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Tropical Cyclones (TCs) inflict substantial coastal damages, making it pertinent to understand changing storm characteristics in the important nearshore region. Past work examined several aspects of TCs relevant for impacts in coastal regions. However, few studies explored nearshore storm intensification and its response to climate change at the global scale. Here, we address this using a suite of observations and numerical model simulations. Over the historical period 1979–2020, observations reveal a global mean TC intensification rate increase of about 3 kt per 24‐hr in regions close to the coast. Analysis of the observed large‐scale environment shows that stronger decreases in vertical wind shear and larger increases in relative humidity relative to the open oceans are responsible. Further, high‐resolution climate model simulations suggest that nearshore TC intensification will continue to rise under global warming. Idealized numerical experiments with an intermediate complexity model reveal that decreasing shear near coastlines, driven by amplified warming in the upper troposphere and changes in heating patterns, is the major pathway for these projected increases in nearshore TC intensification. Plain Language Summary: Tropical cyclones (TCs) that intensify close to the coast pose a major socio‐economic threat and are a substantial challenge from an operational standpoint. Therefore understanding historical trends in nearshore storm intensification and how they may change in future is of considerable significance. Despite this, few studies examined this key aspect of TCs at the global scale. Here we show, using an analysis of observations and atmospheric reanalyses, that the mean TC intensification rate has increased significantly over the period 1979–2020 primarily aided by increases in relative humidity and decreases in vertical wind shear. Further, high‐resolution climate models, which explicitly resolve TCs, suggest that nearshore TC intensification will continue to increase in future. These increases in coastal TC intensification rates can mainly be attributed to stronger projected decreases in vertical wind shear. To better understand wind shear projections, a suite of idealized numerical experiments with an intermediate complexity model were conducted. The experiments indicate that enhanced warming in the upper‐troposphere and changing heating patterns are likely responsible. Key Points: Tropical cyclone (TC) intensification rates have increased in near coastal regions over the 42‐year period 1979‐2020Increases in relative humidity along with decreases in vertical wind shear are responsibleClimate models project a continued increase in nearshore TC intensification rates with decreasing wind shear playing a crucial role [ABSTRACT FROM AUTHOR]

Anthropogenic climate change has already affected drought severity and risk across many regions, and climate models project additional increases in drought risk with future warming. Historically, droughts are typically caused by periods of below‐normal precipitation and terminated by average or above‐normal precipitation. In many regions, however, soil moisture is projected to decrease primarily through warming‐driven increases in evaporative demand, potentially affecting the ability of negative precipitation anomalies to cause drought and positive precipitation anomalies to terminate drought. Here, we use climate model simulations from Phase Six of the Coupled Model Intercomparison Project (CMIP6) to investigate how different levels of warming (1, 2, and 3°C) affect the influence of precipitation on soil moisture drought in the Mediterranean and Western North America regions. We demonstrate that the same monthly precipitation deficits (25th percentile relative to a preindustrial baseline) at a global warming level of 2°C increase the probability of both surface and rootzone soil moisture drought by 29% in the Mediterranean and 32% and 6% in Western North America compared to the preindustrial baseline. Furthermore, the probability of a dry (25th percentile relative to a preindustrial baseline) surface soil moisture month given a high (75th percentile relative to a preindustrial baseline) precipitation month is 6 (Mediterranean) and 3 (Western North America) times more likely in a 2°C world compared to the preindustrial baseline. For these regions, warming will likely increase the risk of soil moisture drought during low precipitation periods while simultaneously reducing the efficacy of high precipitation periods to terminate droughts. Plain Language Summary: Regional warming associated with climate change is already making droughts worse in many places. This trend is expected to continue as global temperatures increase in response to continued emissions of greenhouse gases. With increasing temperatures, soils are projected to lose water to the atmosphere as it draws more and more moisture from the land surface, increasing drought risk. At the same time, climate change is also projected to shift precipitation patterns. This means it is important to understand how changes in rainfall will cause and end droughts in the future. Using state‐of‐the‐art climate models, we investigate how future warming will impact the influence of precipitation on soil moisture in the Mediterranean and Western North America. Our results suggest that current levels of monthly precipitation will be insufficient to alleviate drought conditions in a warmer world. Key Points: Warming causes declines in soil moisture in the Mediterranean and Western North AmericaThe probability of a moderate soil moisture drought increases during both low and high precipitation anomalies, even at 1°C of warmingThe ability of large positive precipitation anomalies to terminate soil moisture droughts will be substantially reduced in a warmer world [ABSTRACT FROM AUTHOR]

We make use of the Community Atmosphere Model version 5 Green's function q‐flux perturbation experiments to explore the most effective remote forcing in driving U.S.‐wide summer heat extremes. We find that positive q‐flux forcing over the western North Pacific Ocean is the most effective in causing an increased heat extreme frequency. This works by driving increased sea surface temperature and precipitation over western North Pacific and an eastward propagating Rossby wave train with an anomalous ridge over the contiguous U.S. In comparison, negative q‐flux forcing over the eastern tropical Pacific and its resulting surface cooling also leads to an increased heat extreme frequency but is less effective. Furthermore, guided by the Green's function results, we separate the role of western North Pacific warming and eastern tropical Pacific cooling in U.S. heat extremes in prescribed sea surface temperature experiments and ERA5 reanalysis data and find overall consistent conclusions. Plain Language Summary: We study summertime U.S. heat extremes and their precursors in ocean surface forcing. Using a novel approach, we find that western North Pacific warming and wetting is the most effective way to cause an increased frequency of U.S.‐wide heat extremes. In comparison, eastern tropical Pacific cooling can also lead to an increased heat extreme frequency but is less effective. Our work advances understanding of U.S. heat extremes with important implications for their predictability. Key Points: Green's function experiments separate the role of western North Pacific warming and tropical Pacific cooling in causing U.S. heat extremesGreen's function experiments highlight the dominant role of western North Pacific warming in causing U.S.‐wide heat extremesComposite analysis based on prescribed sea surface temperature experiments and ERA5 supports the conclusion [ABSTRACT FROM AUTHOR]

Ozone in the troposphere is a pollutant and greenhouse gas, and it is crucial to better understand its transport from the ozone‐rich stratosphere. Tropopause folding, wherein stratospheric air intrudes downward into the troposphere, enables stratosphere‐to‐troposphere ozone transport (STT). However, systematic analysis of the relationship between folding and tropospheric ozone, using data that can both capture folding's spatial scales and accurately represent tropospheric chemistry, is limited. Here, we compare folding in high‐resolution reanalysis ERA5 (0.25° horizontal, <21 hPa vertical) and low‐resolution chemical reanalysis CAMSRA (0.75°, <40 hPa), against CAMSRA ozone, over 1 year. Folding becomes dramatically more frequent at high resolution, with vertical resolution overwhelmingly responsible. Deeper, more filamentary folding is almost entirely unrepresented at low resolution. Higher‐resolution folding is better‐correlated with tropospheric ozone (especially along midlatitude storm tracks, where deep folding is most common); STT is therefore likely more attributable to tropopause folding than coarsely‐resolved folding can capture. Plain Language Summary: "Tropopause folding" refers to high‐altitude atmospheric events wherein the "tropopause" (the boundary separating the troposphere, the lowest atmospheric layer, from the stratosphere above it) is perturbed, "folding" downward and allowing stratospheric air to intrude into the troposphere. These intrusions enable stratosphere‐to‐troposphere transport (STT) of ozone, a pollutant and greenhouse gas in the troposphere—however, a thorough understanding of the relationship between folding and ozone STT has been limited due to the difficulty of combining global yet detailed meteorological data with high‐fidelity chemical data. Here, we identify folding occurrences in both high‐ and low‐resolution representations of atmospheric motion throughout 1 year, and assess how strongly each folding data set is related to estimated tropospheric ozone. A high‐resolution view reveals that folding events are much more frequent and widespread—and penetrate further into the troposphere, becoming thinner—than possible to represent at lower resolution. Moreover, folding at higher resolution is more closely correlated with nearby tropospheric ozone amounts. Folding may therefore exert influence over a larger proportion of ozone STT than is suggested by coarse representations of folding. Furthermore, our results identify the importance of vertical (rather than horizontal) resolution in representing such features and atmospheric transport. Key Points: Tropopause folding is much more frequent at higher reanalysis resolution, due almost entirely to vertical resolutionNearly 90% of folding in ERA5 (nearly 100% of Deep folding) is unrepresented in CAMSRA (at the resolution of ERA‐Interim)Higher‐resolution folding is more strongly correlated with tropospheric ozone, driven by deeper and more filamentary folding [ABSTRACT FROM AUTHOR]

Rainfall extremes are one of the most difficult features to simulate using global climate models. Using pairs of high‐ and low‐resolution atmosphere‐only simulations from the High‐Resolution Model Intercomparison Project, we assess whether and how the horizontal grid spacing influences model's skill in simulating the Asian summer monsoon rainfall extremes. Despite the large inter‐model spread, improved model performance is seen across most models as resolution increases, both in terms of spatial distribution and intensity of monsoon rainfall extremes. Rainfall extremes are enhanced at the windward side of steep topography such as the foothills of Himalayas, and in the heavy monsoon regions such as central India, southern China, and western North Pacific. The improvements are found to be associated with a better representation of the vertical ascents and models' ability in simulating the rain‐producing synoptic low‐pressure systems, which are more frequent and stronger at higher resolutions. Plain Language Summary: Adequate simulations of rainfall extremes are essential for climate models to be useful for assessing regional climate impacts. However, notable biases exist in climate model simulations of Asian monsoon rainfall extremes and their spatial distributions. In the current work, we find that models with finer horizontal resolutions (smaller grid spacing) correct substantially the dry biases that are common in coarse resolution model simulations. The improvement is mainly through better simulations of rainfall extremes along the windward side of the steep topography such as the foothills of Himalayas, and over heavy monsoon regions including central India, southern China, the East Asian shorelines, and western North Pacific. Further analysis indicates that the enhanced extreme rainfall in high‐resolution models is associated with intensified vertical ascents during the occurrence of rainfall events and is attributable to more frequent and stronger monsoon low‐pressure systems, which are among the most important heavy rain‐bearing meteorological systems in the Asian summer monsoon regions. Key Points: Finer resolution models reduce dry biases in modeled Asian monsoon rainfall extremesImproved simulation of monsoon rainfall extremes attributed to better representation of monsoon low‐pressure systems (LPSs)Higher resolution models tend to produce more frequent and stronger LPSs [ABSTRACT FROM AUTHOR]

Prior studies that argue external forcings drive Atlantic Multidecadal Variability (AMV) use linear detrending to remove the global warming trend from North Atlantic sea surface temperatures (SSTs). The linear detrending method, however, aliases residual nonlinear, global‐scale warming, affecting the interpretation of results. Here we revisit the role of external forcing in AMV using large climate model ensembles, examining the influences of greenhouse gases (GHGs) and industrial aerosols—the two dominant 20th‐century external forcing agents—on North Atlantic SSTs separately from their respective global response. Our approach shows that GHGs have little to no influence unique to the North Atlantic, while industrial aerosols exert only modest influences as part of a broader loading over the Northern Hemisphere. We demonstrate that a prominent role for external forcings on AMV is an artifact of the linear detrending method that disappears when their global responses common to the World Ocean are correctly accounted for. Plain Language Summary: The origin of Atlantic Multidecadal Variability (AMV) is widely debated, with discussions focused on whether it is an internal mode of the coupled climate system or a response to external radiative forcings. Most studies that identify a dominant role for external forcings in driving AMV apply linear detrending to remove the global warming trend from North Atlantic SSTs. This is problematic because the linear detrending method does not sufficiently remove the global warming signal. In this study, we re‐examine the role of external forcing in 20th‐century AMV, correctly accounting for global forcing influences common to the entire World Ocean. Separate analysis of the roles of greenhouse gases (GHGs) and industrial aerosols on AMV, independent of their respective planetary influence, shows that GHGs have little to no influence unique to the North Atlantic, while industrial aerosols exert only modest influences as part of a broader impact on the Northern Hemisphere oceans. Our results suggest that linear signal separation methods that do not fully isolate the influences of global forcing on North Atlantic temperatures incorrectly inflate the role played by external forcing. When these approaches are avoided, our results indicate that external forcing has little influence on AMV compared to variability internal to the climate system. Key Points: Linearly detrending North Atlantic sea surface temperatures conflate variability unique to the Atlantic with aliased residual, nonlinear, global warmingThe role of external forcing on Atlantic Multidecadal Variability (AMV) is greatly diminished when global forcing signals are sufficiently removedRoles for historical forcing on AMV previously identified with the linear detrending method are likely an unphysical artifact [ABSTRACT FROM AUTHOR]

The boreal summer climate is of significant societal importance and is trending toward increased risks of extreme climate events such as heatwaves. The summer North Atlantic Oscillation, as the primary mode of atmospheric variability in the northern hemisphere, has been long considered lacking predictability on seasonal time scales. Here we show that the summer North Atlantic Oscillation is predictable with a 2‐month lead for the recent decades. The primary predictor is the March North Atlantic jet strength, which is correlated with the summer North Atlantic Oscillation index at a correlation coefficient of 0.66 over 1979–2018. Spring stratosphere‐troposphere coupling plays a critical role in this extended predictability from spring to summer, in contrast to the common knowledge that this dynamical coupling is relatively inactive outside the winter season. These results may bring sound prospects for summer seasonal prediction of boreal climate that benefits the energy and public health sectors. Plain Language Summary: The summer climate in the northern hemisphere is of significant societal importance. Extreme climate events such as heatwaves occur more and more frequently under global warming. The summer North Atlantic Oscillation is a good representation of atmospheric motions in the North Atlantic region and can influence the atmospheric flow of the entire northern hemisphere. Scientists have not made accurate seasonal forecast of this mode and hence boreal summer climate. In this study, we discovered a key predictor in March whose signal can be transmitted by the stratosphere back to the summer surface to make the summer North Atlantic Oscillation predictable 2 months in advance. Our findings can help the society better prepared for the extreme summer climate in the northern hemisphere. Key Points: The summer North Atlantic Oscillation is found to become predictable in the recent decadesMarch North Atlantic jet strength is the key predictor for the seasonal forecast of the summer North Atlantic OscillationStratosphere‐troposphere dynamical coupling leads to this extended seasonal predictability of the summer North Atlantic Oscillation [ABSTRACT FROM AUTHOR]

The largest sea surface temperature (SST) anomalies associated with Atlantic Multidecadal Variability (AMV) occur over the Atlantic subpolar gyre, yet it is the tropical Atlantic from where the global impacts of AMV originate. Processes that communicate SST change from the subpolar Atlantic gyre to the tropical North Atlantic thus comprise a crucial mechanism of AMV. Here we use idealized model experiments to show that such communication is accomplished by an "atmospheric bridge." Our results demonstrate an unexpected asymmetry: the atmosphere is effective in communicating cold subpolar SSTs to the north tropical Atlantic, via an immediate extratropical atmospheric circulation change that invokes slower wind‐driven evaporative cooling along the Eastern Atlantic Basin and into the tropics. Warm subpolar SST anomalies do not elicit a robust tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites. Plain Language Summary: To investigators of Atlantic Multidecadal Variability (AMV), one outstanding conundrum is the discrepancy between where the largest AMV changes occur (subpolar North Atlantic) and where the remote impacts of AMV originate (tropical North Atlantic). Processes that communicate sea surface temperature (SST) changes from the subpolar gyre region to the tropical North Atlantic are a potentially crucial but poorly understood aspect of AMV. Here, using idealized model experiments, we investigate the idea that such communication is accomplished by an "atmospheric bridge"—a fast, basin‐wide atmospheric response to subpolar gyre SST anomalies that produces a tropical SST change. Our results demonstrate an unexpected, but robust asymmetry: The atmosphere is effective in communicating, via a fast circulation change that invokes wind‐driven evaporative cooling along the Eastern Basin, only cold SST anomalies from the Atlantic subpolar gyre to the tropical North Atlantic. In contrast, imposed warm subpolar SST anomalies do not elicit a tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites. Key Points: A fast, basin‐wide atmospheric response to Atlantic subpolar gyre sea surface temperature (SST) anomalies connects the subpolar and tropical Atlantic regionsThis "atmospheric bridge" communicates, via wind‐driven evaporative cooling, only cold subpolar Atlantic SSTs to the tropical AtlanticThe atmospheric bridge does not effectively communicate warm subpolar Atlantic SSTs, highlighting an important asymmetry of AMV [ABSTRACT FROM AUTHOR]

Extreme heat research has largely focused on dry‐heat, while humid‐heat that poses a substantial threat to human‐health remains relatively understudied. Using hourly high‐resolution ERA5 reanalysis and HadISD station data, we provide the first spatially comprehensive, global‐scale characterization of the magnitude, seasonal timing, and frequency of dry‐ and wet‐bulb temperature extremes and their trends. While the peak dry‐ and humid‐heat extreme occurrences often coincide, their timing differs in climatologically wet regions. Since 1979, dry‐ and humid‐heat extremes have become more frequent over most land regions, with the greatest increases in the tropics and Arctic. Humid‐heat extremes have increased disproportionately over populated regions (∼5.0 days per‐person per‐decade) relative to global land‐areas (∼3.6 days per‐unit‐land‐area per‐decade) and population exposure to humid‐heat has increased at a faster rate than to dry‐heat. Our study highlights the need for a multivariate approach to understand and mitigate future harm from heat stress in a warming world. Plain Language Summary: Combined high temperature and humidity can be more dangerous to humans and wildlife relative to high temperature alone. There are known physiological limits to humid‐heat and adverse impacts on human‐health and performance can be felt well below those limits. Thus, it is important we understand their climatological characteristics and recent changes. Prior research on heat extremes has largely focused on dry‐heat. Using two high‐spatial resolution datasets, we provide a global‐scale characterization of the magnitude, seasonal timing, and frequency of dry‐ and humid‐heat extremes and their historical trends. While dry‐ and humid‐heat extremes occur at the same time of the year in most regions, their timing differs by multiple months in tropical regions with high rainfall. Over the past four decades, dry‐ and humid‐heat extremes have become more frequent over most land regions, with the greatest increases in the tropics and Arctic. Since 1979, each person, on average, has experienced an increase of approximately five additional extreme humid‐heat days per‐decade. These increases are concentrated over densely populated regions in the tropics and sub‐tropics, where humid‐heat levels are already high. Our results emphasize the increasing risk from dangerous heat stress, particularly in vulnerable regions of the world. Key Points: Global patterns and trends in magnitude, timing, and frequency of dry and humid‐heat extremes in ERA5 compare well with HadISD station dataLargest increases in frequency, intensity, and months of occurrence of dry‐ and humid‐heat extremes in tropical and Arctic regionsStronger trends in population exposure to extreme humid‐heat relative to dry‐heat and greater amplification during recent El Niños [ABSTRACT FROM AUTHOR]

The core Indian monsoon region receives more than half of the rainfall extremes from low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies. Yet, the relationship between the trends of LPSs and rainfall extremes remains uncertain. Using two tracking algorithms and reanalyses‐derived LPSs, we find that LPS activity and extreme rainfall exhibit coherent trends during the post‐1979 satellite era. Over time, the LPSs propagate preferentially into south‐central India rather than north‐central India, imparting a corresponding dipole footprint in rainfall extremes. Consistent with existing theories that the diabatic heating is instrumental in shifting the LPSs west‐southwestward, the LPSs traveling through south‐central India have stronger updrafts on their west‐southwestern flank than those passing through north‐central India. The increased frequency of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which favors stronger storm ascents. Plain Language Summary: The South Asian synoptic‐scale low‐pressure systems (LPSs), which typically form over the Bay of Bengal and propagate upstream against the time‐mean low‐level westerlies, produce more than half of the summer rainfall extremes over the densely populated central India. Given the vulnerability of societies in this region to rainfall extremes, investigating the connection between LPSs and extreme rainfall regarding their long‐term trends has important implications for climate adaptation. Using two different tracking algorithms and reanalyses‐derived LPS tracks, we find that the trends of extreme rainfall and LPS activity exhibit a strong coherence during the post‐1979 satellite era. Specifically, the LPSs prefer to propagate into south‐central India than north‐central India over time, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward‐propagating component due to horizontal nonlinear adiabatic advection and the southwestward‐propagating component due to diabatic heating, we notice that the LPSs migrating through the south‐central India have stronger updrafts on their west‐southwestern flank compared to those passing through north‐central India. Our results indicate that the increasing number of LPSs propagating into south‐central India is likely due to a strengthened cross‐equatorial moisture transport, which provides a wetter environment and favors stronger storm ascents. Key Points: An extreme rainfall dipole with positive trends over south‐central India and negative trends over north‐central India is observed since 1979The extreme rainfall dipole aligns with the trends in the number of Indian monsoon low‐pressure systems passing through the two regionsThe changing LPS translation is likely associated with a wetter environment owing to a strengthened cross‐equatorial moisture transport [ABSTRACT FROM AUTHOR]

Trace gases and aerosols play an important role in Arctic chemistry and climate. As most Arctic tracers and aerosols are transported from midlatitude source regions, long‐range transport into the Arctic is one of the key factors to understand the current and future states of Arctic climate. While previous studies have investigated the airmass fraction and transit time distribution in the Arctic, the actual transport pathways and their underlying dynamics and efficiencies are yet to be understood. In this study, we implement a large ensemble of idealized tagged pulse passive tracers in the Whole Atmosphere Community Climate Model version 5 to identify and analyze summertime transport pathways from different Northern Hemisphere surface regions into the Arctic. Three different transport pathways are identified as those associated with fast, intermediate and slow time scales. Midlatitude tracers can be transported into the Arctic in the troposphere via the fast transport pathway (∼8 days), which moves tracers northward from the source region mainly through transient eddies. For the intermediate transport pathway, which happens on 1–3 weeks' time scales, midlatitude tracers are first zonally transported by the jet stream, and then advected northward into the Arctic over Alaska and northern North Atlantic. Tropical and subtropical tracers are transported into the Arctic lower stratosphere via the slow transport pathway (1–3 months), as the tracers are lifted upward into the tropical and subtropical lower stratosphere, and then transported into the Arctic following the isentropic surfaces. Key Points: Identified fast, intermediate and slow transport pathways from various Northern Hemisphere surface regions into the ArcticMidlatitude tracers are transported via the fast and intermediate pathways while low latitude tracers are transported by the slow pathwayAnalyzed and quantified dynamical processes, such as transient versus mean transport, associated with different pathways [ABSTRACT FROM AUTHOR]

The contribution of individual aerosol species and greenhouse gases to precipitation changes during the South Asian summer monsoon is uncertain. Mechanisms driving responses to anthropogenic forcings need further characterization. We use an atmosphere‐only climate model to simulate the fast response of the summer monsoon to different anthropogenic aerosol types and to anthropogenic greenhouse gases. Without normalization, sulfate is the largest driver of precipitation change between 1850 and 2000, followed by black carbon and greenhouse gases. Normalized by radiative forcing, the most effective driver is black carbon. The precipitation and moisture budget responses to combinations of aerosol species perturbed together scale as a linear superposition of their individual responses. We use both a circulation‐based and moisture budget‐based argument to identify mechanisms of aerosol and greenhouse gas induced changes to precipitation and find that in all cases the dynamic contribution is the dominant driver to precipitation change in the monsoon region. Plain Language Summary: Small particles suspended in the atmosphere and emitted by human activities, known as atmospheric aerosols, contribute to changes in precipitation in South Asia, yet the exact importance of individual components is not well understood. Further, the impact of human‐caused greenhouse gas emissions is also uncertain. We use a computer model to simulate the monsoon and find that aerosols are the largest driver of precipitation change compared to greenhouse gases. We also breakdown the precipitation changes by their components and find that changes in atmospheric vertical and horizontal motion of air dominate over increases in moisture for explaining the total rainfall changes. These results help further understand the impact of climate change on precipitation in South Asia, which is important to the lives of billions of people. Key Points: Per unit radiative forcing, black carbon is the most effective driver of precipitation change during the South Asian summer monsoonAnomalous sinking and rising explain modeled precipitation responses to anthropogenic greenhouse gases and aerosolsThe dynamic component dominates the mean column‐integrated moisture flux convergence over the thermodynamic component [ABSTRACT FROM AUTHOR]

The unexpected dry 2015–2016 El Niño winter and extremely wet 2016–2017 La Niña winter in California challenged current seasonal prediction systems. Using the Met Office GloSea5 forecast ensemble, we study the precipitation and circulation differences between these seasons and identify processes relevant to California precipitation predictions. The ensemble mean accurately predicts the midlatitude atmospheric circulation differences between these years, indicating that these differences were predictable responses to the strong oceanic forcing differences. The substantial California precipitation differences were poorly predicted with large uncertainty. Notable differences in high‐latitude circulation anomalies associated with internal variability distinguish the ensemble members that successfully simulate precipitation from those that do not. Specifically, accurate representation of the Arctic Oscillation phase differences improves the accuracy of simulated precipitation differences but these differences were not well predicted in the ensemble mean for these seasons. Improved representation of high‐latitude processes such as the Arctic Oscillation and polar‐midlatitude teleconnections could therefore improve California seasonal predictions. Plain Language Summary: California recently experienced two unusual winter seasons. Following a failed rainy season despite the strong 2015–2016 El Niño that typically brings heavy rains, California unexpectedly experienced one of its wettest winter seasons on record during the 2016–2017 La Niña. The seasonal forecast systems were unable to predict these unusual winter precipitation patterns. We examine the ability of a state‐of‐the‐art forecast system to capture the large‐scale atmospheric circulation patterns that influence the track of storms, which bring a majority of winter precipitation to California. We show that the seasonal forecast systems can reproduce the large‐scale circulation differences in the North Pacific‐American domain, but random atmospheric variability can still easily prevent the accurate prediction of regional scale precipitation in extratropical regions such as California. Further, we identify the role of a natural pattern of large‐scale variability in the atmosphere that affects weather in the middle and high latitudes, referred to as the Arctic Oscillation, in controlling the accuracy of California precipitation forecasts. Prioritizing improvements in the representation of these patterns, the processes by which they are predicted and their influence over other regions in the forecasts systems can help improve seasonal predictions, which is important for the management of California's water resources and infrastructure. Key Points: Large‐scale circulation differences across the Pacific Basin are predictable, but California precipitation has high internal variabilityHigher skill in predicting upper‐level (200 mb) circulation anomalies relative to the lower‐level (850 mb) anomalies over the Pacific BasinEnsemble subset with correct Arctic Oscillation phase improves accuracy of predicting California precipitation differences between these seasons [ABSTRACT FROM AUTHOR]

Abstract: Anthropogenic aerosols are a major factor contributing to human‐induced climate change, particularly over the densely populated Asian monsoon region. Understanding the physical processes controlling the aerosol‐induced changes in monsoon rainfall is essential for reducing the uncertainties in the future projections of the hydrological cycle. Here we use multiple coupled and atmospheric general circulation models to explore the physical mechanisms for the aerosol‐driven monsoon changes on different time scales. We show that anthropogenic aerosols induce an overall reduction in monsoon rainfall and circulation, which can be largely explained by the fast adjustments over land north of 20∘N. This fast response occurs before changes in sea surface temperature (SST), largely driven by aerosol‐cloud interactions. However, aerosol‐induced SST feedbacks (slow response) cause substantial changes in the monsoon meridional circulation over the oceanic regions. Both the land‐ocean asymmetry and meridional temperature gradient are key factors in determining the overall monsoon circulation response. [ABSTRACT FROM AUTHOR]

Winter storms pose numerous hazards to the Northeast United States, including rain, snow, strong wind, and flooding. These hazards can cause millions of dollars in damages from one storm alone. This study investigates meteorological intensity and impacts of winter storms from 2001 to 2014 on coastal counties in Connecticut, New Jersey, and New York and underscores the consequences of winter storms. The study selected 70 winter storms on the basis of station observations of surface wind strength, heavy precipitation, high storm tide, and snow extremes. Storm rankings differed between measures, suggesting that intensity is not easily defined with a single metric. Several storms fell into two or more categories (multiple-category storms). Following storm selection, property damages were examined to determine which types lead to high losses. The analysis of hazards (or events) and associated damages using the Storm Events Database of the National Centers for Environmental Information indicates that multiple-category storms were responsible for a greater portion of the damage. Flooding was responsible for the highest losses, but no discernible connection exists between the number of storms that afflict a county and the damage it faces. These results imply that losses may rely more on the incidence of specific hazards, infrastructure types, and property values, which vary throughout the region. [ABSTRACT FROM AUTHOR]

Climate models suggest that anthropogenic aerosol‐induced drying dominates the historical rainfall changes over the heavily populated South Asian monsoon region. The regional response depends on both the aerosol fast radiative effect and the slow process through sea surface temperature (SST) cooling. Two atmospheric general circulation models, NCAR‐CAM5 and GFDL‐AM3, are used to investigate the monsoon response to prescribed aerosol‐forced SSTs. The total SST is separated into uniform cooling and a spatially varying component characterized by interhemispheric asymmetry. The monsoon rainfall is predominantly controlled by the nonuniform SSTs, in the local Indian Ocean, South, and East China Seas (IO‐CSs). The reduced meridional SST gradient in the IO‐CSs leads to weakened monsoon circulation, which drives a north‐south dipole rainfall change. The latitudinal location of the dipole shows model dependence due to differences in local SSTs and their meridional gradient, which determines the latitudinal location of the meridional overturning circulation responses. Plain Language Summary: South Asian monsoon rainfall is an important part of the region's economy as it affects the agriculture and water supply in this densely populated region. Anomalously weak monsoon rainfall can lead to catastrophic crop failures and famine. Recent increases in aerosol emission not only caused severe air pollution problems in the region, but also led to monsoon circulation and rainfall changes. Previous studies have shown that aerosols can lead to drying in a region due to interaction with radiation and clouds. Additionally, the northern hemisphere centric aerosol emission led sea surface temperatures to cool more in northern than southern hemispheres. This sea surface temperature mediated responses to aerosol can play an important role in the South Asian monsoon response to aerosols. In this study, we demonstrate that this effect is mainly through the north‐south temperature gradient in the Indian Ocean, South, and East China Seas, which leads to atmospheric circulation that weakens the monsoon overturning and drying in South Asia. However, the exact latitudes of this drying tend to be highly model‐dependent due to different sea surface temperatures in the local ocean basins. Key Points: Predominant impact of aerosol‐forced sea surface temperatures on South Asian monsoon is the local spatially nonuniform partLarge intermodel differences in monsoon responses are due to regional aerosol‐forced SST differencesDynamical mechanisms dominate South Asian drying due to SST‐mediated aerosol forcing [ABSTRACT FROM AUTHOR]

This study investigates the leading pattern of Eurasian summer heat waves (HWs) using observed and simulated data sets and reveals an intensified mode of variability that bridges the HWs in eastern Europe (EE) and northern China (NC). The concurrent variability of the HWs in EE and NC is primarily driven by an atmospheric circum‐global teleconnection that induces anomalous anticyclones over the two regions. The observed upward trends in EE and NC HW days could be related to the warm sea surface temperatures around Greenland Island, which may weaken the Atlantic westerly jet stream and lead to amplified wave trains at the exit of the jet, resulting in strengthened anticyclones over EE and NC that favor the occurrences of HWs. The Geophysical Fluid Dynamics Laboratory high‐resolution atmospheric model fails to simulate the EE and NC HWs, due probably to the model's poor representation of the South Asian summer rainfall. Plain Language Summary: Heat waves (HWs) in eastern Europe (EE) and northern China (NC) are found to be bridged via an atmospheric teleconnection, which induces abnormal highs over EE and NC. As a result, the soil conditions in EE and NC become drier and hotter due to the decreased precipitation and increased solar radiation, which favors the occurrences of HWs. The North Atlantic warming may lead to the increases in EE and NC HWs by altering the Atlantic westerly jet stream and associated wave trains. The Geophysical Fluid Dynamics Laboratory high‐resolution atmospheric model fails to simulate the EE and NC HWs, due probably to model's poor representation of the South Asian summer rainfall. Key Points: Summer heat waves (HWs) in eastern Europe (EE) and northern China (NC) are modulated by an intensified mode of variabilityThe concurrent variability of HWs in EE and NC is primarily driven by an atmospheric circum‐global teleconnection (CGT)The GFDL high‐resolution atmospheric model shows a poor representation of the EE and NC HWs [ABSTRACT FROM AUTHOR]