained by several hypothetical mechanisms. Here, we found that meteorological conditions were important drivers of the seasonality across diverse temperate regions particularly Asia through various indirect effects. Meteorological conditions can explain approximately 47%, 43%, and 67% of the seasonal amplitudes at the North American (NA) remote sites, European remote sites, and Asian monsoon sites, respectively. Surface air temperature, solar radiation, and surface wind collectively contributed significantly to the seasonality at the NA and European sites through vegetation resistance and oceanic evasion, while monsoon wind and precipitation promoted the summertime removal of Hg at the Asian sites through atmospheric transport and wet deposition, respectively. The findings predicted further changes in atmospheric Hg seasonality under future global warming. Plain Language Summary: Mercury is a neurotoxic pollutant that can be transported globally by atmospheric circulation before being removed from the atmosphere by deposition. Deposited mercury poses potential risks to ecosystems and human health owing to methylation and bioaccumulation. Strong seasonality of atmospheric mercury concentrations has been observed in the temperate Northern Hemisphere. Several hypotheses have been proposed to explain this strong seasonality, including vegetation uptake, chemical transformations, and anthropogenic emissions. In this study, we found that meteorological conditions were important drivers of the seasonality across diverse temperate regions particularly Asia through various indirect effects. Typical monsoon climate in Asia led to differences in the crucial meteorological factors between Asia and other temperate regions. Key Points: Meteorological conditions contributed significantly to atmospheric mercury seasonality across diverse temperate regions particularly AsiaTypical monsoon climate in Asia led to differences in the crucial meteorological factors between Asia and other temperate regionsSignificant contributions of meteorological factors predicted further changes in atmospheric mercury seasonality under future global warming [ABSTRACT FROM AUTHOR]

Understanding influence of vegetation on past temperature changes in the Arctic region would help isolate uncertainty and build understanding of its broader climate system, with implications for paleoclimate reconstructions and future climate change. Using an Earth system model EC‐Earth, we conduct a series of simulations to investigate the impact of vegetation‐climate feedback on the Arctic climate during the mid‐Holocene. Results show Arctic greening induced by the warming resulting from stronger orbital forcing, further amplifies the Arctic warming. The increased vegetation contributes 0.33°C of Arctic warming and 0.35 × 106 km2 of Arctic sea ice loss. Increased Arctic vegetation leads to reduced land surface albedo and increased evapotranspiration, both of which cause local warming in spring and summer. The resultant sea ice loss causes warming in the following seasons, with atmospheric circulation anomalies further amplifying the warming. Our results highlight the significant contribution of vegetation‐climate feedback to Arctic climate under natural conditions. Plain Language Summary: Vegetation‐climate feedback is an essential process between the terrestrial biosphere and the atmosphere. Greening of the Arctic can be invoked by increased solar radiation, for example, during the mid‐Holocene (6,000 years ago), or increased greenhouse gas, for example, ongoing global warming. Here we perform simulations of vegetation and climate interactions during the mid‐Holocene and demonstrate the impact of vegetation‐climate feedback on the Arctic climate. We show that compared to the pre‐industrial period (1850 CE), the Arctic region experiences an expansion of vegetation under a warm climate due to increased solar radiation during the mid‐Holocene. The greening leads to reduced land albedo and enhanced evapotranspiration. These changes and resultant sea ice loss, in combination with associated atmospheric circulation changes, further amplify the Arctic warming. These changes alone are responsible for 0.33°C increase in surface air temperature and 0.35 × 106 km2 increase in sea ice loss. Given the facts of observed ongoing Arctic greening in recent decades, our modeling results suggest that the effect of the vegetation‐climate feedback will amplify the future Arctic warming and sea ice loss even more. Key Points: The Earth system model EC‐Earth reproduced the expansion of vegetation in northern high latitudes during the mid‐Holocene compared to the PIThe mid‐Holocene Arctic greening contributes 0.33°C increase in annual Arctic warming and 0.35 × 106 km2 of sea ice lossThe increased evapotranspiration and decreased surface albedo due to the Arctic greening amplify the Arctic warming in the growing season [ABSTRACT FROM AUTHOR]

Australia experienced record‐breaking wildfires during 2019–2020, which emitted large amounts of carbonaceous aerosols (CAs) to the atmosphere. In this study, we explored the atmospheric dynamics and thermodynamic mechanism of the long‐range transport of CAs during November 2019–February 2020. The results indicate that the emitted CAs had a ternary advective spreading pattern across the Pacific Ocean in the Southern Hemisphere (SH), influenced mainly by the westerlies and anticyclonic systems. Upward vertical motion over the convergence zone and heat released from smoke pyro‐cumulonimbus clouds and strong light absorption of CAs provided strong vertical lift as the predominant injection pathway of the CAs into the stratosphere in the central Pacific Ocean in the SH. The abundant tropospheric CAs induced atmospheric heating (and potentially surface cooling), with increasing air temperature and decreasing of surface temperature. The anomalous thermal structure led to changes in atmospheric circulation, which weakened the upward vertical motion. Plain Language Summary: During the summer of 2019–2020, Australia suffered unprecedented wildfires. These wildfires emitted large amounts of pollutants into the free atmosphere. The pollutants are mainly composed of carbonaceous aerosols (CAs). Under the effects of meteorological conditions, the CAs experienced a long‐range spread expanding from the western Pacific Ocean to the eastern Pacific Ocean even arriving at the Atlantic Ocean. During the transportation, the CAs were accumulated over the central Pacific Ocean under the trapping effect of the anticyclone and then exhibited significant lift owing to upward motion over the convergence zone between westerlies and anticyclone and heat buoyancy released from smoke pyro‐cumulonimbus clouds and strong light absorption of CAs. As a response, the strong absorption of CAs resulted in the warming of the atmosphere and cooling of the surface, which enhanced atmospheric stratification and constrained the development of atmospheric instability. Thus, the upward vertical motion was weakened. Key Points: The westerlies and anticyclonic system dominated the advective spread of carbonaceous aerosols (CAs)The upward vertical motion, heat released from smoke pyro‐cumulonimbus clouds, and strong CAs light absorption contributed to CAs liftThe CAs caused warming of air temperature and cooling of surface temperature, which weakened the upward vertical motion [ABSTRACT FROM AUTHOR]

The isotopic values of rainfall in the southern parts of India exhibit strong seasonality. Such a contrast in isotopic values arises mainly due to the seasonal reversal of the monsoon winds. During the summer monsoon season, moisture is primarily transported from the Arabian Sea, while during the winter, it is sourced mainly from the Bay of Bengal. Additionally, atmospheric processes contribute to these differences. We collected precipitation samples from a southern Indian site and two neighboring island locations to study their isotopic characteristics from intra‐seasonal to seasonal timescale. Oxygen isotopes in the marine environment seem to respond differently to the surface and tropospheric temperature than their counterparts from the land region. The seasonal isotopic gradient in the marine environment appears to be modulated by the tropospheric temperature anomaly. Oxygen isotopic values showed a strong association with the Webster‐Yang monsoon index. Two distinct clusters were formed when the oxygen isotopes were plotted against the above‐mentioned monsoon index. The formation of these clusters indicates the precipitation isotopes' response to moisture dynamics, which significantly change during the transition phase of the southwest to the northeast monsoon. Finally, we examine the potential of the precipitation isotopic records in studying the monsoon process, viz, the use of precipitation isotopes in characterizing the thermodynamical properties of the atmosphere and estimating the summer monsoon withdrawal phase. Plain Language Summary: Atmospheric parameters such as temperature, rainfall, humidity, wind circulation, etc., vary significantly across the seasons. Such kind of variations or the seasonality also varies from place to place. The heavy to light isotopic distributions of hydrogen and oxygen atoms in atmospheric water also differ considerably from summer to winter, termed isotopic seasonality (defined here as Δδ18O). This feature manifests well in southern peninsular India, wherein the summer monsoon season is characterized by higher isotopic values relative to the winter season. Monsoon circulation is south‐westerly during the summer and north‐easterly during the winter. These two opposing circulation regimes cause isotopic seasonality in south peninsular India. Meteorological parameters such as rainfall, wind speed, etc., display sharp changes during the summer monsoon onset in early June. But they change rather slowly during the transition from summer to winter monsoon season around late September–early October. As a result, the estimation of the summer monsoon withdrawal date may have considerable uncertainty. It is proposed that applying the isotopic technique may be potentially helpful in reducing such an uncertainty. Key Points: Precipitation isotopic time series at the northern Indian Ocean reveal strong seasonality, driven by monsoon thermodynamicsTwo well‐defined clusters are formed when δ18O is plotted against the Webster‐Yang circulation indexThe formation of clusters is indicative of isotopic response to moisture dynamics [ABSTRACT FROM AUTHOR]

We investigated interdecadal relationship changes between the Arctic oscillation (AO) and Aleutian low (AL) for the period 1980–2021 along with the linkage on surface air temperature (SAT) over Eastern Eurasia during the boreal winter (December–January–February). The AO–AL relationship has weakened since the mid‐1990s. To explore a possible explanation, we compared atmospheric fields for two contrasting periods, one when the AO–AL relationship is the strongest (1986–2006) and one when it is the weakest (2000–2020). In both periods, atmospheric circulation over the Arctic region for the two AO–AL in‐phase relationships (i.e., either +AO/+AL or –AO/−AL) evolves with negative AO‐like patterns, contributing to a weakening AO–AL relationship in concurrent with a cool SAT in Eastern Eurasia. We found that the correlation between AL and SAT over Eastern Eurasia decreases, while the AO and SAT correlation remains unchanged. This implies that low‐frequency atmospheric circulation changes such as those of the AO and AL should be considered when studying SAT changes in Eastern Eurasia. [ABSTRACT FROM AUTHOR]

In sharp contrast to rapid global warming, the surface air temperature (SAT) in Northeast Asia (NEA) has exhibited strong cooling in June since 2010. Our results show that this cooling is a possible response to declining Bering Sea sea ice concentrations (BSICs) in March. A physical process analysis shows that decreasing BSICs provoke a south‐eastward‐propagating Rossby wave train and induce a strong anomalous anticyclone over the midlatitude Northwest Atlantic, resulting in tripolar anomalous sea surface temperature (SST) pattern with warm anomalies in the midlatitude Northwest Atlantic. These persistent tripolar SST anomalies generate a new Rossby wave across Eurasia in June, which enhances the cyclonic atmosphere circulation anomaly over NEA and leads to the cold SAT anomalies observed in this area. Correspondingly, in response to the sharp decline in BSIC in March, the NEA has exhibited an unprecedented cooling trend in June over the most recent decade. Plain Language Summary: In contrast to rapid global warming, the surface air temperature (SAT) over Northeast Asia (NEA) in June has exhibited a strong cooling trend since 2010, which has had an important impact on local agriculture. Here we show that reduced Bering Sea sea ice concentrations (BSICs) in March promote cold SATs in the following June in NEA. Declining BSICs in March excite a Rossby wave train that propagates eastward, causing a strong anomalous anticyclone (cyclone) over the midlatitude North Atlantic (subpolar Atlantic) and a tripolar anomalous sea surface temperature (SST) pattern by modifying the surface heat flux. The tripolar SST pattern in the North Atlantic develops in April and May due to the downstream development of the wave train; it persists in June mainly due to the ocean memory effect and subsequently generates a new Rossby wave across Eurasia via eddy‐mediated processes, provoking a cyclonic atmospheric circulation anomaly over NEA and leading to the observed cold SAT anomalies. Accompanying the significant BSIC decline in March since 2010, the atmospheric regime and SST variations over the North Atlantic in association with the cold SAT anomalies observed in NEA in June have strengthened and thus induced strong cooling over NEA. Key Points: Unusual cooling has been observed in June over Northeast Asia (NEA) under the most recent global warmingThis June cooling in NEA is consistent with declining Bering Sea sea ice concentrations (BSICs) in MarchThe Rossby wave train and the sea surface temperature anomalies in Northwest Atlantic bridge the declining BSIC in March and the cooling over NEA in June [ABSTRACT FROM AUTHOR]

This study employs ERA5 reanalysis and CERES/EBAF Ed4.1 data to evaluate the dominant features of summer shortwave cloud radiative effect (SWCRE) variability and associated atmospheric circulation anomalies. Our findings suggest that the greatest variability in summer SWCRE occurs over the Barents, Kara, and Laptev seas and northern Eurasia. We also observe a close relationship between summertime SWCRE, particularly in northern Russia, and atmospheric circulation variability over East Asia. Significant positive SWCRE anomalies over northern Russia favor the generation of the Ural blocking, and dynamically trigger the emergence of positive Eurasian (EU) pattern, resulting in positive (negative) precipitation anomalies in northern (southeastern) China and persistent East Asian heatwaves between 20°N and 40°N. In contrast to previous work, which has focused mainly on local atmospheric responses to SWCRE, this study provides a broader perspective, thereby helping bridge summertime circulation between high latitudes and East Asia. Plain Language Summary: Clouds help balance Earth's radiation budget and thus play a vital role in global climate. During summer, clouds tend to produce shortwave cloud radiative effect (SWCRE) by reflecting incoming solar radiation and exert a net cooling cloud radiative effect (CRE). Our investigation reveals that the greatest variability in summer SWCRE occurs over the Barents, Kara, and Laptev seas and northern Eurasia. Moreover, summer SWCRE in northern Russia appears to play a role in surface temperature and precipitation over East Asia. For instance, significant positive SWCRE anomalies over northern Russia favor the generation of Ural blocking and can stimulate the emergence of positive Eurasian (EU) teleconnection, resulting in increased (decreased) rainfall in northern (southeastern) China and more frequent hot summers in East Asia between 20°N and 40°N. In contrast to previous work, our results are not limited to the local circulation response to SWCRE. Furthermore, our findings provide new evidence for a high‐latitude influence on East Asian weather and climate. Key Points: The greatest variability in summer shortwave cloud radiative effect (SWCRE) is observed over the Barents, Kara, and Laptev seas and northern EurasiaSummertime SWCRE over northern Russia potentially influences precipitation and air temperature over East AsiaPositive SWCRE anomalies in northern Russia favor the development of Ural blocking and dynamically trigger the emergence of positive Eurasian (EU) [ABSTRACT FROM AUTHOR]

Major low‐latitude volcanic eruptions cool Earth's climate, and can lead to a positive phase of the North Atlantic Oscillation (NAO) during winter. However, the question of the climate and circulation impact of Northern Hemisphere high‐latitude eruptions has received less attention. Here we show that, contrary to low‐latitude eruptions, the response to high‐latitude eruptions can be associated with negative NAO both winter and summer. We furthermore demonstrate that also the response to low‐latitude eruptions prevails during summer months, and corroborates previous findings of an extended impact on winter circulation lasting up to 5 years. Our analysis of novel climate field reconstructions supports this extended response, with the addition of showing a positive NAO during summer after low‐latitude eruptions. The differences in the effect of high‐ and low‐latitude eruptions on atmospheric circulation and regional temperature provide important insights for the understanding of past and future climate changes in response to volcanic forcing. Plain Language Summary: Large volcanic eruptions cool the climate as volcanic particles scatter and absorb part of the solar radiation before reaching the surface. In addition, tropical volcanic eruptions have been shown to strengthen the westerly winds across the North Atlantic for several years after the eruptions. Volcanic eruptions in the high northern latitudes have been generally less strong and less frequent over the last centuries than eruptions in the tropics. In our study we use reconstructions of seasonal temperature and atmospheric circulation to show that high‐latitude eruptions have an opposite effect on the circulation than tropical volcanic eruptions as they weaken the westerly winds. This gives large regional differences in the climate impact between high‐ and low‐latitude volcanic eruptions both for summer and winter. Key Points: Contrary to low‐latitude eruptions, high‐latitude eruptions are associated with a negative North Atlantic Oscillation (NAO)The NAO response to low‐latitude and high‐latitude eruptions is seen during both summer and winterConsistent results are shown for independent NAO reconstructions and results from a chemistry climate model simulating a Laki‐type eruption [ABSTRACT FROM AUTHOR]

Vegetation growth in northern high‐latitudes during springtime is strongly temperature limited, and thus anomalously warm springs are expected to result in an increased drawdown of carbon dioxide (CO2). However, a recent analysis of the relationship between spring temperature anomalies and atmospheric CO2 anomalies at Point Barrow, Alaska, suggests that the link between spring carbon uptake by northern ecosystems and temperature anomalies has been weakening over recent decades due to a diminishing control of temperature on plant productivity. Upon further analysis, covering the 1982–2015 period, we found no significant change in the relationship between spring vegetation productivity derived from remote sensing data and air temperature. We showed that a reduction in spatial coherence of temperature anomalies, alongside a significant sensitivity to atmospheric transport, is likely responsible for the apparent weakening. Our results, therefore, suggest that spring temperature remains as an important control of northern high‐latitude CO2 uptake. Plain Language Summary: The timing of the vegetation growing season is strongly linked to the quantity of carbon dioxide (CO2) absorbed each year, however, it is unclear how this will evolve. The spring zero crossing (SZC) of atmospheric CO2 is an indicator of how early the growing season starts. This is because the growing season is characterized by a significant photosynthetic drawdown of CO2 by vegetation. This quantity has historically been strongly controlled by temperature in northern high‐latitude ecosystems. A recent study analyzed surface CO2 data at a site in Alaska and suggested that this control has broken down. We show here, using satellite‐based vegetation data, that there has been no change in the control of temperature on vegetation productivity across the high northern latitudes. We then demonstrate that the temperature anomalies became more spatially variable across the northern high latitudes in recent decades and that variations in atmospheric circulation lead to different regions of influence over the Alaskan CO2 site. These influences play a key role in the year‐to‐year variability of the SZC and its relationship with temperature. Our study demonstrates that the control of temperature on the year‐to‐year variations in the timing and magnitude of spring CO2 uptake has remained strong. Key Points: No significant change in the overall temperature sensitivity of high northern latitude spring greenness in the past 40 yearsDecreased spatial coherence of spring temperature anomalies has increased the influence of atmospheric transport on surface CO2 dataTemperature sensitivity of spring carbon uptake remains strong when accounting for atmospheric transport [ABSTRACT FROM AUTHOR]

The article focuses on cities are urban heat islands that not only increase mortality, but also exacerbate climate change, but new roofing and pavement materials could help to cool things down. Topics include the large majority of cities in the world have an urban heat island, and the urban heat island is directly proportional to the size of the city.

Stable isotope ratios of precipitation trace mechanisms of hydroclimatic change in the modern and paleoclimate record. Patterns and drivers of isotopic change at multidecadal timescales have remained unclear, however, due to limitations in the observational record. Here, we use a 65‐year global data compilation to estimate solstial season δ18Op trends. Spatially organized regions of change suggest divergent controls, and we propose that changes in atmospheric water balance dominate trends in moisture‐limited areas, whereas changes in upwind source region conditions drive trends where atmospheric water flux is large relative to precipitation. Positive trends on windward coasts suggest the latter effect, whereas we attribute the dipole patterns in trends over North America and Europe and decreasing trends in southern Australia during boreal winter to the former. Simulations from the isotope‐enabled Community Atmospheric Model match predictions for water balance, implying that the model may underrepresent the effects of changing vapor source conditions. Plain Language Summary: Impacts of global climate change extend beyond rising temperatures and include shifts in rain and snowfall. The changes to precipitation occur both because warmer ocean and atmospheric temperatures cause increased evaporation and also because changing atmospheric circulation alters patterns of moisture transport. It's difficult to figure out what aspects of the water cycle have changed due to climate change by simply using measurements of rain and snow amounts. However, stable isotopes of oxygen in precipitation can reveal process changes. We compiled a global data set of precipitation oxygen isotope data for the past half century. We identified trends in the data set and found a complex response, characterized by regions of both positive and negative change. In some places this differs from the patterns expected if only the local balance between vapor and precipitation is considered. In the places where the patterns differ, our results suggest that the observed pattern must arise due to changes in evaporation location, conditions or condensation in the air mass prior to reaching the collection site. This occurs most often on the coasts, where precipitation comes from relatively unaltered ocean vapor. Within some continental interiors, changes to vapor flux become more important in governing the oxygen isotope response. However, oxygen isotope trends simulated by an atmospheric circulation model suggest that most of the model's response comes from changes to water balance, including water vapor flux, meaning that the model may miss some water cycle changes. Key Points: Spatial analysis of a multidecadal data set shows strong regional increases and decreases in precipitation δ18Op valuesThe sensitivity of δ18O to local water balance is not ubiquitous and may be greater in vapor limited regionsAtmospheric circulation model δ18O indicates mostly positive trends that match local water balance estimates [ABSTRACT FROM AUTHOR]

The Amundsen Sea Low (ASL) is a distinctive feature of the Southern Hemisphere high latitude atmospheric circulation, regulating regional Antarctic climate, meridional heat transport, ocean circulation, and sea‐ice in the Amundsen‐Bellingshausen Seas. Most previous research on the ASL has focused on its variability with only a few studies attempting to understand why the climatological ASL exists. These studies have proposed different hypotheses to explain the presence of the ASL, however, a clear understanding of the mechanisms responsible for the generation of the ASL remains uncertain. Here we use an atmospheric general circulation model to show that the ASL is a consequence of the interaction between Antarctic topography and the westerly wind jet, with negligible influence from low‐latitude teleconnections. A nonrotating fluid flow simulation further suggests that the ASL can be explained by flow separation resulting from the interaction of westerly winds with the topography of Antarctica. Plain Language Summary: The Amundsen Sea Low (ASL) is a permanent atmospheric low‐pressure center located offshore of western Antarctica. The ASL affects air and ocean temperature, precipitation, sea‐ice, and ice shelves in the West Antarctica region. Various hypotheses have been proposed to explain the mechanisms responsible for the existence of the ASL. However, a clear understanding of the generation mechanisms is not yet available. In this study, we use a state‐of‐the‐art climate model to test the previously suggested mechanisms and show that the ASL is generated by an interaction between the westerly winds with Antarctic orography, with negligible influence from low latitudes. Key Points: The Amundsen Sea Low (ASL) owes its existence to an interaction between the westerly wind jet and Antarctic orographyMountains in the West Antarctic region trap the low‐pressure system in the Amundsen‐Bellingshausen Seas regionTeleconnections from low latitudes have little to no role in generating the climatological ASL [ABSTRACT FROM AUTHOR]

Relationship between Antarctic sea ice and the Southern Annular Mode and tropical climate variability has been widely studied. Focusing on the midlatitude oceanic frontal zones, this study identifies a fingerprint of South Atlantic (SA) variability in the Antarctic summer sea ice. The interaction between spring SA sea surface temperature (SST) and summer storm‐track activity modulates the hemispheric atmospheric circulation, which in turn alters Antarctic summer sea ice concentration (SIC) through thermal and wind‐driven forcing. Specifically, warm SST anomalies in the western SA frontal zone, corresponding to the strengthened SST front, cause the increased SIC in the eastern Ross Sea via anomalous cold air advection and offshore drift of sea ice. The opposite effect (i.e., warm air advection and onshore ice drift) results in the decreased SIC in the northwestern Weddell Sea. The findings also imply a potential impact of the midlatitude SA variability to the South Pole air temperature variability. Plain Language Summary: Sea ice plays an important role in the global climate. In the context of global warming, small temperature changes can be amplified in polar regions by the positive feedback of sea ice. Many studies have revealed the relationship between Antarctic sea ice and major modes of climate variability, such as the Southern Annular Mode and El Niño/Southern Oscillation. A recent study found that warming reaching the South Pole is attributed by the tropical Pacific Ocean variability. However, little attention has been paid to the influence of midlatitude oceanic frontal zones. Here, we show that the interaction between the South Atlantic (SA) sea surface temperature (SST) in spring and storm‐track activity in summer induces anomalous hemispheric‐scale atmospheric circulation, which further modulates the summertime sea ice concentration (SIC) via both thermal and mechanical processes. Particularly, warm SST anomalies in the western SA frontal zone drive the anomalous high and low sea level pressure (SLP) over the Weddell and Ross Seas, resulting in the decreased and increased local SIC, respectively. Such anomalous high SLP over the Weddell Sea also induces anomalous warm air advection penetrating toward the South Pole. This reveals a potential connection between the midlatitude SA SST front and the South Pole air temperature. Key Points: South Atlantic thermal front variations in spring modulate extratropical storm tracks and atmospheric circulation in summerThe planetary‐scale atmospheric response alters Antarctic summer sea ice concentration through thermal advection and wind‐driven driftRemarkable changes of sea ice concentration occur in the Weddell and Ross Seas [ABSTRACT FROM AUTHOR]

The modern history of North Atlantic sea surface temperature shows variability coinciding with changes in air temperature and rainfall over the Northern Hemisphere. There is a debate about this variability and, in particular, whether it is internal to the ocean‐atmosphere system or is forced by external factors (natural and anthropogenic). Here we present a temperature record, obtained using the Sr/Ca ratio measured in a skeleton of a sclerosponge, that shows agreement with the instrumental record over the past 150 years as well as multidecadal temperature variability over the last 600 years. Comparison with climate simulations of the last millennium shows that large cooling events recorded, in the sclerosponge, are consistent with natural (primarily volcanic activity) and anthropogenic forcings. There are, however, multidecadal periods not connected to current estimates of external forcing over the last millennium allowing for alternative explanations, such as internally driven changes in ocean and atmospheric circulation. Key Points: A high‐resolution 600‐year proxy temperature record from the subtropical Atlantic shows multidecadal temperature variabilityIn both models and observations, surface temperatures at this site are reflective of basin‐wide multidecadal variabilityComparison with Last Millennium climate model simulations shows the fingerprint of volcanically induced cooling [ABSTRACT FROM AUTHOR]

The effect of future Arctic amplification (AA) on the extratropical atmospheric circulation remains unclear in modeling studies. Using a collection of coordinated atmospheric and coupled global climate model perturbation experiments, we find an emergent relationship between the high‐latitude 1,000–500 hPa thickness response and an enhancement of the Siberian High in winter. This wave number‐1‐like sea level pressure anomaly pattern is linked to an equatorward shift of the eddy‐driven jet and a dynamical cooling response in eastern Asia. Additional simulations, where AA is imposed directly into the model domain by nudging, demonstrate how the sea ice forcing is insufficient by itself to capture the vertical extent of the warming and by extension the amplitude of the response in the Siberian High. This study demonstrates the importance of the vertical extent of the tropospheric warming over the polar cap in revealing the "warm Arctic, cold Siberia" anomaly pattern in future projections. Plain Language Summary: Surface air temperature averaged within the Arctic Circle is rising more than twice as fast as the rate of the globally averaged temperature. This is known as "Arctic amplification" and is the result of numerous feedbacks in the Earth system. This warming is not restricted to the surface, as the temperature is also increasing at least to the middle troposphere in the Arctic. The effects of this warming on weather patterns in the midlatitudes remain uncertain in observations and model simulations. In this study, we conduct a series of climate model experiments to better understand how the vertical extent of warming in the Arctic atmosphere connects to a strengthening of the Siberian High in boreal winter. This high‐pressure response leads to colder air temperatures in eastern Asia through advection. We find that climate models prescribed with only Arctic sea ice loss do not fully capture the vertical extent of warming in the atmosphere, and thus underestimate the linkage between projected Arctic amplification and Eurasian climate. Key Points: Climate models forced only by sea ice anomalies do not capture the vertical extent of Arctic warming during winterIncrease in thickness of the winter 1,000–500 hPa layer is linked to a strengthening of the Siberian High and cold anomalies in eastern AsiaRole of the stratosphere is unclear due to large internal variability at future global warming of 2° C above preindustrial levels [ABSTRACT FROM AUTHOR]

Western Ireland lies at the Atlantic fringe of Europe and is thus well positioned to record changes in climate linked to North Atlantic circulation. Lake‐sediment oxygen and carbon isotopes are sensitive to changes in the atmosphere, the lake catchment and the lake itself, and thus are valuable proxies for long‐term environmental change. Here we present Younger Dryas and Holocene stable‐isotope records from endogenic and biogenic carbonates from An Loch Mór, a karstic lake on Inis Oírr, Aran Islands, western Ireland. The bulk carbonate and ostracod‐derived stable‐isotope records are a complex response to regional climatic and local factors. Low oxygen‐isotope and high carbon‐isotope values in the Younger Dryas most likely reflect low air temperatures and dominance of bedrock‐derived carbon from the poorly vegetated catchment, but input of detrital carbonate cannot be ruled out. Holocene variations in oxygen‐isotope values are likely a response mainly to changes in the isotopic composition of rainfall, evaporative enrichment of lake water and, during the past millennium, incursion of seawater as a result of regional sea‐level rise. A reduction in carbon‐isotope values at the Younger Dryas/Early Holocene transition reflects increased input of soil‐derived carbon to the lake, modified by changes in aquatic productivity. During the past millennium a shift to less negative values is attributed mainly to increasing saline influence. Broad similarities with stable‐isotope records from other sites in western Ireland suggest regional climatic controls dominated by North Atlantic atmospheric circulation patterns, although abrupt events, that were possibly unique to An Loch Mór, point to local factors also being important. [ABSTRACT FROM AUTHOR]

We show that in the tropics, tropical atmospheric dynamics force the subcloud moist static energy (MSE) over land and ocean to be very similar in, and only in, regions of deep convection. Using observed rainfall as a proxy for convection and reanalysis data to calculate MSE, we show that subcloud MSE in the nonconvective regions may differ substantially between land and ocean but is uniform across latitudes in convective regions even on a daily timescale. This result holds also in CMIP5 model simulations of past cold and future warm climates. Furthermore, the distribution of rainfall amount in subcloud MSE is very similar over land and ocean with the peak at 343 J/g and a half width at half maximum of 3 J/g. Our results demonstrate that the horizontally uniform free tropospheric temperature forces the highest subcloud MSE values to be similar over land and ocean. Plain Language Summary: An extremely idealized picture of the tropical atmospheric dynamics is that deep convection sets a horizontally uniform free tropospheric temperature profile. Here, we show that despite the idealization, this simple picture is very useful in explaining the observations; convection occurs at very similar, spatially uniform subcloud MSE regardless of over land or ocean. Key Points: The utility of quasi‐equilibrium and weak temperature gradient theories (QE‐WTG) can be demonstrated by a rainfall‐weighting methodObserved convection occurs at very similar subcloud moist static energy across all latitudes in the inner tropics as a result of QE‐WTGThe highest moist static energy values are tightly coupled over land and ocean, while the lower values are free to differ [ABSTRACT FROM AUTHOR]

The tropical tropopause layer (TTL) temperature balance is of considerable interest for its control over the amount of water entering the stratosphere. The upwelling branch of the Brewer‐Dobson circulation (BDC) directly affects these temperatures through adiabatic cooling. BDC upwelling also indirectly affects TTL temperatures through the influence of ozone transport on radiative heating. We investigate this latter feedback using a single‐column radiative‐convective equilibrium model coupled with a model of simplified stratospheric ozone chemistry and vertical transport. We find that BDC ozone transport is of first‐order importance for TTL temperatures. Additionally, we estimate the effect of ozone transport on cold point tropopause temperature responses to changes in upwelling. We find that the feedback is responsible for approximately 20% of the response to perturbations on time scales longer than about half a year but that this contribution can be neglected for time scales shorter than about a week. Key Points: Interactive ozone chemistry is applied in a single‐column radiative‐convective equilibrium model to investigate transport processesOzone transport by Brewer‐Dobson circulation upwelling is of first‐order importance for temperature structure of tropical tropopause layerTransport‐radiation feedback: increased (decreased) upwelling reduces (increases) ozone −20% of the total cooling (warming) from upwelling [ABSTRACT FROM AUTHOR]

We assess the reliability of an indirect method of inferring the atmospheric response to projected Arctic sea ice loss from CMIP5 simulations, by comparing the response inferred from the indirect method to that explicitly simulated in sea ice perturbation experiments. We find that the indirect approach works well in winter, but has limited utility in the other seasons. We then apply a modified version of the indirect method to 11 CMIP5 models to reveal the robust and non‐robust aspects of the wintertime atmospheric response to projected Arctic sea ice loss. Despite limitations of the indirect method, we identify a robust enhancement of the Siberian High, weakening of the Icelandic Low, weakening of the westerly wind on the poleward flank of the eddy‐driven jet, strengthening of the subtropical jet, and weakening of the stratospheric polar vortex. The surface air temperature response to projected Arctic sea ice loss over mid‐latitude continents is non‐robust across the models. Plain Language Summary: The continued melt of Arctic sea ice will likely affect weather and climate in places far from the Arctic. To better understand the far‐flung implications of sea ice loss, scientists can perform bespoke climate model experiments in which sea ice is reduced, but all other climate drivers are fixed. In our paper, we test the reliability of an indirect method to infer the atmospheric response to sea ice loss, which makes use of a large set of climate model experiments originally performed for other purposes. We find that the indirect approach works well in winter, but not so well in other seasons. We then apply the indirect method to 11 climate models to reveal the robust and non‐robust aspects of the wintertime atmospheric response to projected Arctic sea ice loss—the largest such comparison to date. We found that the models agreed on quite a few aspects of the response, including warming over the Arctic, regional surface pressure changes over Siberia and Iceland, and a slowing of the mid‐latitude jet stream. On the other hand, the models disagreed on whether surface air temperatures over Europe, North America, and East Asia will warm or cool in response to future Arctic sea ice loss. Key Points: Wintertime atmospheric response to projected Arctic sea‐ice loss can be well‐estimated indirectly from CMIP5 simulationsRobust changes across models including stronger Siberian High, weaker Icelandic Low, slower eddy‐driven jet and faster subtropical jetNon‐robust changes in surface air temperature over mid‐latitude continents and sea level pressure over the North Pacific [ABSTRACT FROM AUTHOR]

This study explores the "tug of war" between the effects of Arctic amplification ("AA") and upper‐troposphere tropical warming ("UTW") on the response of the future North Atlantic atmospheric circulation. The late 21st century AA and UTW temperature anomalies are imposed in a high‐top atmospheric model by nudging the temperature. Two sets of experiments are performed, with and without feedback of the polar stratosphere to highlight its role in the response to UTW, AA, and both combined. With interactive polar stratosphere, UTW forces an equatorward shift of the eddy‐driven jet that reinforces the response to AA. However, when the polar stratosphere feedback is suppressed, the response to UTW is opposite and reflects the previously identified tug of war between the effects of UTW and AA in midlatitudes. This study highlights that the polar stratosphere is a key component for future changes in the North Atlantic atmospheric circulation and that it must be accurately represented in climate change scenarios. Plain Language Summary: The jet streams are bands of strong westerly winds that drive weather patterns in midlatitudes. With climate change, they are expected to migrate poleward in association with an expansion of the tropical belt and amplified upper‐level tropical warming (UTW). However, recent studies have pointed out that the fast warming in the Arctic (or Arctic Amplification, AA) may counteract this effect in winter. In this study we explore the respective and combined influence of UTW and AA by prescribing temperature anomalies in an atmospheric model that resolves the stratosphere. The focus is on the North Atlantic sector, where previous work had found a pronounced tug of war effect. In fall, in the absence of any significant response in the polar stratosphere, we retrieve well‐established results, that is, a poleward shift of the midlatitude jet in response to UTW and an equatorward shift in response to AA. However, in winter UTW induces a robust weakening of the stratospheric polar vortex that shifts the jet equatorward and reinforces the effect of AA. Our study highlights that the polar stratosphere may significantly modulate the tropics‐Arctic tug of war in the future, advocating for an accurate representation of stratospheric processes in climate models. Key Points: The late 21st century temperature anomalies in the upper‐level tropics and the Arctic are imposed in a high‐top atmospheric model using nudgingIn winter, warmer tropics induce a weakening of the polar vortex that reinforces the response to Arctic amplification (no tug of war)The polar stratosphere has the potential to shape the tug of war over the North Atlantic in future climate scenarios [ABSTRACT FROM AUTHOR]

In January of 2016, the Ross Sea sector of the West Antarctica ice sheet experienced a 3‐week‐long melting episode. Here we quantify the association of the large‐extent and long‐lasting melting event with the enhancement of the downward longwave (LW) radiative fluxes at surface due to water vapor, cloud, and atmospheric dynamic feedbacks using the ERA‐Interim data set. The abnormally long‐lasting temporal surges of atmospheric moisture, warm air, and low‐level clouds increase the downward LW radiative energy flux at the surface during the massive ice‐melting period. The concurrent timing and spatial overlap between poleward wind anomalies and positive downward LW radiative surface energy flux anomalies over West Antarctica due to warmer air temperature and increases in atmospheric water vapor and cloud coverage provide direct evidence that warm and moist air advection from lower latitudes to West Antarctica causes the rapid long‐lasting warming and vast ice mass loss in January of 2016. Plain Language Summary: The melting of the fringing ice shelves from below by warm ocean water is thought to be the leading factor responsible for the retreat of the West Antarctica ice sheet in the past several decades. The analysis presented here shows that atmospheric processes through the long‐range transport of heat and moisture from lower latitudes to West Antarctica by the atmospheric circulation can lead to temporal surges of downward longwave radiative fluxes at the surface, substantially warming the surface. Atmospheric poleward heat transport plays a key role for individual ice sheet melting events in West Antarctica, such as the ice sheet melting episode over Ross Sea sector of the West Antarctica Ice Sheet in January 2016. Key Points: Pronounced enhancement of downward longwave radiative energy fluxes results in massive melting of the West Antarctica Ice Sheet in January 2016Contributions to the enhancement of downward longwave radiative fluxes are quantified via a linearized radiative transfer modelAtmospheric poleward heat transport plays a key role for the enhancement of downward longwave radiative surface energy fluxes [ABSTRACT FROM AUTHOR]

Ground‐breaking measurements from the ocean observatories initiative Irminger Sea surface mooring (60°N, 39°30′W) are presented that provide the first in situ characterization of multiwinter surface heat exchange at a high latitude North Atlantic site. They reveal strong variability (December 2014 net heat loss nearly 50% greater than December 2015) due primarily to variations in frequency of intense short timescale (1–3 days) forcing. Combining the observations with the new high resolution European Centre for Medium Range Weather Forecasts Reanalysis 5 (ERA5) atmospheric reanalysis, the main source of multiwinter variability is shown to be changes in the frequency of Greenland tip jets (present on 15 days in December 2014 and 3 days in December 2015) that can result in hourly mean heat loss exceeding 800 W/m2. Furthermore, a new picture for atmospheric mode influence on Irminger Sea heat loss is developed whereby strongly positive North Atlantic Oscillation conditions favor increased losses only when not outweighed by the East Atlantic Pattern. Plain Language Summary: The ocean loses heat to the atmosphere in the far northern Atlantic. This is important as heat loss influences how much deep water is formed and the strength of the Atlantic circulation. However, the amount of heat lost is poorly known because measurements are difficult to obtain in the icy, high wind conditions of the subpolar seas. New measurements from a state‐of‐the‐art mooring in the Irminger Sea east of Greenland are presented here. They are the first multiwinter measurements obtained at such high latitudes and reveal strong variability in ocean heat loss. This variability is due to changes between winters in the number of intense heat loss events. The events are caused by the mountainous Greenland terrain which focuses winds into narrow, very strong jets over the ocean. We develop a new picture which explains how changing atmospheric circulation influences the number of events and hence the ocean heat loss. Key Points: Irminger Sea surface flux buoy analysis provides first multiwinter observations of high‐latitude North Atlantic air‐sea heat exchangeObserved net heat loss varies by nearly 50% between successive years due primarily to variations in frequency of Greenland tip jetsPositive North Atlantic Oscillation favors increased Irminger Sea heat loss only when not dominated by stronger East Atlantic Pattern [ABSTRACT FROM AUTHOR]

An iterative dynamic initialization method is presented to produce balanced initial conditions for whole atmospheric global modeling. In this method, a global hydrostatic numerical model is iteratively nudged toward ground‐to‐space wind and temperature profiles at specific date and time. Ground‐to‐space atmospheric profiles are obtained by fitting spline curves to reanalyses below the lower mesosphere and empirical model results in the upper atmosphere. An optimal nudging coefficient is determined by examining if reasonable structure of mesospheric gravity wave (GW) momentum forcing and residual mean meridional circulations can be obtained from balanced initial conditions. Estimated mesospheric GW momentum forcing is found to exhibit a distinctive structure with larger (smaller) values in the lower and upper mesosphere (in the midmesosphere), when compared with parameterized climatological forcing. The iterative dynamic initialization allows for dynamical balance among the model's prognostic variables and reduces excitation of spurious GWs and noises at initial time. However, theoretical imbalances, measured by the ellipticity of the nonlinear balance equation, are not completely eliminated in balanced flows, and they are found in narrow tropospheric frontal regions and over localized areas associated with the large‐scale instability in the midlatitude middle atmosphere. These imbalances are discussed in the context of their potential relation to generation of planetary‐scale and inertia GWs around the middle atmospheric and tropospheric jets. Plain Language Summary: This study reports a method of initializing a whole‐atmosphere global model that covers from the ground to the lower thermosphere. Through this initialization, a whole atmospheric state balanced with the model dynamics is effectively generated. Free coefficient required for the initialization is chosen by estimating gravity wave momentum forcing and meridional mass circulations in the mesosphere and lower thermosphere. In the generated balanced flow, theoretical imbalances, measured by the ellipticity of the nonlinear balance equation, are found. These imbalances can be related to the spontaneous generation of gravity waves and balanced and unbalanced large‐scale instabilities in the middle atmosphere. The large‐scale instabilities can be attributed to generation of the planetary‐scale and inertia gravity waves in the middle atmosphere. Key Points: Iterative dynamic initialization for whole atmospheric global modeling is presentedGravity wave momentum forcing and mass circulations are estimated to constrain nudging coefficientDeviations from nonlinear balance may occur in association with large‐scale instabilities [ABSTRACT FROM AUTHOR]

In energetic equilibrium, the atmosphere's net radiative divergence ( [ABSTRACT FROM AUTHOR]

In energetic equilibrium, the atmosphere's net radiative divergence ( [ABSTRACT FROM AUTHOR]

The trends and interannual variations of summer sea ice extents ( SIEs) in both Hudson Bay and the Arctic Ocean are investigated in association with variations in atmospheric circulation and air temperature. The summer SIE variabilities of both Hudson Bay and the Arctic Ocean are well correlated and have a strong negative trend. The negative SIE trends are associated with a summer atmospheric circulation pattern that is characterized by positive anomalies over the Arctic Ocean and negative anomalies over mid-latitudes. The trends are also connected with warm air temperatures over mid- and high-latitudes. The atmospheric circulation and air temperature regressed on the summer SIEs in both Hudson Bay and the Arctic Ocean are quite similar to these trend patterns. On the other hand, the year-to-year variations of the detrended summer SIEs in both Hudson Bay and the Arctic Ocean are not correlated. The summer atmospheric circulation and air temperature over the Arctic Ocean have contributed to the detrended summer Arctic Ocean SIE. In contrast, the detrended summer Hudson Bay SIE is controlled by atmospheric circulation and air temperature in the previous spring. [ABSTRACT FROM AUTHOR]

Idealized explicit convection simulations of the Met Office Unified Model exhibit spontaneous self-aggregation in radiative-convective equilibrium, as seen in other models in previous studies. This self-aggregation is linked to feedbacks between radiation, surface fluxes, and convection, and the organization is intimately related to the evolution of the column water vapor field. Analysis of the budget of the spatial variance of column-integrated frozen moist static energy (MSE), following Wing and Emanuel (2014), reveals that the direct radiative feedback (including significant cloud longwave effects) is dominant in both the initial development of self-aggregation and the maintenance of an aggregated state. A low-level circulation at intermediate stages of aggregation does appear to transport MSE from drier to moister regions, but this circulation is mostly balanced by other advective effects of opposite sign and is forced by horizontal anomalies of convective heating (not radiation). Sensitivity studies with either fixed prescribed radiative cooling, fixed prescribed surface fluxes, or both do not show full self-aggregation from homogeneous initial conditions, though fixed surface fluxes do not disaggregate an initialized aggregated state. A sensitivity study in which rain evaporation is turned off shows more rapid self-aggregation, while a run with this change plus fixed radiative cooling still shows strong self-aggregation, supporting a 'moisture-memory' effect found in Muller and Bony (2015). Interestingly, self-aggregation occurs even in simulations with sea surface temperatures (SSTs) of 295 and 290 K, with direct radiative feedbacks dominating the budget of MSE variance, in contrast to results in some previous studies. [ABSTRACT FROM AUTHOR]

The diurnal hydrologie cycle, a sequence of évapotranspiration, boundary layer growth, moist convection, and precipitation, is described in a thermodynamic framework, assuming an atmosphere composed solely of water. This idealized cycle is shown to be equivalent to an abbreviated version of the classical Rankine cycle where not all the water vapor is condensed. Energy and entropy fluxes of the processes involved in the cycle are quantified using the reversible approximation as a function of the quality of the liquid-vapor mixture (the ratio of the residual background vapor and the total mass of water) and the different temperatures at which evaporation and condensation take place. The proposed framework allows quantitative estimates of the net work (which is used by the cycle to drive the atmospheric circulation and dissipated by various frictional forces and nonidealities) as well as of the thermodynamic efficiency of the cycle. Possible extensions of the idealized framework relating to the role of dry air and the inclusion of various irreversible processes are also discussed. [ABSTRACT FROM AUTHOR]

Southern California's Mediterranean‐type hydroclimate is highly variable on interannual time scales due to teleconnected climate forcings such as the El Niño–Southern Oscillation (ENSO). Here we present subannually resolved scanning X‐ray fluorescence Ti counts from deep‐sea cores in Santa Barbara Basin, California, recording 2,000 years of hydroclimate variability. The reconstructed Southern California precipitation record contains interannual variability in the 2‐ to 7‐year band that could be driven by changes in tropical Pacific ENSO variability and/or the strength of the ENSO teleconnection modulated by extratropical pressure systems. Observed interannual precipitation variance increased and was associated with longer periodicities (5–7 years) when the Intertropical Convergence Zone migrated southward (1370–1540 CE) and the Aleutian Low strengthened creating a robust ENSO teleconnection. Weak interannual precipitation variance with shorter periodicity (2–3 years) was observed when the Intertropical Convergence Zone shifted northward (700–900 CE) and/or the Aleutian Low was weak (1540–1680 CE). Plain Language Summary: El Niños occur when the rising branch of atmospheric circulation in the tropical Pacific shifts eastward, driving changes in air temperature and rainfall around the globe. Rainfall in Southern California often increases during El Niño events causing rivers to carry extra sediment to the ocean. We reconstructed Southern California rainfall for every year of the last 2,000 years using the elemental signature of river sediment deposited in Santa Barbara Basin. We found that after ~1350 CE, when the Aleutian Low was strong, interannual rainfall in Southern California varied more and with longer cycles (5 to 7 years). During this time, the region of rising air at the equator was further south and storms over the North Pacific Ocean were stronger and occurred further east. Both of these changes in atmospheric circulation increased the Southern California rainfall response to El Niño events in the tropical Pacific Ocean. Key Points: The interannual (2‐ to 7‐year band) precipitation in Southern California is closely related to ENSO variance originating from the tropical PacificExtratropical pressure systems modulate the interannual precipitation changes in Southern California by influencing the ENSO teleconnectionThe magnitude and frequency of interannual precipitation variance in Southern California changes throughout the past 2,000 years [ABSTRACT FROM AUTHOR]

A coordinated set of large ensemble atmosphere‐only simulations is used to investigate the impacts of observed Arctic sea ice‐driven variability (SIDV) on the atmospheric circulation during 1979–2014. The experimental protocol permits separating Arctic SIDV from internal variability and variability driven by other forcings including sea surface temperature and greenhouse gases. The geographic pattern of SIDV is consistent across seven participating models, but its magnitude strongly depends on ensemble size. Based on 130 members, winter SIDV is ~0.18 hPa2 for Arctic‐averaged sea level pressure (~1.5% of the total variance), and ~0.35 K2 for surface air temperature (~21%) at interannual and longer timescales. The results suggest that more than 100 (40) members are needed to separate Arctic SIDV from other components for dynamical (thermodynamical) variables, and insufficient ensemble size always leads to overestimation of SIDV. Nevertheless, SIDV is 0.75–1.5 times as large as the variability driven by other forcings over northern Eurasia and Arctic. Plain Language Summary: Changing Arctic sea ice conditions since the late 1970s have exerted profound impacts on environment and ecosystem at the high latitudes and have been suggested to affect midlatitude weather and climate, although this topic has been controversial. In order to improve our understanding on how Arctic sea ice changes influence local and remote weather and climate, a coordinated set of experiments has been performed using various state‐of‐the‐art atmosphere‐only models to study the linkages between the Arctic climate change and lower latitudes. This study uses seven models following a common experimental protocol to investigate the atmospheric circulation changes forced by Arctic sea ice variability. The protocol allows the Arctic sea ice‐driven variability (SIDV) to be singled out. In boreal winter, the Arctic SIDV is ~0.18 hPa2 and accounts for only ~1.5% of the total variance for sea level pressure, while it is ~0.35 K2 and accounts for ~21% for surface air temperature. The results also suggest that using insufficient ensembles always leads to an overestimation of SIDV, and more than 100 and 40 ensemble members are needed for sea level pressure and surface air temperature within the Arctic, respectively, to separate the SIDV from the variability due to other factors, primarily the atmospheric internal variability. Key Points: Arctic sea ice variations drive ~1.5% of boreal winter Arctic SLP variance during 1979–2014 in coordinated 130‐member AGCM experimentsAn accurate quantification of the impacts of Arctic sea ice‐driven SLP variability requires an ensemble size greater than 100 membersInsufficient ensemble size always leads to an overestimation of the Arctic sea ice‐driven variance within and outside Arctic [ABSTRACT FROM AUTHOR]

The article discusses a study conducted which explores the effects of hydroclimatic changes at the North America Monsoon System (NAMS) region. It reveals that warming occurred during dry season while minimum temperatures happened during wet season. Moreover, remote-sensing data and regional modeling experiments are needed to determine the changes in atmospheric circulation.