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Summer drought over northern Asia (NA) seriously threatens the local fragile ecological environment and social economy development. In this study, using the standardized precipitation evapotranspiration index (SPEI), we first identify the dominant modes of interannual variability in summer drought condition over NA and then explore the atmospheric patterns responsible for the formation of the modes. The results show that the first empirical orthogonal function (EOF1) mode of summer SPEI over NA exhibits a meridional dipole pattern, which is influenced primarily by the polar–Eurasian teleconnection (POL) and circumglobal teleconnection (CGT) patterns. Under the influence of negative POL and positive CGT patterns, there is an anomalous anticyclone (cyclone) over northwestern Siberia (Lake Baikal to Northeast China). Such atmospheric circulations lead to meridional dipole patterns in air temperature, moisture condition, vertical motion, and cloud cover over NA, favoring decreased (increased) precipitation and increased (decreased) potential evapotranspiration over northern (southern) NA, finally contributing to the formation of EOF1. The second EOF (EOF2) shows an approximate zonal dipole pattern, which is influenced by the British–Baikal Corridor (BBC) and Scandinavia (SCA) teleconnection patterns. The positive BBC and SCA patterns can lead to an anomalous anticyclone over the Ural Mountains and a cyclone over Lake Baikal. Such atmospheric circulations result in a zonal dipole pattern in precipitation and potential evapotranspiration over NA through changing the local moisture condition, air temperature, and radiation, consequently favoring the formation of EOF2. Fitting analysis indicates that the aforementioned atmospheric factors can explain 76% (55%) of the interannual variability of EOF1 (EOF2). [ABSTRACT FROM AUTHOR]

A main source of regional climate change uncertainty is the large disparity across models in simulating the atmospheric circulation response to global warming. Using the latest suite of global climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), a storyline approach is adopted to derive physically plausible scenarios of Antarctic climate change for 2070–99, according to Shared Socioeconomic Pathway SSP5-8.5. These storylines correspond to differences in the simulated amount of seasonal sea ice loss and either (i) the delay in the summertime stratospheric polar vortex (SPV) breakdown or (ii) wintertime SPV strengthening, which together constitute robust drivers of the response pattern to future climate change. Such changes combined are known to exert a strong control over the Southern Hemisphere midlatitude jet stream, which we quantify as collectively explaining up to 70% of the variance in jet response in summer and 35% in winter. For summer, the expected strengthening and displacement of the tropospheric jet stream varies between a ∼1 and 2 m s−1 increase and ∼2°–4° poleward shift, respectively, across storylines. In both seasons, a larger strengthening of the jet is correlated with less Antarctic warming. By contrast, the response in precipitation is more consistent but still strongly attenuated by large-scale dynamics. We find that an increase in high-latitude precipitation around Antarctica is more pronounced for storylines characterized by a greater poleward jet shift, particularly in summer. Our results highlight the usefulness of the storyline approach in illustrating model uncertainty and understanding the processes that determine the spread in projected Antarctic regional climate response. Significance Statement: Uncertainty in future climate predictions for the Antarctic is dominated by the unknown response of the large-scale (global) atmospheric circulation. In characterizing such uncertainty, plausible outcomes of climate response (storylines) are generated from the organization of model projections according to the amount of simulated seasonal sea ice loss and the delay in summertime breakdown/winter strengthening of the stratospheric westerly circulation (polar vortex). The intensity and location of the tropospheric jet stream is strongly dependent on both factors, which strongly influences the near-surface climate response over Antarctica. We find that the simulated amount that Antarctic air temperatures increase by in the future (to the end of the century) is intrinsically related to the projected intensification of the Southern Hemisphere tropospheric jet, varying by a factor of 2 or more across storylines for summer. Storylines with greater jet strengthening are associated with less Antarctic warming (reduced poleward advection of air masses from lower latitudes). Similar differences are found for changes in jet position, which we note has a much stronger control on mid- to high-latitude precipitation response. This includes both an enhanced wetting response around Antarctica and drying response farther equatorward, for storylines characterized by a greater poleward jet shift. [ABSTRACT FROM AUTHOR]

This study identifies a significantly positive relationship between summer surface air temperature (SAT) anomalies over two remote regions in the Eurasian continent and North America during the period 1979–2021 on the interannual time scale. The former region includes the East European Plain and the West Siberian Plain, and the latter region includes central and eastern North America. The regionally averaged summer SAT anomalies show a correlation coefficient of 0.66 between these two regions, which is significant at the 99% confidence level. This intercontinental SAT relationship can be explained by a wavelike pattern of circulation anomalies, which is the leading mode of upper-tropospheric circulation anomalies over the middle and high latitudes of the Northern Hemisphere in summer. Further analysis suggests that the sea surface temperature (SST) anomalies over the Pacific and North Atlantic in the preceding spring, being coupled with the leading mode of atmospheric circulation anomalies over the Pacific–Atlantic sector, persist into summer and affect the SATs in the two remote regions, resulting in the intercontinental SAT connection. Significance Statement: Summer surface air temperature (SAT) has profound effects on public health and agricultural production. Here we find a significantly positive relationship between interannual variations of summer SATs over two remote regions, one in the Eurasian continent and the other in North America. This intercontinental relationship in SATs can be explained as a result of atmosphere–ocean coupling over the Pacific and North Atlantic in the preceding spring. The result is likely to be a critical implication for the seasonal forecast of SAT variations over the two regions. In addition, the concurrence of higher or lower temperatures in these two regions may have impacts on global grain production, since these two regions include many major grain-producing areas in the world. [ABSTRACT FROM AUTHOR]

El Niño is known to affect Arctic temperature. However, the robustness of the observed relationship between El Niño and Arctic temperature remains debated. Here we reexamine the impacts of El Niño on the Arctic temperature in boreal winter [December–February (DJF)] using reanalysis datasets and atmospheric model experiments. This study shows that extreme El Niño events are accompanied by significant boreal winter cooling over northeastern Canada and Greenland (NECG), while moderate eastern Pacific (MEP) El Niño events are accompanied by significant boreal winter warming in this region. For central Pacific (CP) El Niño events, a cold signal appears in NECG, but with no statistical significance. During extreme El Niño winters, a positive Pacific–North America (PNA)-like pattern is seen in the Pacific, and anomalously negative 200-hPa geopotential height (Z200) strengthening occurs over NECG, which is a response to anomalous strong wave activity originating in the tropical Pacific. El Niño–induced circulation anomalies can further induce NECG cooling via cold temperature advection and decreased downward longwave radiation. In contrast, for the MEP El Niño, the subtropical jet extends zonally from the North Pacific to the North Atlantic, which is accompanied by increased baroclinicity anomalies and favors the propagation of synoptic eddies into the Atlantic, leading to a negative North Atlantic Oscillation (NAO)-like pattern. This in turn could further enhance the positive Z200 anomalies over NECG, resulting in anomalous warming in NECG through warm temperature advection and enhanced downward longwave radiation. A series of atmospheric model experiments simulates the observed circulation changes and associated warming over NECG. Significance Statement: This work investigates the different impacts of the three El Niño types on regional Arctic wintertime temperature anomalies based on observations and model experiments. The impacts of El Niño events on northeastern Canada and Greenland temperatures during boreal winter show distinct differences between extreme El Niño and moderate EP El Niño events. These distinct differences can be attributed to the different atmospheric circulation patterns induced by different SST patterns, which can lead to warm (cold) temperature advection and enhanced (decreased) downward longwave radiation. These results highlight the different impacts of extreme and moderate EP El Niños on Arctic temperatures and provide an improved understanding of the impact of El Niños on the Arctic climate. [ABSTRACT FROM AUTHOR]

The South Asian high (SAH) location and intensity are linked with the latent heating of the Tibetan Plateau (TP) and Yangtze River basin. The relationship between SAH variability and its impact on South Asian monsoon (SAM) onset is rarely linked with TP soil moisture. This study uses remotely sensed soil moisture and reanalysis products to quantify the relationship between the TP spring (April–June) soil moisture with SAH and SAM onset during 1988–2008. The results show that the TP spring soil moisture and monsoon onset indices are negatively correlated (R < −0.65), whereas the SAH exhibits a significant positive correlation (R ≥ 0.70) with TP soil moisture. The monsoon onset shows a difference of 20–25 days between the early- and late-onset composites. Significant positive (negative) soil moisture anomalies persist over the TP during the early (late) onset followed by positive (negative) LH (SH) anomalies during early (late)-onset composites. The TP thermal forcing exhibited positive anomalies during the early (late)-onset composites implying significant soil moisture control over the diabatic heating, which favors vertical ascent over the eastern plateau. Such a pattern leads to an earlier formation and movement of the SAH toward the Bay of Bengal (BOB) and southwestward of the TP. Before the early and late monsoon onset composites, the SAH pentad evolution drives the lower-tropospheric westerlies/easterlies toward continental SA. In the Indian Ocean the wind shear and transition from prevailing easterlies into westerlies during the pre-onset, onset, and post-onset pentad results in strong/weak ascent affecting the onset timing over the Arabian Sea and continental SA with less influence over the BOB monsoon onset. Significance Statement: The Tibetan Plateau heating is one of the key drivers of the Asian monsoon precipitation in the surrounding regions, which has been previously studied in detail. This study explored the Tibetan Plateau spring soil moisture's effect on South Asian monsoon onset timing. The monsoon onset timing is calculated using changes in wind direction, atmospheric temperature, and relative precipitation magnitude. Results found that the spring soil moisture substantially affects the TP thermal heating and the SA monsoon onset timing and highlights the physical processes leading to changes in the monsoon onset timing. The inclusion of soil moisture in estimating the monsoon onset timing can provide a tangible way of improving our understanding of the monsoon and associated water resources management practices. [ABSTRACT FROM AUTHOR]

A strong coherence of the summer (June–August) surface air temperature (SAT) anomalies exists over eastern Eurasia (EE) and central Eurasia (CE) in association with a remarkable intraseasonal (10–90-day) variability. The intraseasonal SAT over Eurasia shows a negative correlation between EE and CE, which is closely related to an intraseasonal wave train at the upper troposphere over the mid–high latitudes of Eurasia. The wave train propagates eastward and results in the intraseasonal variation of SAT in a seesaw pattern. The column-integrated temperature budget suggests that both the horizontal and vertical advection are important for the intraseasonal SAT dipole. More specifically, the climatological westerly, intraseasonal meridional wind anomaly, and the anomalous vertical motion play dominant roles in perturbing temperature over the CE and EE regions, while the diabatic heating is a negative feedback. The propagation of the Rossby wave train in the upper troposphere helps to maintain the circulation anomalies associated with the SAT seesaw. The leading modes of meridional wind anomalies at 200 hPa over the Eurasian high latitudes resemble the intraseasonal wave train, indicating that the wave train associated with the SAT seesaw is an atmospheric intrinsic mode over the mid–high latitudes. This study also investigates the impact of the intraseasonal SAT seesaw on regional heat wave events. It is suggested that the intraseasonal SAT seesaw has a major influence on the variability of heat wave events over both EE and CE, which may provide great implications for regional subseasonal forecasts of extreme weather events. [ABSTRACT FROM AUTHOR]

To investigate patterns of horizontal atmospheric latent energy (LE) transport toward the Arctic, we applied the self-organizing maps (SOM) method to the daily vertically integrated horizontal LE flux from ERA5 in winter (January–March) during 1979–2021. A clear picture depicting the LE transport to the Arctic at a synoptic scale then emerged, with four primary transport pathways identified: the northern Europe, the Davis Strait, the Greenland Sea, and the Bering Strait pathways. The four primary pathways occurred at a comparable frequency, and noticeable interannual variability was observed in their time series of frequency during 1979–2021. Further analysis suggested that the northward LE transport through all these pathways is significantly modulated by cyclones, with the northern Europe and the Greenland Sea pathways being mostly affected. Generally, more frequent and stronger cyclones were observed near the entry regions of LE transport compared to other regions. Moreover, this study provides a comprehensive picture of how atmospheric LE transport is related to air temperature, moisture, surface heat flux, and sea ice anomalies over the Arctic Ocean in winter. Through a thermodynamic perspective, we argue that the deleterious impacts of poleward LE transport on Arctic sea ice are to a large extent attributable to the enhanced local atmosphere–ice interactions, which increase downward longwave radiation (DLR) plus turbulent fluxes, consequently warming the surface and promoting the loss of sea ice. According to the quantitative results, among the four primary pathways, LE transport through the Davis Strait and the Greenland Sea could cause the loss of Arctic sea ice most efficiently. [ABSTRACT FROM AUTHOR]

The premonsoon circulation over South Asia in May shows remarkable interannual variations, and it can modulate the onset of the Asian summer monsoon. The present study derives two dominant stratosphere–troposphere modes over the North Atlantic in April via applying principal component (PC) analysis to regional geopotential height during 1979–2015. Both of the modes reflect the North Atlantic Oscillation (NAO)-like circulation at the midtroposphere, but the circulation patterns and planetary wave activity between the two modes are quite different. The first mode represents a stratosphere–troposphere-coupled mode. Further analysis indicates that during the years with a positive phase of the first mode, the low-level anomalous southerlies over the northern Barents Sea (BS) lead to a reduction in local sea ice, which could persist into May through the ice-albedo feedback. The BS sea ice anomalies in May could generate a southeastward propagating Rossby wave train, producing an anomalous anticyclonic circulation to the west of Qinghai–Tibet Plateau. This anomalous anticyclone induces increases in local air temperature and the meridional temperature gradient, resulting in a strengthened premonsoon circulation. Given that prediction of the premonsoon circulation over South Asia remains a challenging issue, the stratosphere–troposphere-coupling modes in April provide another potential predictability source. On the other hand, ENSO has a significant effect on the South Asian premonsoon circulations in May, and can also lead to the NAO-like circulation by influencing atmospheric Rossby waves. Such an NAO-like circulation is closely related to the second mode. [ABSTRACT FROM AUTHOR]

As the understanding of decadal variability in climate systems deepens, there is a growing interest in investigating the decadal variability of seasonal-mean or monthly mean variables. This study aims to understand the seasonality observed in the amplitude of decadal variability. To accomplish this, we analyze the decadal variability of the monthly mean North Atlantic Oscillation (NAO) index and North Pacific index (NPI) over the past decades using two different calculating processes: the full smoothing (F) process and the seasonal-specific (SS) process. Our findings suggest that the F process only captures the decadal variability of annual-mean variables, whereas the SS process is suited for capturing the seasonality of decadal variability. We find that the seasonality in decadal variability aligns with the seasonality in interannual variability. Additionally, we explore the seasonality in decadal variability in atmospheric and oceanic variables. The seasonality in oceanic decadal variability, including sea surface temperature and salinity, is found to be weak and small. The amplitude of decadal variability in the Pacific decadal oscillation (PDO) is similar across different months, indicating weak seasonality in the PDO. On the other hand, decadal variability of lower-tropospheric atmospheric circulation, including horizontal wind, geopotential height, and surface air temperature, exhibits significant seasonality in the extratropics, with the strongest decadal variability occurring in winter. Moreover, the significant seasonality in decadal variability of precipitation is observed in the tropics, with the strongest decadal variability occurring in summer. Our study provides insights into understanding the seasonality of decadal variability, which can aid in the improvement of decadal prediction of climate variability. Significance Statement: The amplitude of decadal variability of seasonal-mean or monthly mean variables exhibits seasonality. Our results show that the seasonality in decadal variability is consistent with the seasonality in interannual variability. We also identified that the seasonality in oceanic decadal variability is weak and smaller than that in atmospheric decadal variability. Decadal variability of lower-tropospheric atmospheric circulation exhibits significant seasonality in the extratropics, with the strongest decadal variability occurring in winter. However, the significant seasonality in decadal variability of precipitation occurs in the tropics, with the strongest decadal variability occurring in summer. Our study provides insights into the seasonality of decadal variability in climate systems. [ABSTRACT FROM AUTHOR]

A new wintertime surface air temperature (SAT) pattern, called the Asia–Kuroshio and Oyashio Extension–North America (AKNA) pattern, is identified over the pan–North Pacific region (85°E–85°W, 25°–65°N) based on NCEP reanalysis data. The AKNA pattern is likely to influence the climate of the extratropical area of Asia and North America via two SAT dipoles and has a significant impact on the wintertime extremely cold weather along the eastern coastal regions of East Asia. Simulations using an atmospheric general circulation model indicate that wintertime sea surface temperature anomalies (SSTa) in the Kuroshio and Oyashio Extension (KOE) region can force an equivalent barotropic atmospheric ridge downstream and weaken the Siberian high and Alaska atmospheric ridge, resulting in the formation of the AKNA pattern. This circulation pattern tends to intensify the midlatitude (40°–60°N) westerlies over East Asia, which inhibits the southward invasion of the cold air into southern East Asia. Further diagnostic analysis indicates that the KOE SSTa can modulate the variation of storm track and westerlies by affecting baroclinic instability and eddy–mean flow interaction. Moreover, the KOE SSTa can provide a favorable environment for the development of the local atmospheric ascending motion and secondary circulation across the KOE SSTa, thereby affecting variability of the free atmosphere. Significance Statement: This study aims to build a connection between the wintertime extratropical climate and the variation of the Kuroshio and Oyashio Extension. This work isolated a new wintertime surface air temperature (SAT) pattern over mid–high-latitude Asia and North America, which explains a considerable proportion of cold extremes over the eastern regions of East Asia. The reanalysis data and model simulations indicate that the temporal variability of the SAT pattern is influenced by the change of sea surface temperature in the Kuroshio and Oyashio Extension. These findings emphasize the important role of midlatitude air–sea interaction in the modulation of the mid–high-latitude climate. [ABSTRACT FROM AUTHOR]

Recent analysis of pantropical interactions suggests that after 1980 the tropical Atlantic Ocean’s (TAO) influence on the tropical Pacific Ocean (TPO) appears to have become much more pronounced while the tropical Indian Ocean’s (TIO) influence appears to have weakened. This study explores whether and how decadal changes in TAO and TPO SSTs modulate these pantropical connections in an attempt to explain the recent dominance of the TAO. To this end, we carry out a series of idealized atmosphere-only experiments using the ACCESS atmospheric general circulation model where the magnitude and sign of the decadal TAO SST signal are varied, presenting various warm and cool Atlantic scenarios. To understand further if these pantropical connections are influenced by changes in TPO SST, we carry out the above TAO experiments with both warm and cool phases of Pacific decadal variability (PDV). We find that an imposed TAO warming leads to increases in TPO atmospheric temperature and stability, which lead to a decrease in average TPO precipitation, with the most prominent changes occurring in June–August. These changes in TPO precipitation induced by TAO warming are largely mirrored when TAO cooling is added, whereas the TPO rainfall response to TAO anomalies remains relatively unchanged for the different phases of PDV. In contrast to the precipitation response, the wind response did display some asymmetries between different phases of TAO SST variability. Specifically, surface winds in the western half of the Niño-4 region exhibited a significantly different response to positive versus negative Atlantic multidecadal variability (AMV), whereas the surface winds in the western equatorial Pacific were significantly stronger (roughly 40% larger) in the positive phase of PDV than in the negative phase. These results suggest that the phases of PDV and AMV may modulate pantropical interactions through their effect on zonal wind stress. [ABSTRACT FROM AUTHOR]

It is well known that El Niño–Southern Oscillation (ENSO) can influence the East Asian winter climate by modifying the atmospheric circulation over the western North Pacific (WNP). While the impact on precipitation in southeastern China has been extensively studied, the ENSO signal in surface air temperature (SAT) remains overlooked. In this paper, we identify robust ENSO footprints in the winter daily minimum SAT in southeastern China, with El Niño winters generally accompanied by warmer-than-normal minimum SAT anomalies. In contrast, the responses of maximum SAT are weak and negligible, suggesting a diurnal cycle–dependent ENSO influence. Further analysis indicates that this diurnal cycle dependence stems primarily from the disparate surface radiative heating between day and night induced by ENSO-related local total cloud cover (TCC) change. The warmer minimum SAT occurring in the early morning of El Niño winters is mainly caused by the enhanced surface downward longwave radiative heating as a result of the TCC increase. However, in the afternoon of El Niño winters, although the anomalous horizontal advection of warm air plays a role, there is surface radiative cooling as the weakening of solar radiation due to TCC reflection overwhelms the increase in downward longwave radiation, which leads to a weakened sensible heat flux and thus has a cooling effect on the SAT. Ultimately, these two processes effectively cancel each other out and together produce insignificant maximum SAT responses. Our conclusions carry important implications for the seasonal to interannual winter climate prediction in southeastern China. Significance Statement The purpose of this study is to better understand how ENSO influences winter surface air temperature in southeastern China. Despite insignificant responses of winter daily maximum temperature to ENSO, we find here a robust ENSO influence on the daily minimum temperature, with warm anomalies tending to occur during El Niño events. Further evidence suggests that this diurnal cycle–dependent ENSO influence is mainly caused by ENSO-induced change in local cloudiness, which has a different radiative heating effect between day and night. Our findings are of great importance for improving regional climate prediction, which will benefit social and economic activities. [ABSTRACT FROM AUTHOR]

We investigate the land-ocean warming contrast mechanisms, f, defined as the land-mean surface air temperature (SAT) change divided by the ocean-mean SAT change, in a transient climate response (TCR) obtained from the Coupled Model Intercomparison Project phase 6 (CMIP6) 1% per year CO2 increase experiments (1pctCO2). The energy budget framework devised in Part I is applied to 15 CMIP6 1pctCO2 simulations, and the climate response in year 140 when the CO2 concentration was quadrupled was compared with a near-equilibrium climate response (NEQ), defined as the last 30-yr mean in the abrupt CO2 quadrupling (abrupt43CO2) experiments. It is shown that f is larger in TCR than in NEQ by approximately 4%, although the difference is not statistically significant. In TCR, effective radiative forcing is large over land compared to the ocean, and this is the main contributor to f as in NEQ. The time evolution of f in 1pctCO2 can be reconstructed by means of the fast and slow components of climate response in abrupt43CO2, indicating that the essential mechanism for the land-ocean warming contrast shown in Part I applies to TCR. Furthermore, our analyses reveal a compensation between land-to-ocean atmospheric energy transport that decreases f and ocean heat uptake that increases f. Regardless of the time scale of the response, these two processes are linked by the change in atmospheric circulation, leading to the small combined effect. As a result, the multimodel mean f in 1pctCO2 is roughly time invariant at approximately 1.5 despite the continuous increase in CO2. [ABSTRACT FROM AUTHOR]

The influence of mid–high-latitude intraseasonal variability (ISV) on the occurrence frequency of the Northeast China cold vortex (NCCV) in early summer was examined through statistical analysis and thermal–dynamic diagnostics. A multivariable empirical orthogonal function (MVEOF) was employed to extract the thermal–pressure coupled ISV mode. Our results show that the geopotential height and air temperature over the NCCV active region exhibit a statistically significant intraseasonal periodicity of 20–60 days. The dominant ISV mode features a westward-propagated zonal dipole pattern, which is generated over the Lake Baikal region and triggered by intraseasonal wave energy accumulation. By dividing the ISV cycle into eight phases, it is found that more NCCVs with a large scope occur in phases 5–8 than those in phases 1–4. The positive (negative) geopotential height and air temperature tendencies in phases 1–4 (5–8) act to suppress (facilitate) the NCCV activity. The thermodynamic tendency budget and scale decomposition reveal that when an anomalous intraseasonal cyclonic circulation propagates westward from Lake Baikal to the Ural Mountains, the anomalous southwesterly transports mean negative vorticity from the north side of the Tibetan Plateau to Northeast Asia and transports mean warm air temperature from low latitudes to high latitudes, leading to the positive geopotential height and air temperature tendencies and thereby restraining the NCCV activity. The opposite is also true for the facilitation of the NCCV modulated by the negative geopotential height and air temperature tendencies. Significance Statement: The purpose of this study is to better understand the factors controlling the Northeast China cold vortex (NCCV) activity in early summer. It is important because influences of the subtropical monsoonal circulation are usually confined to southern China in this season and the anomalous atmospheric circulation from the mid–high latitudes plays a more important role in the generation of the NCCV. Our results provide a guide on how intraseasonal variability at mid–high latitudes controls the occurrence frequency of the NCCV and highlight the process of thermal and dynamical modulation in the NCCV. [ABSTRACT FROM AUTHOR]

The increase of atmospheric CO2 concentrations changes the atmospheric temperature distribution, which in turn affects the circulation. A robust circulation response to CO2 forcing is the strengthening of the stratospheric Brewer-Dobson circulation (BDC), with associated consequences for transport of trace gases such as ozone. Ozone is further affected by the CO2-induced stratospheric cooling via the temperature dependency of ozone chemistry. These ozone changes in turn influence stratospheric temperatures and thereby modify the CO2-induced circulation changes. In this study, we perform dedicated model simulations to quantify the modification of the circulation response to CO2 forcing by stratospheric ozone. Specifically, we compare simulations of the atmosphere with preindustrial and with quadrupled CO2 climate conditions, in which stratospheric ozone is held fixed or is adapted to the new climate state. The results of the residual circulation and mean age of air show that ozone changes damp the CO2-induced BDC increase by up to 20%. This damping of the BDC strengthening is linked to an ozone-induced relative enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the Southern Hemisphere, ozone reduces the CO2-induced delay of the final warming date by 50%. [ABSTRACT FROM AUTHOR]

Both South Asia and East Asia are the most polluted regions of the world. Unlike East Asia, the aerosol optical depth (AOD) over South Asia keeps increasing for all recent years, which calls for more attention. This study investigates the impacts of anthropogenic emissions over South Asia on the downstream regional climate during spring with the Community Earth System Model 2 (CESM2). The model results suggest that South Asian pollutants have significant impacts on East Asian spring climate, and the impacts could be even larger than locally emitted aerosols. Two possible dynamical pathways (i.e., the northern and the southern pathways) bridging South Asian aerosol forcing and East Asian climate are proposed, and both ways are associated with the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP). The northern pathway is mainly due to the TP warming induced by the BC snow darkening effect (SDE), which significantly reduces the surface air temperature (SAT) over northern East Asia. BC-induced TP warming increases the meridional thermal gradient and accelerates the midlatitude jet stream, which favors the cold-air advection over northern East Asia. The southern pathway is associated with the BC "elevated heat pump" hypothesis, which mainly affects the precipitation in southern East Asia. BC from South Asia accumulates near the south slope of TP, inducing an abnormal ascending motion near the Bay of Bengal. A compensating anomalous sinking motion is then forced in South China, which suppresses the precipitation there. A primary observational analysis is also performed to verify both dynamical pathways. [ABSTRACT FROM AUTHOR]

The Maritime Continent disrupts eastward propagation of the Madden–Julian oscillation (MJO). This study surveys the impact of the disruption—often known as the barrier effect—on the MJO teleconnections. The MJO propagation may be broadly categorized based on whether the MJO precipitation crosses the Maritime Continent (MC) during extended boreal winter seasons: successfully propagating across the MC (MJO-C) or being blocked by the MC (MJO-B). Compositing atmospheric circulation upon these two categories reveals that precipitation anomalies of MJO-C are stronger and more coherent than those of MJO-B, while their phase speed and lifetime are comparable. MJO-C and MJO-B excite distinct extratropical responses due to their diabatic heating in the deep tropics. Midlatitude circulation displays stronger and long-lasting negative geopotential anomalies in the northern Pacific Ocean 5–14 days after phase 7–8 of MJO-C, but significantly weaker anomalies from MJO-B. The extratropical water vapor transport during MJO-B and MJO-C differs markedly after phase 2. The Pacific–North American (PNA) pattern and North Atlantic Oscillation (NAO) both show significant response after phase 6 of MJO-C as its precipitation anomaly over the tropical Pacific during this period is stronger, while MJO-B has little impact on both. Surface air temperatures (SAT) at high latitudes during MJO-B and MJO-C are also significantly different. SAT is weaker and delayed in MJO-B in comparison to MJO-C, likely due to different meridional eddy heat fluxes. [ABSTRACT FROM AUTHOR]

Observing system experiments were undertaken within the 4D-Var data assimilation of the Météo-France global numerical weather prediction (NWP) model. A 6-month period was chosen (October 2019–March 2020) where 40 million observations per day were assimilated. The importance of in situ observations provided by aircraft, radiosondes, and surface weather stations, despite their small fractional amount (7%), has been confirmed particularly in the Northern Hemisphere. Moreover, the largest impact over Europe in terms of root-mean-square error (RMSE) scores comes from surface observations. Satellite data play a dominant role over tropical regions and the Southern Hemisphere. Microwave radiances have a more pronounced impact on the long range and on the humidity field than infrared radiances, despite being less numerous (10% versus 80%). Bending angles impact significantly the quality of the upper-troposphere–lower-stratosphere temperature of the tropics and Southern Hemisphere. Atmospheric motion vectors (AMVs) are beneficial in wind forecasts at low and high levels in the tropics and the Southern Hemisphere, but also in the humidity field. Such impacts are only significant during the first 48 h of the forecasts. Scatterometer winds have an impact restricted to low levels that is kept at longer ranges. A comparison with forecast sensitivity–observation impact studies over a 3-month period using the same measure of short-range (24-h) forecast errors reveals that the ranking between the major observing systems is kept between these two ways of measuring observation impact in NWP. From our conclusions, recommendations are provided on possible evolutions of the global observing system for NWP. [ABSTRACT FROM AUTHOR]

Over the past decades, Arctic climate has exhibited significant changes characterized by strong pan-Arctic warming and a large-scale wind shift trending toward an anticyclonic anomaly centered over Greenland and the Arctic Ocean. Recent work has suggested that this wind change is able to warm the Arctic atmosphere and melt sea ice through dynamically driven warming, moistening, and ice drift effects. However, previous examination of this linkage lacks a capability to fully consider the complex nature of the sea ice response to the wind change. In this study, we perform a more rigorous test of this idea by using a coupled high-resolution modeling framework with observed winds nudged over the Arctic that allows for a comparison of these wind-induced effects with observations and simulated effects forced by anthropogenic forcing. Our nudging simulation can well capture observed variability of atmospheric temperature, sea ice, and the radiation balance during the Arctic summer and appears to simulate around 30% of Arctic warming and sea ice melting over the whole period (1979–2020) and more than 50% over the period 2000–12, which is the fastest Arctic warming decade in the satellite era. In particular, in the summer of 2020, a similar wind pattern reemerged to induce the second-lowest sea ice extent since 1979, suggesting that large-scale wind changes in the Arctic are essential in shaping Arctic climate on interannual and interdecadal time scales and may be critical to determine Arctic climate variability in the coming decades. Significance Statement: This work conducts a set of new CESM1 nudging simulations to quantify the impact of the observed evolution of large-scale high-latitude atmospheric winds on Arctic climate variability over the past four decades. Variations in climate parameters, including sea ice, radiation, and atmospheric temperatures are well replicated in the model when observed winds are imposed in the Arctic. By investigating simulated sea ice melting processes in the simulation, we illustrate and estimate how large-scale winds in the Arctic help melt sea ice in summer. The nudging method has the potential to make Arctic climate attribution more tangible and to unravel the important physical processes underlying recent abrupt climate change in the Arctic. [ABSTRACT FROM AUTHOR]

The warm Arctic–cold continent pattern (WACC) of near-surface air temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents. Whether the existence of this pattern is due to variability of sea ice or is caused by atmospheric internal dynamics is subject to debate. Based on a long integration of a primitive equation atmospheric model (SGCM), this study examines the origin of the warm Arctic–cold North American pattern (WACNA), which is characterized by a pair of opposite surface air temperature anomalies over the high-latitude Chukchi–Bering Sea region and the North American continent, in boreal winter on the intraseasonal time scale. The model atmosphere is maintained by a time-independent forcing, so that atmospheric internal dynamics is the only source of variability. It is found that the SGCM model simulates well the behavior of the observed WACNA pattern. The WACNA pattern develops by interacting with the time-mean flow and synoptic-scale transient eddies. Two pathways of Rossby wave propagation are associated with WACNA. The northern pathway originates from eastern Siberia moving eastward across the Bering Strait to Canada, and the southern pathway is rooted in the subtropical waveguide propagating across the eastern North Pacific. Our simulation of this pattern implies that tropospheric dynamics alone can generate the WACNA, and the predictability associated with this pattern is likely limited by its internal dynamics nature. Significance Statement: The warm Arctic–cold continent pattern of temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents, which implies possible impacts of polar warming on midlatitude climate. There has been debate on whether the existence of this pattern depends on variability of sea ice or can be caused by processes within the atmosphere. In this study, we use a simple atmospheric model, which has a constant forcing; thus, atmospheric internal dynamics is the only source of variability. We show that atmospheric internal dynamics alone can generate the warm Arctic–cold North American pattern. The result has implications for our understanding of the impact of global warming. [ABSTRACT FROM AUTHOR]

Droughts and associated near-surface temperature anomalies can be attributed to amplified vertical subsidence and anomalous anticyclonic circulations from dynamic perspectives. However, two open and interesting issues remain unknown: 1) whether hydrometeorological situations under droughts can be reproduced directly utilizing variability of atmospheric dynamics and 2) what specific roles atmospheric dynamics play in drought reconstruction. To explore these questions, this study employs three kinds of dynamic features (i.e., vertical velocity, relative vorticity, and horizontal divergence) for hydrometeorological reconstruction (e.g., precipitation and near-surface air temperature) under drought situations through a so-called XGBoost (extreme gradient boosting) ensemble learning method. The study adopts two different reconstruction schemes (i.e., statistically preexisting dynamic–hydrometeorological relationships and interannual variability) and finds dynamically based reconstruction feasible. The three main achievements are as follows. 1) Regarding different hydrometeorological situations reconstructed with preexisting dynamic–hydrometeorological relationships, good reconstruction performance can be captured with the same or different lead times, depending on whether the evolution of dynamic anomalies (e.g., vertical motion and relative vorticity) is temporally asynchronous. 2) Reconstruction on the interannual scale performs relatively well, seemingly regardless of seasonality and drought-inducing mechanisms. 3) More importantly, from interpretable perspectives, global-scale analysis of dynamic contributions helps discover unexpected dynamic drought-inducing roles and associated latitudinal modulation. That is, low-level cyclonic/anticyclonic anomalies contribute to drought development in the northern middle and high latitudes, while upper-level vertical subsidence contributes significantly to tropical near-surface temperature anomalies concurrent with droughts. These achievements could provide guidance for dynamically based drought monitoring and prediction in different geographic regions. Significance Statement: It is common sense that severe drought events are physically attributable to amplified vertical subsidence and anomalous anticyclonic circulations. However, the specific contributions of atmospheric dynamics, together with the feasibility of dynamically based drought reconstruction, are crucial components that are seldom investigated. To our knowledge, this manuscript is the first to reconstruct drought utilizing atmospheric dynamics and to interpret quantified dynamic contributions; it also represents a new interdisciplinary attempt to reproduce hydrological variability based on routine atmospheric dynamic variables. [ABSTRACT FROM AUTHOR]

This study examines the evolution of the interannual warm Arctic–cold continents (WACC) pattern over the North American sector, which refers to the warm Arctic–cold North American pattern (WACNA), and explores its driving mechanism. WACNA features a pair of opposite surface air temperature anomalies centered over the Chukchi–Bering Seas and the North American Great Plains. A negative phase of the warm Arctic–cold Eurasia (WACE) pattern tends to lead a positive phase of the WACNA pattern by about 25 days. Negative Asian–Bering–North American (ABNA)- and Pacific–North American (PNA)-like atmospheric circulation patterns also appear upstream and precede a positive WACNA by about 25 days, gradually develop, reach their peaks when both circulation patterns lead the WACNA by 5 days, and weaken afterward. The negative ABNA-like pattern can be driven by the Siberian snow decline that is related to a negative WACE pattern and its featured Eurasian warming, whereas the negative PNA-like pattern is influenced by negative SST anomalies over the tropical central-eastern Pacific Ocean that resemble the tropical ENSO variability. The surface signatures of both patterns highlight a horseshoe-shaped high pressure anomaly straddling over the Gulf of Alaska, Alaska, and northwestern Canada. The anomalous warm advection from the North Pacific and cold advection from the Arctic that follow the circulation anomalies, as well as sea ice declines over the Chukchi–Bering Seas and growth over Hudson Bay, lead to the formation of the positive WACNA pattern. Processes with circulation anomalies of opposite signs will likewise lead to the negative WACNA pattern. [ABSTRACT FROM AUTHOR]

Record-breaking heatwaves and wildfires immersed Siberia during the boreal spring of 2020 following an anomalously warm winter. Springtime heatwaves are becoming more common in the region, with statistically significant trends in the frequency, magnitude, and duration of heatwave events over the past four decades. Mechanisms by which the heatwaves occur and contributing factors differ by season. Winter heatwave frequency is correlated with the atmospheric circulation, particularly the Arctic Oscillation, while the frequency of heatwaves during the spring months is highly correlated with aspects of the land surface including snow cover, albedo, and latent heat flux. Idealized AMIP-style experiments are used to quantify the contribution of suppressed Arctic sea ice and snow cover over Siberia on the atmospheric circulation, surface energy budget, and surface air temperature in Siberia during the winter and spring of 2020. Sea ice concentration contributed to the strength of the stratospheric polar vortex and Arctic Oscillation during the winter months, thereby influencing the tropospheric circulation and surface air temperature over Siberia. Warm temperatures across the region resulted in an earlier-than-usual recession of the winter snowpack. The exposed land surface contributed to up to 20% of the temperature anomaly during the spring through the albedo feedback and changes in the ratio of the latent and sensible heat fluxes. This, in combination with favorable atmospheric circulation patterns, resulted in record-breaking heatwaves in Siberia in the spring of 2020. [ABSTRACT FROM AUTHOR]

In this study, 2-m or near-surface air temperature (T2m) products from atmospheric reanalysis ERA5 and the regional climate model RACMO2.3p2 over Greenland are compared with observations from staffed stations and Automated Weather Stations (AWS). The results show the following: 1) Greenland experienced decadal periods of both cooling and warming during 1958–2020, with an inflection point around the mid-1990s, and no significant warming after ∼2005 except in the north and northeast. 2) In the full time series, the magnitude of the warming increases gradually from south to north, with peak warming found along the northeastern coast. 3) The most intense warming occurred in autumn and winter, notably in the northeast. 4) The correlations of T2m with the large-scale circulation indices NAO and GBI are highly significant, but they gradually weaken from southwestern to northeastern Greenland. Under the background of Greenland rapidly warming, the shift from positive to negative NAO (negative to positive GBI) is critical to the sudden warming in Greenland since the mid-1990s. [ABSTRACT FROM AUTHOR]

Interdecadal variations of the relationship between El Niño–Southern Oscillation (ENSO) and the Indo-China Peninsula (ICP) surface air temperature (SAT) in winter are investigated in the study. Generally, there exists a positive correlation between them during 1958–2015 because the ENSO-induced anomalous western North Pacific anticyclone (WNPAC) is conducive to pronounced temperature advection anomalies over the ICP. However, such correlation is unstable in time, having experienced a high-to-low transition around the mid-1970s and a recovery since the early 1990s. This oscillating relationship is owing to the anomalous WNPAC intensity in different decades. During the epoch of high correlation, the anomalous WNPAC and associated southwesterly winds over the ICP are stronger, which brings amounts of warm temperature advection and markedly heats the ICP. In contrast, a weaker WNPAC anomaly and insignificant ICP SAT anomalies are the circumstances for the epoch of low correlation. It is also found that substantial southwesterly wind anomalies over the ICP related to the anomalous WNPAC occur only when large sea surface temperature (SST) anomalies over the northwest Indian Ocean (NWIO) coincide with ENSO (viz., when the ENSO–NWIO SST connection is strong). The NWIO SST anomalies are capable of driving favorable atmospheric circulation that effectively alters ICP SAT and efficiently modulates the ENSO–ICP SAT correlation, which is further supported by numerical simulations utilizing the Community Atmospheric Model, version 4 (CAM4). This paper emphasizes the non-stationarity of the ENSO–ICP SAT relationship and also uncovers the underlying modulation factors, which has important implications for the seasonal prediction of the ICP temperature. [ABSTRACT FROM AUTHOR]

Despite the development of sophisticated statistical and dynamical climate models, a relative long-term and reliable prediction of the Indian summer monsoon rainfall (ISMR) has remained a challenging problem. Toward achieving this goal, here we construct a series of dynamical and physical climate networks based on the global near-surface air temperature field. We show that some characteristics of the directed and weighted climate networks can serve as efficient long-term predictors for ISMR forecasting. The developed prediction method produces a forecasting skill of 0.54 (Pearson correlation) with a 5-month lead time by using the previous calendar year's data. The skill of our ISMR forecast is better than that of operational forecasts models, which have, however, quite a short lead time. We discuss the underlying mechanism of our predictor and associate it with network–ENSO and ENSO–monsoon connections. Moreover, our approach allows predicting the all-India rainfall, as well as the rainfall different homogeneous Indian regions, which is crucial for agriculture in India. We reveal that global warming affects the climate network by enhancing cross-equatorial teleconnections between the southwest Atlantic, the western part of the Indian Ocean, and the North Asia–Pacific region, with significant impacts on the precipitation in India. A stronger connection through the chain of the main atmospheric circulations patterns benefits the prediction of the amount of rainfall. We uncover a hotspot area in the midlatitude South Atlantic, which is the basis for our predictor, the southwest Atlantic subtropical index (SWAS index). Remarkably, the significant warming trend in this area yields an improvement of the prediction skill. [ABSTRACT FROM AUTHOR]

The anticyclonic anomaly over the Ural Mountains, or the Ural high (UH), has recently received much attention as a factor related to weather anomalies across Eurasia. Here we studied how the UH affects the occurrence of cold wintertime episodes over eastern Europe and northern China. By employing three methods to identify the UH, we found that a method based on the sea level pressure anomaly captures a stronger cooling signal over eastern Europe and this method includes nonblocking cases associated with low-level anticyclones that do not affect the upper troposphere. However, with UH occurrence, a stronger cooling over northern China is detected by a method based on 500-hPa geopotential height fields. Cold events over eastern Europe typically occur when UH formation is associated with a Rossby wave breaking in the upper level. Our results show that the horizontal temperature advection plays an important role in formation of cold episodes in both eastern Europe and northern China. The advection is balanced by diabatic processes, which show an opposite sign to the temperature advection in both regions. Also adiabatic warming contributes to balancing the advection in northern China. We find that the exact location of the positive SLP anomaly during UH episodes is the most important factor controlling whether or not eastern Europe or northern China will experience a cold episode. If the positive SLP anomaly develops more northwest than usual, eastern Europe will experience a cold episode. When the anomaly moves eastward, northern China will be cold. [ABSTRACT FROM AUTHOR]

Long-term continuous monitoring of precipitation isotopes has great potential to advance our understanding of hydrometeorological processes that determine stable isotope variability in the monsoon regions. This study presents 4-yr daily precipitation isotopes from Yungui Plateau in southwestern China that are influenced by Indian summer monsoon and East Asian monsoon. The local meteoric water line (LMWL; δ2H = 8.12δ18O + 11.2) was first established at the Tengchong (TC) site, which was close to the global meteoric water line (GMWL; δ2H = 8δ18O + 10), indicating little secondary subcloud evaporation in the falling rain. Precipitation δ18O values exhibited significant inverse relationships with precipitation amount (r = −0.42), air temperature (r = −0.43), and relative humidity (r = −0.41) with lower correlation coefficients throughout the entire period, which indicated that precipitation isotopic variability in TC could not be well explained by the local meteorological factors but influenced by other combined factors of regional precipitation amount and upstream rainout. Precipitation δ18O values showed a clear V-shaped trend throughout the observation period, characterized by higher δ18O values during the premonsoon period whereas lower values during the postmonsoon period. This seasonal variation of precipitation δ18O values was associated with the seasonal movement of the intertropical convergence zone and seasonal changes in moisture transport. Combined with backward trajectory analysis, precipitation δ18O values were estimated by a Rayleigh distillation model showing that upstream rainout processes from the Bay of Bengal (BoB) toward land (Myanmar) and recycling moisture over land were key factors affecting the isotopic compositions of the TC precipitation. These findings could enhance our understanding of atmospheric dynamics and moisture source in the monsoon regions and will potentially facilitate the interpretation of numerous isotopic proxy records from this region. SIGNIFICANCE STATEMENT: The variability of the summer monsoon and its onset, duration, and failure directly determine the strong rainfall and drought in a given region and have great impacts on regional societies and agriculture. To better understand this variability, this study presented a 4-yr daily dataset of precipitation isotopes on the Yungui Plateau of southwestern China to explore atmospheric processes and moisture sources that drive isotopic variability in this region. Precipitation δ18O exhibited remarkably seasonal variability, with higher values in premonsoon period and lower values in the postmonsoon period. During the Indian summer monsoon period, moisture sources primarily originated from the BoB toward the TC site, experiencing rainout processes and local moisture recycling over land using a Rayleigh fractionation model. These findings shed new light on the temporal variations of precipitation stable isotopes and facilitate our understanding of hydrological cycle in the monsoon regions. [ABSTRACT FROM AUTHOR]

To better understand the dynamics and impacts of blocking events, their 3D structure needs to be further investigated. We present a comprehensive composite analysis of the 3D structure of blocks and its response to future climate change over the North Pacific, the North Atlantic, and Russia in summers and winters using reanalysis and two large-ensemble datasets from CESM1 and GFDL-CM3. In reanalysis, over both ocean and land, the anomalous winds are equivalent-barotropic in the troposphere and stratosphere, and temperature anomalies are positive throughout the troposphere and negative in the lower stratosphere. The main seasonal and regional differences are that blocks are larger and/or stronger in winters; over oceans, the temperature anomaly is shifted westward due to latent heating. Analyzing the temperature tendency equation shows that in all three sectors, adiabatic warming due to subsidence is the main driver of the positive temperature anomaly; however, depending on season and region, meridional thermal advection and latent heating might have leading-order contributions too. Both GCMs are found to reproduce the climatological 3D structure remarkably well, but sometimes disagree on future changes. Overall, the future summertime response is weakening of all fields (except for specific humidity), although the impact on near-surface temperature is not necessarily weakened; for example, the blocking-driven near-surface warming over Russia intensifies. The wintertime response is strengthening of all fields, except for temperature in some cases. Responses of geopotential height and temperature are shifted westward in winters, most likely due to latent heating. Results highlight the importance of process-level analyses of blocks' 3D structure for improved understanding of the resulting temperature extremes and their future changes. [ABSTRACT FROM AUTHOR]

As a result of the high mountains to the west and north of the plateau and the control by westerly mean flow in spring, hot and dry conditions are often observed over the southeastern edge of the Tibetan Plateau (SETP), favoring occurrences of extreme heat events there. Indeed, maximum centers and remarkable increasing trends of extreme heat (EH) days in spring are found over the region. Springtime EH events over the SETP also exhibit strong interannual variability and are closely linked to a spring-type circumglobal teleconnection (SCGT) pattern, which is the second leading mode of 200-hPa meridional wind over the North Hemisphere in spring. This SCGT shows distinctive features from the traditional circumglobal teleconnection patterns found in boreal summer and winter. It manifests as a circumglobally navigated Rossby wave train along the mid–high latitudes, which splits to a north branch along the polar jet and a south branch along the subtropical jet over Eurasia after propagating through the North Atlantic Ocean. The two branches eventually reach the SETP, forming an anomalous anticyclonic circulation over the region. Hence, conditions in the SETP are controlled by significant anomalous subsidence and a clearer sky, resulting in below-normal rainfall and above-normal air temperature, favoring more EH events in the region. The SETP EH events are also closely linked to the spring-type CGT-like pattern in April and May but not in March. In addition, the influence of the foehn effect on the SETP EH is discussed. [ABSTRACT FROM AUTHOR]

Superimposed on a warming trend, Arctic winter surface air temperature (SAT) exhibits substantial interannual variability, the underlying mechanisms of which are unclear, especially with regard to the role of sea ice variations and atmospheric processes. Here, atmospheric reanalysis data and idealized atmospheric model simulations are used to reveal the mechanisms by which sea ice variations and atmospheric anomalous conditions affect interannual variations in wintertime Arctic SAT. Results show that near-surface interannual warming in the Arctic is accompanied by comparable warming throughout large parts of the Arctic troposphere and large-scale anomalous atmospheric circulation patterns. Within the Arctic, changes in large-scale atmospheric circulations due to internal atmospheric variability explain a substantial fraction of interannual variation in SAT and tropospheric temperatures, which lead to an increase in moisture and downward longwave radiation, with the rest likely coming from sea ice–related and other surface processes. Arctic winter sea ice loss allows the ocean to release more heat and moisture, which enhances Arctic warming; however, this effect on SAT is confined to the ice-retreat area and has a limited influence on large-scale atmospheric circulations. [ABSTRACT FROM AUTHOR]

Glacier changes on the Tibetan Plateau (TP) have been spatially heterogeneous in recent decades. The understanding of glacier mass changes in western Tibet, a transitional area between the monsoon-dominated region and the westerlies-dominated region, is still incomplete. For this study, we used an energy–mass balance model to reconstruct annual mass balances from October 1967 to September 2019 to explore the effects of local climate and large-scale atmospheric circulation on glacier mass changes in western Tibet. The results showed that Xiao Anglong Glacier is close to a balanced condition, with an average value of −53 ± 185 mm water equivalent (w.e.) yr−1 for 1968–2019. The interannual mass balance variability during 1968–2019 was primary driven by ablation-season precipitation, which determined changes in the snow accumulation and strongly influenced melt processes. The interannual mass balance variability during 1968–2019 was less affected by ablation-season air temperature, which only weakly affected snowfall and melt energy. Further analysis suggests that the southward (or northward) shift of the westerlies caused low (or high) ablation-season precipitation, and therefore low (or high) annual mass balance for glaciers in western Tibet. In addition, the average mass balance for Xiao Anglong Glacier was 83 ± 185, −210 ± 185, and −10 ± 185 mm w.e. yr−1 for 1968–90, 1991–2012, and 2013–19, respectively. These mass changes were associated with the variations in precipitation and air temperature during the ablation season on interdecadal time scales. [ABSTRACT FROM AUTHOR]

Using a high-resolution atmospheric general circulation model simulation of unprecedented ensemble size, we examine potential predictability of monthly anomalies under El Niño–Southern Oscillation (ENSO) forcing and background internal variability. This study reveals the pronounced month-to-month evolution of both the ENSO forcing signal and internal variability. Internal variance in upper-level geopotential height decreases (~10%) over the North Pacific during El Niño as the westerly jet extends eastward, allowing forced signals to account for a greater fraction of the total variability, and leading to increased potential predictability. We identify February and March of El Niño years as the most predictable months using a signal-to-noise analysis. In contrast, December, a month typically included in teleconnection studies, shows little to no potential predictability. We show that the seasonal evolution of SST forcing and variability leads to significant signal-to-noise relationships that can be directly linked to both upper-level and surface variable predictability for a given month. The stark changes in forced response, internal variability, and thus signal-to-noise across an ENSO season indicate that subseasonal fields should be used to diagnose potential predictability over North America associated with ENSO teleconnections. Using surface air temperature and precipitation as examples, this study provides motivation to pursue "windows of forecast opportunity" in which statistical skill can be developed, tested, and leveraged to determine times and regions in which this skill may be elevated. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between the central tropical Pacific (CTP) sea surface temperature (SST) and the surface air temperature (SAT) variability unrelated to canonical El Niño–Southern Oscillation (ENSO) over mid–high-latitude Eurasia during boreal summers over the past half-century. The results show that their relationship experienced a decadal shift around the early 1980s. Before the early 1980s, the Eurasian SAT–CTP SST connection was weak; after that time, the relationship became stronger, and the SAT anomalies exhibited a significant wavelike pattern over Eurasia. Such a decadal change in the Eurasian SAT–CTP SST relationship could be attributed to decadal changes in the mean state and variability of CTP SST. The warmer mean state and enhanced SST variability after the early 1980s reinforced the convective activities over the tropical Pacific, leading to significantly anomalous divergence/convergence and Rossby wave sources over the North Pacific. This outcome further excited the wave train propagating along the Northern Hemisphere zonal jet stream to northern Eurasia and then affected the surface heat fluxes and atmospheric circulations over the region, resulting in wavelike SATs over Eurasia. However, during the period before the early 1980s, the CTP SST had a weak impact on the North Pacific atmospheric circulation and was consequently not able to excite the wave train pattern to impact the Eurasian atmospheric circulation and SATs. The physical processes linking the CTP SST and Eurasian SAT are further confirmed by numerical simulations. The results of this study are valuable to understanding the variability of summer Eurasian SATs. [ABSTRACT FROM AUTHOR]

Decadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability. [ABSTRACT FROM AUTHOR]

The summer British–Baikal Corridor pattern (BBC) and the Silk Road pattern (SRP) manifest as zonally oriented teleconnections in the high and middle latitudes, respectively, of the Eurasian continent. In this study, we investigate the combined effects of the BBC and SRP on surface air temperatures over the Eurasian continent. It is found that the combination of the BBC and SRP results in two kinds of well-organized, large-scale circulation anomalies: the zonal tripole pattern and the Ω-like pattern in the 200-hPa geopotential height anomalies. The zonal tripole pattern is characterized by opposite variations between western Siberia/western Asia and Europe/central Asia/central Siberia, and the Ω-like pattern manifests as consistent variations over midlatitude Europe, western Siberia, and central Asia. Correspondingly, the resultant large-scale surface air temperature anomalies feature the same zonal tripole pattern and Ω-like pattern, respectively. Further results indicate that these two patterns resemble the two leading modes of surface air temperature anomalies over the middle to high latitudes of Eurasia. This study indicates that the temperature variations in the middle and high latitudes of Eurasia can be coordinated and evidently explained by the combination of the BBC and SRP, and it contributes to a more comprehensive understanding of the large-scale Eurasian climate variability. [ABSTRACT FROM AUTHOR]

Atmospheric circulation associated with the Arctic dipole (AD) pattern plays a crucial role in modulating the variations of summertime sea ice concentration (SIC) within the Pacific Arctic sector (PAS). Based on reanalysis data and satellite observations, we found that the impacts of atmospheric circulation associated with a positive AD (AD+) on SIC change over different regions of the PAS [including the East Siberian Sea (ESS), Beaufort and Chukchi Seas (BCS), and Canadian Arctic Archipelago (CAA)] are dependent on the phase shifts of Pacific decadal oscillation (PDO). Satellite observations reveal that SIC anomalies, influenced by AD+ during PDO− relative to that during PDO+, varies significantly in summer by 4.9%, −7.3%, and −6.4% over ESS, BCS, and CAA, respectively. Overall, the atmospheric anomalies over CAA and BCS in terms of specific humidity, air temperature, and thereby downward longwave radiation (DLR), are enhanced (weakened) in the atmospheric conditions associated with AD+ during PDO− (PDO+). In these two regions, the larger (smaller) increases in specific humidity and air temperature, associated with AD+ during PDO− (PDO+), are connected to the increased (decreased) poleward moisture flux, strengthened (weakened) convergence of moisture and heat flux, and in part to adiabatic heating. As a consequence, the DLR and surface net energy flux anomalies over the two regions are reinforced in the atmospheric scenarios associated with AD+ during PDO− compared with that during PDO+. Therefore, smaller SIC anomalies are identified over CAA and BCS in the cases related to AD+ during PDO− than during PDO+. Essentially, the changes of the DLR anomaly in CAA and BCS are in alignment with geopotential height anomalies, which are modulated by the anticyclonic circulation pattern in association with AD+ during varying PDO phases. In contrast, the SIC changes over ESS is primarily attributed to the variations in mechanical wind forcing and sea surface temperature (SST) anomalies. The cloud fraction anomalies associated with AD+ during different PDO phases are found not to be a significant contributor to the variations of sea ice anomaly in the studied regions. Given the oscillatory nature of PDO, we speculate that the recent shift to the PDO+ phase may temporarily slow the observed significant decline trend of the summertime SIC within PAS of the Arctic. [ABSTRACT FROM AUTHOR]

The wintertime (December–February) 1990–2016 Arctic surface air temperature (SAT) trend is examined using self-organizing maps (SOMs). The high-dimensional SAT dataset is reduced into nine representative SOM patterns, with each pattern exhibiting a decorrelation time scale of about 10 days and having about 85% of its variance coming from intraseasonal time scales. The trend in the frequency of occurrence of each SOM pattern is used to estimate the interdecadal Arctic winter warming trend associated with the SOM patterns. It is found that trends in the SOM patterns explain about one-half of the SAT trend in the Barents and Kara Seas, one-third of the SAT trend around Baffin Bay, and two-thirds of the SAT trend in the Chukchi Sea. A composite calculation of each term in the thermodynamic energy equation for each SOM pattern shows that the SAT anomalies grow primarily through the advection of the climatological temperature by the anomalous wind. This implies that a substantial fraction of Arctic amplification is due to horizontal temperature advection that is driven by changes in the atmospheric circulation. An analysis of the surface energy budget indicates that the skin temperature anomalies as well as the trend, although very similar to that of the SAT, are produced primarily by downward longwave radiation. [ABSTRACT FROM AUTHOR]

The variability of spring snow cover over Eurasia can have notable impacts on the current and following season climate, but the causes of it are poorly understood. This study investigates the potential drivers and the associated physical processes for the first two empirical orthogonal function (EOF) modes of the Eurasian spring snow-cover variability during 1967–2018, which are characterized by a continent-wide coherent pattern and a west–east dipole structure, respectively. Analyses show that the spring surface air temperature and snowfall are the direct factors influencing the two modes. We further examined the contributions to the snow-cover variability of atmospheric teleconnection patterns, sea surface temperature (SST) anomalies, and variations of Arctic sea ice during spring. The results indicate that circulation anomalies associated with the Arctic Oscillation, Polar–Eurasian pattern, and West Pacific pattern can partly explain the formation of the EOF1 mode, while the EOF2 mode has a close relationship with the East Atlantic–Western Russia pattern. In addition, a horseshoe-like monopole structure of SST anomalies over the North Atlantic plays an important role in regulating the EOF2 mode by inducing a wave train circulation. Moreover, the EOF2 mode is also affected by anomalous circulations induced by the sea ice anomalies in the Barents–Kara Seas. An empirical model using these drivers satisfactorily reproduced the temporal variations of the two EOF modes, implying that our results can substantially improve comprehension of the variability of Eurasian spring snow cover. [ABSTRACT FROM AUTHOR]

The dominant mode of wintertime interdecadal covariability between subseasonal surface air temperature (SAT) variability and midtropospheric circulation over the North Atlantic sector is identified through maximum covariance analysis applied to century-long reanalysis data. This mode highlights a tendency for subseasonal temperature variability over Europe and eastern North America to be enhanced during decades when the negative phase of the North Atlantic Oscillation (NAO) prevails. This interdecadal NAO is characterized by a stationary Rossby wave train that originates from the subtropical Atlantic, propagates northward into the subpolar Atlantic, and finally refracts toward Europe and the Middle East. A decadal increase in precipitation in the subtropics under the enhanced supply of heat and moisture from the Gulf Stream and its surroundings appears to act as a source for this wave train. The influence of the interdecadal NAO on subseasonal SAT variability is explained primarily by the modulated efficiency of baroclinic conversion of available potential energy from the background atmospheric flow to subseasonal eddies. The combination of enhanced subseasonal variability and low winter-mean temperature anomalies associated with the negative phase of the interdecadal NAO increases the frequency of cold extremes affecting Europe and the eastern United States. [ABSTRACT FROM AUTHOR]

This study reveals a pronounced out-of-phase relationship between surface air temperature (SAT) anomalies over northeast Eurasia in boreal winter and the following summer during 1980–2017. A colder (warmer) winter over northeast Eurasia tends to be followed by a warmer (cooler) summer of next year. The processes for the out-of-phase relation of winter and summer SAT involve the Arctic Oscillation (AO), the air–sea interaction in the North Atlantic Ocean, and a Eurasian anomalous atmospheric circulation pattern induced by the North Atlantic sea surface temperature (SST) anomalies. Winter negative AO/North Atlantic Oscillation (NAO)-like atmospheric circulation anomalies lead to continental cooling over Eurasia via anomalous advection and a tripolar SST anomaly pattern in theNorth Atlantic. The North Atlantic SST anomaly pattern switches to a dipolar pattern in the following summer via air–sea interaction processes and associated surface heat flux changes. The summer North Atlantic dipolar SST anomaly pattern induces a downstream atmospheric wave train, including large-scale positive geopotential height anomalies over northeast Eurasia, which contributes to positive SAT anomalies there via enhancement of downward surface shortwave radiation and anomalous advection. Barotropic model experiments verify the role of the summer North Atlantic SST anomalies in triggering the atmospheric wave train over Eurasia. Through the above processes, a colder winter is followed by a warmer summer over northeast Eurasia. The above processes apply to the years when warmer winters are followed by cooler summers except for opposite signs of SAT, atmospheric circulation, and SST anomalies. [ABSTRACT FROM AUTHOR]

Rossby wave propagation along the Asian jet during boreal summer, such as the Silk Road pattern, frequently causes wave breaking near the Asian jet exit region. This study examines the statistical relationship between interannual variability of the Rossby wave breaking frequency near Japan and large-scale atmospheric circulation during the boreal summer. The Rossby wave breaking frequency in the midlatitudes climatologically shows its maximum near Japan, and significantly increases during La Niña years. The uppertropospheric circulation regressed onto the Rossby wave breaking frequency near Japan in August shows large-scale anomalous convergence from the tropical central to eastern Pacific and divergence around the Indian Ocean. The consequent northward anomalous divergent wind over Eurasia contributes to enhancement and northward shift of the Asian jet. The Asian jet also shows meridional meandering with a phase of anomalous anticyclonic circulation near Japan accompanied by the frequent Rossby wave breaking, which is associated with the Silk Road pattern. The frequent Rossby wave breaking is related to southwestward intrusion of anomalous low potential temperature air mass toward the subtropical western North Pacific associated with an enhanced mid-Pacific trough. West of the southwestward cold-air intrusion, enhanced cumulus convection is seen around the northern Philippines, and the Pacific–Japan pattern is significantly seen in the lower troposphere. This result is consistent with a previous study that revealed a linkage mechanism between the Rossby wave breaking near Japan and the Pacific–Japan pattern through dynamically induced ascent resulting in an intrusion of high potential vorticity associated with the Rossby wave breaking. [ABSTRACT FROM AUTHOR]

Surface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, fromwinter to spring. The combined circulation anomalies result inSATwarming overmost of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to themaintenance of the NorthAtlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) inwinter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes. [ABSTRACT FROM AUTHOR]

The Northern Hemisphere (NH) has experienced winter Arctic warming and continental cooling in recent decades, but the dominant patterns in winter surface air temperature (SAT) are not well understood. Here, a self-organizing map (SOM) analysis is performed to identify the leading patterns in winter daily SAT fields from 1979 to 2018, and their associated atmospheric and ocean conditions are also examined. Three distinct winter SAT patterns with two phases of nearly opposite signs and a time scale of 7–12 days are found: one pattern exhibits concurrent SAT anomalies of the same sign over North America (NA) and northern Eurasia, while the other two patterns show SAT anomalies of opposite signs between, respectively, NA and the Bering Sea, and the Kara Sea and East Asia (EA). Winter SAT variations may arise from changes in the SOM frequencies. Specifically, the observed increasing trends of winter cold extremes over NA, central Eurasia, and EA during 1998–2013 can be understood as a result of the increasing occurrences of some specific SAT patterns. These SOMs are closely related to poleward advection of midlatitude warm air and equatorward movements of polar cold airmass. These meridional displacements of cold and warm airmasses cause concurrent anomalies over different regions not only in SAT but also in water vapor and surface downward longwave radiation. Anomalous sea surface temperatures in the tropical Pacific, midlatitude North Pacific, and North Atlantic and anomalous Arctic sea ice concentrations also concur to support and maintain the anomalous atmospheric circulation that causes the SAT anomalies. [ABSTRACT FROM AUTHOR]

Previous studies revealed the influence of the autumn–winter Tibetan Plateau snow cover on atmospheric circulation and climate in the North American region. The present study documents the relationship between the eastern Tibetan Plateau snow cover and the North American air temperature in spring and the associated physical processes using satellite-based snow cover, reanalysis atmospheric and surface variables, observation-based surface air temperature (SAT), and sea surface temperature (SST). A stable relationship is identified between the eastern Tibetan Plateau snow cover and the North American SAT in spring before the mid-2000s. Positive snow-cover anomalies over the eastern Tibetan Plateau induce cooling in the local atmospheric column. The atmospheric cooling stimulates a large-scale atmospheric wave pattern at the upper level that extends northeastward from the eastern Tibetan Plateau via northeast Asia and the North Pacific to North America. An anomalous high forms over North America, accompanied by anomalous descent. In the northwestern part, the horizontal advection by anomalous southerly winds along the west flank of anomalous anticyclone induces SAT increase. In the central part, the enhanced surface sensible heat flux following anomalous descent-induced downward shortwave radiation increase leads to SAT increase. The relationship between the eastern Tibetan Plateau snow cover and the North American SAT is weakened after the mid-2000s. The weakened relationship is attributed to an intensified impact of tropical central Pacific SST anomalies on the North American SAT variations through a Pacific–North America-like atmospheric circulation pattern, which overcomes the influence of the Tibetan Plateau snow-cover anomalies. [ABSTRACT FROM AUTHOR]

We analyze the temporal structure of the climatological seasonal cycle in surface air temperature across the globe. We find that, over large regions of Earth, the seasonal cycle of surface temperature departs from an annual harmonic: the duration of fall and spring differ by as much as 2 months. We characterize this asymmetry by the metric ASYM, defined as the phase lag of the seasonal maximum temperature relative to the summer solstice minus the phase lag of the seasonal minimum temperature relative to winter solstice. We present a global analysis of ASYM from weather station data and atmospheric reanalysis and find that ASYM is well represented in the reanalysis. ASYM generally features positive values over land and negative values over the ocean, indicating that spring has a longer duration over the land domain whereas fall has a longer duration over the ocean. However, ASYM also features more positive values over North America compared to Europe and negative values in the polar regions over ice sheets and sea ice. Understanding the root cause of the climatological ASYM will potentially further our understanding of controls on the seasonal cycle of temperature and its future/past changes. We explore several candidate mechanisms to explain the spatial structure of ASYM including 1) modification of the seasonal cycle of surface solar radiation by the seasonal evolution of cloud thickness, 2) differences in the seasonal cycle of the atmospheric boundary layer depth over ocean and over land, and 3) temperature advection by the seasonally evolving atmospheric circulation. [ABSTRACT FROM AUTHOR]

Global warming projections show an anomalous temperature increase both at the Arctic surface and at lower latitudes in the upper troposphere. The Arctic amplification decreases the meridional temperature gradient, and simultaneously decreases static stability. These changes in the meridional temperature gradient and in the static stability have opposing effects on baroclinicity. The temperature increase at the upper tropospheric lower latitudes tends to increase the meridional temperature gradient and simultaneously increase static stability, which have opposing effects on baroclinicity as well. In this study, a dry idealized general circulation model with a modified Newtonian cooling scheme, which allows any chosen zonally symmetric temperature distribution to be simulated, is used to study the effect of Arctic amplification and lower-latitude upper-level warming on eddy activity. Due to the interplay between the static stability and meridional temperature gradient on atmospheric baroclinicity changes, and their opposing effect on atmospheric baroclinicity, it is found that both the Arctic amplification and lower-latitude upper-level warming could potentially lead to both decreases and increases in eddy activity, depending on the exact prescribed temperature modifications. Therefore, to understand the effect of global warming–like temperature trends on eddy activity, the zonally symmetric global warming temperature projections from state-of-the-art models are simulated. It is found that the eddy kinetic energy changes are dominated by the lower-latitude upper-level warming, which tends to weaken the eddy kinetic energy due to increased static stability. On the other hand, the eddy heat flux changes are dominated by the Arctic amplification, which tends to weaken the eddy heat flux at the lower levels. [ABSTRACT FROM AUTHOR]

The lake ice phenology response to climate change has been receiving growing concern in recent years. However, most studies have put emphasis on the spatial and temporal variability of lake ice phenology, and relatively few studies have been devoted to investigating the physical mechanisms of changes in lake ice phenology from the perspective of climatic dynamics. This study investigates the possible impact of the Antarctic Oscillation (AAO) on the variations in lake ice phenology over the Tibetan Plateau (TP). The results show that there is an intimate relationship between the AAO and the variations in break-up/ice duration during the period 2003-15. Further analysis indicates that the preceding boreal spring AAO-induced atmospheric circulation anomalies are favorable for generating tropical South Atlantic Ocean SST anomalies through air-sea interaction. Then the tropical SST anomalies strengthen the anomalous local-scale meridional-vertical circulation that projects into the Azores high and further induce the extratropical portion of the North Atlantic SST tripole. The anomalous warm core in the North Atlantic serves as the source of wave activity flux and stimulates a stationary wave train along the Eurasian continent to change the downstream atmospheric circulation. As a response, an abnormal cyclone and enhanced updraft are triggered over the TP, which are favorable for the formation of snowfall and then lower the surface air temperature according to the snow-albedo feedback mechanism, and thus result in the prolonged lake ice duration events. This study provides a new insight to link the AAO influence and climate over the TP and is helpful to understand the changes in lake ice phenology in response to climate change in recent years. [ABSTRACT FROM AUTHOR]

Composite analysis is used to examine the physical processes that drive the growth and decay of the surface air temperature anomaly pattern associated with the North Atlantic Oscillation (NAO). Using the thermodynamic energy equation that the European Centre for Medium-Range Weather Forecasts implements in their reanalysis model, we show that advection of the climatological temperature field by the anomalous wind drives the surface air temperature anomaly pattern for both NAO phases. Diabatic processes exist in strong opposition to this temperature advection and eventually cause the surface air temperature anomalies to return to their climatological values. Specifically, over Greenland, Europe, and the United States, longwave heating/cooling opposes horizontal temperature advection while over northern Africa vertical mixing opposes horizontal temperature advection. Despite the pronounced spatial correspondence between the skin temperature and surface air temperature anomaly patterns, the physical processes that drive these two temperature anomalies associated with the NAO are found to be distinct. The skin temperature anomaly pattern is driven by downward longwave radiation whereas stated above, the surface air temperature anomaly pattern is driven by horizontal temperature advection. This implies that the surface energy budget, although a useful diagnostic tool for understanding skin temperature changes, should not be used to understand surface air temperature changes. [ABSTRACT FROM AUTHOR]

This article presents the "screen-level and surface analysis only" (SSA) system at the European Centre for Medium-Range Weather Forecasts (ECMWF). SSA is a simplification of the operational land–atmosphere weakly coupled data assimilation (WCDA). The goal of SSA is to provide 1) efficient research into land surface developments in NWP and 2) land reanalyses with land–atmosphere coupling. SSA maintains a coupled forecast model between assimilation cycles, but the atmospheric analysis is not performed; rather, it is forced from an archived analysis. Hence, SSA is much faster than WCDA, although it lacks feedback between the land and atmospheric analyses. A global sensitivity analysis was performed over one year to compare the WCDA and SSA systems. Prescribed proxy 2-m temperature/humidity screen-level observation errors were approximately doubled in the soil moisture data assimilation, thereby reducing the average size of the root-zone soil moisture analysis increments by about 60%. The systematic impact of these changes on the WCDA surface and near-surface atmospheric dynamics was effectively captured by SSA, although the short-term impact was underestimated. Importantly, the SSA forecast verification scores accurately reflected those of WCDA: atmospheric 1–10-day temperature/humidity forecasts were degraded in the tropics and lower midlatitudes up to about 700 hPa. The soil moisture analysis performance was not significantly impacted. These results endorse SSA as an NWP research tool and confirm the role of assimilating proxy screen-level observations in the soil moisture analysis to improve weather forecasts. Appropriate use and limitations of SSA are considered. [ABSTRACT FROM AUTHOR]