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This study reveals that haze pollution (HP) over the North China Plain (NCP) in winter can persist to following spring during most years. The persistence of HPNCP is attributed to maintenance of an anticyclonic anomaly (AA) over northeast Asia and southerly wind anomalies over the NCP. Southerly wind anomalies over the NCP reduce surface wind speed and increase relative humidity, which are conducive to above-normal HPNCP both in winter and spring. However, there exist several years when above-normal HPNCP in winter are followed by below-normal HPNCP in the following spring. The reversed HPNCP in winter and spring in these years is due to the inverted atmospheric anomalies over northeast Asia. In particular, AA over northeast Asia in winter is replaced by a cyclonic anomaly (CA) in the following spring. The resultant spring northerly wind anomalies over NCP are conducive to below-normal HPNCP. These two distinctive evolutions of HPNCP and atmospheric anomalies over northeast Asia from winter to spring are attributed to the different evolutions of sea surface temperature anomalies (SSTA) in the North Atlantic. In the persistent years, warm North Atlantic SSTA in winter maintains to following spring via positive air-sea interaction process and induces a negative spring North Atlantic Oscillation (NAO)-like pattern, which contributes to the AA over northeast Asia via atmospheric wave train. By contrast, in the reverse years, cold SSTA in the North Atlantic is maintained from winter to spring, which induces a positive spring NAO-like pattern and leads to CA over northeast Asia via atmospheric wave train. The findings suggest that North Atlantic SSTA plays crucial roles in modulating the distinct evolutions of HPNCP from winter to succedent spring, which can be served as an important preceding signal for haze pollution evolution over the North China Plain. Keywords: Evolution of Haze pollution; North China Plain; North Atlantic sea surface temperature; North Atlantic Oscillation; Atmospheric circulation [ABSTRACT FROM AUTHOR]

This study examines the relationship between the spring northern tropical Atlantic (NTA) sea surface temperature (SST) and its following summer western North Pacific subtropical high (WNPSH) in historical simulations of 20 coupled models of the IPCC AR6. These models well simulate the spring NTA SST, as well as the WNPSH‐related atmospheric and precipitation anomalies over the Asian monsoon region. Ensemble mean of these models reproduces the observed relation and processes linking the spring NTA SST to the summer WNPSH. In the ensemble mean, spring NTA SST warming persists to the following summer, and induces an anomalous Walker circulation with an ascent over the tropical Atlantic and a descent over the tropical central Pacific. The associated precipitation decrease over the tropical central Pacific induces an anomalous low‐level anticyclone over the WNP via a Rossby wave atmospheric response. Meanwhile, spring NTA SST warming leads to easterly wind anomalies over the Indian Ocean (IO) via a Kelvin wave atmospheric response and causes SST warming there via reducing wind speeds. Then, the precipitation anomalies induced by the IO SST warming result in an enhancement of the WNPSH. There exists a large diversity of the NTA SST‐WNPSH relationship among the models. This diversity is related to the difference in the climatological mean IO precipitation. The models with large mean IO precipitation provide a favourable condition for the IO SST warming to bring large local precipitation anomalies and trigger strong easterly wind anomalies over the tropical WNP, and thus exert a strong impact on the WNPSH. [ABSTRACT FROM AUTHOR]

Observations indicate that late-summer precipitation over the East Asian transitional climate zone (TCZ) showed a pronounced decreasing trend during 1951–2005. This study examines the relative contributions of anthropogenic [including anthropogenic aerosol (AA) and greenhouse gas (GHG)] and natural forcings to the drying trend of the East Asian TCZ based on simulations from CMIP5. The results indicate that AA forcing plays a dominant role in contributing to the drying trend of the TCZ. AA forcing weakens the East Asian summer monsoon via reducing the land-sea thermal contrast, which induces strong low-level northerly anomalies over eastern China, suppresses water vapor transport from southern oceans and results in drier conditions over the TCZ. In contrast, GHG forcing leads to a wetting trend in the TCZ by inducing southerly wind anomalies, thereby offsetting the effect of the AA forcing. Natural forcing has a weak impact on the drying trend of the TCZ due to the weak response of atmospheric anomalies. [ABSTRACT FROM AUTHOR]

Significant uncertainties exist in climate change projections, and these mainly originate from the internal variability of the climate system. Using 40-member ensemble projections of the National Center for Atmospheric Research Community Climate System Model Version 3, we explored the uncertainties in the projected change of the Northeast Asian summer precipitation during 2006–2060 and examined the sources of the internal variability-generated uncertainties. The projected summer precipitation trends show large diversities from member to member, attributable to the superposition of the internal climate variability on the external forcing. Except for Northeast China, the signal-to-noise ratios (SNRs) are less than one, indicating the internally induced precipitation trends dominate the externally forced trends over large parts of Northeast Asia. The first mode of the internally induced precipitation trends displays widespread negative trends over Northeast Asia, and the second mode is characterized by a dipole trend pattern between Northeast China and Siberia. The atmospheric circulation trend patterns related to the first and second modes bear resemblances to those related to the Arctic Oscillation and Polar-Eurasian pattern, respectively. A dynamical adjustment was further performed on the summer precipitation trends to reduce the impact of the internal atmospheric variability. After the dynamical adjustment, the level of similarity in the residual precipitation trends across the 40 ensemble members increased significantly. In particular, the SNRs increased notably, with values larger than one over a vast area of Northeast Asia, which was in sharp contrast to values larger than one only being noticeable within Northeast China prior to the adjustment. [ABSTRACT FROM AUTHOR]

The first two leading modes of the North Pacific atmospheric variability, the Aleutian Low (AL) and North Pacific Oscillation (NPO), in boreal winter and their relations to the North American and Eurasian surface temperature, El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Victoria Mode (VM) are explored in 20 coupled climate models which participated in the sixth Assessment Report of the Intergovernmental Panel on Climate Change. The historical simulations of these models can well reproduce spatial structures and amplitudes of the winter AL and NPO, as well as their associations with North American and Eurasian surface air temperatures. The close connections of the winter AL with ENSO and PDO, as well as the linkage between the NPO and VM could also be well simulated. However, most of the models lack the capability in simulating the impact of the winter ENSO on the NPO. This deficiency is mainly attributed to westward shifts of the ENSO-related sea surface temperature and precipitation anomalies in the tropics and ENSO-induced atmospheric teleconnections over the North Pacific in the models. Spread in the ENSO's amplitude also contributes partly to the diversity of the ENSO–NPO relation among the models. Under the SSP2-RCP4.5 forced climate change projection, projected changes in the amplitudes and centers of the AL and NPO exhibit large uncertainties across the 20 models. The close connections of the AL with ENSO and PDO, and the NPO with VM are still robust in the warming climate. Most models project an increase (a decrease) in the AL–PDO (NPO–VM) relationship. However, there exists a large uncertainty in the projected changes of the AL–ENSO relationship, which is partly attributed to the large divergence in the projected changes of the ENSO's amplitude among the models. [ABSTRACT FROM AUTHOR]

Previous studies suggested that autumn Arctic sea ice concentration (SIC) change around the Kara–Laptev seas has a marked impact on the following winter Siberian High (SH). This analysis reveals that the autumn Arctic SIC‐winter SH connection experienced a marked interdecadal weakening around the late‐1990s according to observational and reanalysis data. Before the late‐1990s, autumn Arctic SIC loss induces pronounced Arctic warming via modulating surface heat fluxes and reduces poleward meridional temperature gradient, which leads to weakening of westerly winds over mid‐latitude Eurasia and contributes to formation of a high‐pressure ridge around Ural mountain. The Ural high‐pressure ridge provides a favourable dynamical condition for the amplification of SH. Consequently, the autumn Arctic SIC‐winter SH connection is strong. In addition, the North Atlantic is dominated by a marked triple sea surface temperature anomaly pattern from autumn to winter, which also partly contributes to a strengthened SH via modulating the Ural high‐pressure ridge. After the late‐1990s, Arctic warming and atmospheric anomalies over Eurasia related to the autumn Arctic SIC loss are much weaker. Accordingly, the autumn Arctic SIC‐winter SH connection is very weak. Changes in the autumn Arctic SIC‐related Arctic warming and atmospheric anomalies are suggested to be related to differences in the autumn Arctic SIC anomalies, with a much weaker amplitude and northward retreat after than before the late‐1990s. Further analysis indicates that the weaker autumn Arctic SIC anomalies over Kara–Laptev seas after the late‐1990s is related to the northward retreat of the mean Arctic SIC. [ABSTRACT FROM AUTHOR]

This study evaluates the ability of 35 climate models, which participate in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical climate simulations, in reproducing the connection between boreal spring Arctic Oscillation (AO) and its following winter El Niño–Southern Oscillation (ENSO). The spring AO–winter ENSO correlations range from − 0.41 to 0.44 among the 35 models for the period of 1958–2005. Ensemble means of the models with positive and negative AO–ENSO correlations both show strong spring sea surface temperature (SST) cooling in the subtropical North Pacific during a positive phase of spring AO, which is conducive to occurrence of a La Niña event in the following winter. However, the models with positive AO–ENSO relations produce a pronounced spring cyclonic anomaly over the subtropical northwestern Pacific and westerly anomalies over the tropical western Pacific (TWP). These westerly wind anomalies bring SST warming and positive precipitation anomalies in the tropical central-eastern Pacific (TCEP) during the following summer, which would maintain and develop into the following winter that support an El Niño-like pattern in the TCEP via a positive air-sea feedback mechanism. By contrast, the models with negative AO–ENSO connections fail to reproduce the spring AO-related cyclonic anomaly over the subtropical northwestern Pacific and westerly wind anomalies in the TWP. Thus, these models would produce a La Niña-like pattern in the subsequent winter. Difference in the spring AO-associated atmospheric anomalies over the subtropical North Pacific among the CMIP5 models may be attributed to biases of the models in simulating the spring climatological storm track. [ABSTRACT FROM AUTHOR]

This study reveals that the South China Sea summer monsoon (SCSSM) onset is closely linked to the preceding March Arctic oscillation (AO), the dominant mode of atmospheric circulation over extratropical Northern Hemisphere. The physical processes for the impact of the March AO on the SCSSM onset are further examined. March AO leads to an anomalous low‐level cyclone over subtropical western North Pacific (WNP) via eddy–mean flow interaction. The southwesterly wind anomalies to the southeastern side of the anomalous cyclone weaken the climatological trade winds, which contribute to warm sea surface temperature (SST) anomalies over tropical western‐central North Pacific via reduction of surface evaporation and latent heat flux. These warm SST anomalies excite an atmospheric Rossby wave response and reinforce the anomalous cyclone over the subtropical WNP. Via the above positive atmosphere–ocean interaction, the anomalous cyclone induced by the March AO could persist through the whole spring and shift equatorward. This anomalous cyclone creates favourable environment for the SCSSM onset via weakening the WNP subtropical high and is conducive to the transition of the low‐level zonal wind. Further analysis indicates that the March AO‐SCSSM onset relationship is independent of El Niño‐Southern Oscillation (ENSO). This result suggests that, besides the dominant tropical system (i.e., ENSO), interannual variation of the SCSSM onset also has a close relation with the dominant extratropical atmospheric system (i.e., AO), which may provide additional source for the seasonal prediction of the monsoon onset. [ABSTRACT FROM AUTHOR]

This paper analyses the different relationships between the two types of South Asian highs (SAH)—the southern mode of the SAH (SSAH) and the traditional SAH in its entirety (ESAH)—with respect to the variations in summer rainfall over East Asia. The ESAH is closely related to summer rainfall over northeastern China, whereas the SSAH is closely linked to summer rainfall over the Yangtze River valley, northern China, and Japan. Above‐normal rainfall tends to occur over northeastern China in strong ESAH years, whereas strong SSAH years tend to yield above‐normal rainfall over the Yangtze River valley and Japan together with below‐normal rainfall over northern China. The westerly jet and the western Pacific subtropical high (WPSH) shifts northward associated with a strengthened ESAH. The related anomalous southwesterly winds to the northwestern side of the WPSH transports abundant water vapour to northeastern China and leads to above‐normal rainfall there. In contrast, the westerly jet shifts southward and WPSH shifts southwestward to the South China Sea associated with a strengthened and southeastward‐shifted SSAH. Accordingly, a cyclonic anomaly appears over Northeast Asia and mid‐latitude western northern Pacific and an anticyclonic anomaly occurs over subtropical western North Pacific. The associated southwesterly wind anomalies over the Yangtze River valley and Japan contributes to above‐normal rainfall over these regions via transporting sufficient amount of water vapour. In addition, the anomalous northerly winds to the western flank of the cyclonic anomaly results in below‐normal rainfall over northern China via carrying drier air southward. Furthermore, the preceding winter ENSO events may play a more important role in the SSAH variation via modulating India Ocean SST. [ABSTRACT FROM AUTHOR]

This study reveals an obvious interdecadal change in the summer water vapour budget over Northeast Asia around the late‐1990s accompanied with the abrupt rainfall decrease. After the late‐1990s, water vapour transported into Northeast Asia is apparently reduced under control of a local anomalous anti‐cyclone. The water vapour transport is further explored by decomposing the water vapour flux into stationary and transient components, which are associated with the mean flow and transient eddy, respectively. The results indicate that the stationary component accounts for nearly 84% of the total water vapour budget change, much larger than the contribution from the transient component (16%). The correlation coefficient between the interdecadal variation of net and stationary (transient) water vapour budget is 0.95 (0.14). Hence, the stationary term plays a dominant role in contributing to interdecadal change in the total water vapour transport. In addition, change in the water vapour budget over Northeast Asia around the late‐1990s is primarily attributed to the wind field change at the lower troposphere. A further comparison of the water vapour budgets in four boundaries of Northeast Asia shows that the water vapour budgets via the southern and northern boundaries display remarkable changes around the late‐1990s, exerting significant influences on the total moisture budget change. The anomalous atmospheric water vapour transport circulations in association with the southern and northern boundaries both present a wave‐like teleconnection pattern over the Eurasia, whose formation may be partly due to the combined impacts from the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation. [ABSTRACT FROM AUTHOR]

Previous studies indicated that the wintertime North Pacific Oscillation (NPO) could exert marked impacts on the following winter El Niño-Southern Oscillation (ENSO) via the seasonal footprinting mechanism (SFM). Here, we examine this winter NPO-ENSO relationship in a 50-member ensemble of historical simulations conducted with the Canadian Centre for Climate Modeling and Analysis second generation Canadian Earth System Model (CanESM2) over the period of 1950–2005. The observed NPO pattern, featured by a meridional dipole atmospheric anomaly over the North Pacific, can be well reproduced by all of the 50 ensemble members. The multi-member ensemble (MME) mean can well simulate the observed NPO-ENSO relationship, as well as the SFM process. However, there exists a large spread of the results among the 50 members due to internal climate variability. Internal climate variability influences the winter NPO-ENSO relationship through modulating the subtropical center of the NPO. Specifically, the ensemble members with high NPO-ENSO correlations tend to have strong atmospheric anomalies over the subtropical North Pacific in winter. The atmospheric circulation anomaly brings strong sea surface temperature and precipitation anomalies in the tropical central Pacific and westerly wind anomalies over the tropical western Pacific in the following spring. These anomalies sustain in the following seasons and eventually lead to ENSO events in the following winter. [ABSTRACT FROM AUTHOR]

Previous observational and modeling studies indicate that the wintertime North Pacific Oscillation (NPO) could significantly impact the following winter El Niño-Southern Oscillation (ENSO) variability via the seasonal footprinting mechanism (SFM). This study explores climate projections of this winter NPO-ENSO relation in a warming climate based on a 50-member large ensemble of climate simulations conducted with the second-generation Canadian Earth System Model (CanESM2). The ensemble mean of the 50 members can well reproduce the observed winter NPO pattern, the NPO-ENSO relationship, and the SFM process over the historical period 1950–2003. These 50 members are then employed to examine climate projections of the NPO-ENSO connection over the anthropogenic forced period 2020–2073. Results indicate that there exists a large spread of projected NPO-ENSO connections across these 50 ensemble members due to internal climate variability. Internal climate variability brings uncertainties in the projection of the winter NPO-ENSO connection originally seen in projected changes of the subtropical center of the winter NPO. The spread of projections of winter NPO-associated atmospheric anomalies over the subtropical North Pacific further results in various responses in the projections of winter and spring precipitation anomalies over the tropical North Pacific, as well as spring zonal wind anomalies over the tropical western Pacific, which eventually lead to uncertainties in the projection of the sea surface temperature anomalies in the tropical central-eastern Pacific from the following summer to winter. [ABSTRACT FROM AUTHOR]

Recent studies indicated that the internal climate variability plays an important role in various aspects of projected climate changes on regional and local scales. Here we present results of the spreads in projected trends of wintertime North American surface air temperature and extremes indices of warm and cold days over the next half-century, by analyzing a 50-member large ensemble of climate simulations conducted with CanESM2. CanESM2 simulations confirm the important role of internal variability in projected surface temperature trends as demonstrated in previous studies. Yet the spread in North American warming trends in CanESM2 is generally smaller than those obtained from CCSM3 and ECHAM5 large ensemble simulations. Despite this, large spreads in the climate means as well as climate change trends of North American temperature extremes are apparent in CanESM2, especially in the projected cold day trends. The ensemble mean of forced climate simulations reveals high risks of warm days over the western coast and northern Canada, as well as a weakening belt of cold days extending from Alaska to the northeast US. The individual ensemble members differ from the ensemble mean mainly in magnitude of the warm day trends, but depart considerably from the ensemble mean in spatial pattern and magnitude of the cold day trends. The signal-to-noise ratio pattern of the warm day trend resembles that of the surface air temperature trend, with stronger signals over northern Canada, Alaska, and the southwestern US than the midsection of the continent. The projected cold day patterns reveal strong signals over the southwestern US, northern Canada, and the northeastern US. In addition, the internally generated components of mean and extreme temperature trends exhibit spatial coherences over North America, and are comparable to their externally forced trends. The large-scale atmospheric circulation-induced temperature variability influences these trends. Overall, our results suggest that climate change trends of North American temperature extremes are likely very uncertain and need to be applied with caution. [ABSTRACT FROM AUTHOR]

Our understanding of the factors influencing the interannual variation of autumn haze pollution in the North China Plain (NCP) remains quite limited. Here, we investigate interannual variations of atmospheric haze pollution and associated atmospheric anomalies in the NCP during autumn (September, October, and November). Pronounced anticyclonic anomalies tend to be observed around northeastern Asia when more severe haze pollution occurs over the NCP during this season. However, the processes underlying the impact of the atmospheric anomalies on the NCP haze show considerable intra-seasonal differences. In September, anticyclonic anomalies impact the NCP haze via modulating the surface relative humidity (SRH) and boundary layer height (BLH), while changes in surface wind speed (SWS) are not found to be related to the NCP haze. In contrast, the interannual variation of the NCP haze has a close relationship with the changes of SWS, SRH, and BLH in both October and November. The factors responsible for formation of the anticyclonic anomalies around northeastern Asia are found to differ greatly over the 3 months. In September, the formation of anticyclonic anomalies is related to the East Atlantic (EA) pattern, which triggers an eastward propagating atmospheric wave train extending from Europe to northeastern Asia. In October, the Scandinavia teleconnection is crucial for generating anticyclonic anomalies. While in November, the formation of the anticyclonic anomalies is largely due to the joint effect of the Scandinavia and East Atlantic/Western Russian teleconnections, as well as the non-negligible influence caused by sea surface temperature anomalies in the tropical central-eastern Pacific. [ABSTRACT FROM AUTHOR]

The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia. [ABSTRACT FROM AUTHOR]

This study reveals a pronounced enhancement of the relationship between the winter El Niño and the following late-summer precipitation over East Asia since the early 1990s. Before the early 1990s, precipitation anomalies over East Asia were weak during the decaying phase of El Niño. In contrast, after the early 1990s, an evident dipole pattern of precipitation anomalies was observed, with centers of action over the central and southern parts of East Asia. The El Niño-induced atmospheric convection and anticyclonic anomalies over the western North Pacific (WNP) moved to the west after the early 1990s. Correspondingly, a Pacific-Japan-like atmospheric meridional wave pattern excited by the WNP anomalous convection shifted westward toward East Asia after the early 1990s. This resulted in an increased impact of the winter El Niño on the subsequent East Asian summer precipitation. The westward movement of WNP convection was related to the change in the El Niño decay time, which became shorter after the early 1990s. Specifically, sea surface temperature (SST) warming was still apparent in the equatorial eastern Pacific (EEP) before the early 1990s, while after the 1990s, the EEP was dominated by SST cooling, and significant SST warming could be observed in the maritime continent. The prominent SST warming in the maritime continent was accompanied by marked upward motion and excessive rainfall anomalies, which further contributed to the westward shift of the WNP convective center by modulating the local Hadley circulation. Further analysis indicated that the strengthening of El Niño-induced SST anomalies in the Indian Ocean might have partly contributed to the rapid decline of the El Niño events after the early 1990s. [ABSTRACT FROM AUTHOR]

Previous studies found a seesaw pattern of summer precipitation between northeast Asia and east Siberia on an interannual time scale, which is associated with an eastward-propagating atmospheric wave train over Eurasia and corresponding water vapor transport circulations. Using a general circulation model with an embedded water-tagging module, the main water vapor sources of the two regions, as well as the relative contributions of each source region to the total precipitation for both the climatological mean and interdecadal variation, are further compared in this study. The model simulation results show that local evaporation, the Pacific Ocean, and East Asia are the dominant moisture sources for northeast Asian precipitation. In contrast, for east Siberia, moisture mainly originates from the Pacific Ocean, northeast Asia, west Siberia, and local evaporation. This suggests that the local evaporation and Pacific Ocean are both crucial to the moisture supply of the two regions, implying the important roles of the land processes and adjacent oceanic sources. In addition, northeast Asia appears to be the major moisture source for east Siberia, whereas east Siberia has weak impacts on the moisture input for northeast Asia. Further analysis finds that the model simulation can capture interdecadal changes in summer precipitation over the two regions around the late 1990s. This interdecadal change is mainly manifested in the moisture supplies from the Pacific Ocean, North Atlantic Ocean, and east Siberia, which suggests a link with the circulation anomalies under the combined impacts of the Pacific decadal oscillation and the Atlantic multidecadal oscillation. [ABSTRACT FROM AUTHOR]

The present study reveals a close relation between the interannual variation of Aleutian low intensity (ALI) in March and the subsequent winter El Niño–Southern Oscillation (ENSO). When March ALI is weaker (stronger) than normal, an El Niño (a La Niña)–like sea surface temperature (SST) warming (cooling) tends to appear in the equatorial central-eastern Pacific during the subsequent winter. The physical process linking March ALI to the following winter ENSO is as follows. When March ALI is below normal, a notable atmospheric dipole pattern develops over the North Pacific, with an anticyclonic anomaly over the Aleutian region and a cyclonic anomaly over the subtropical west-central Pacific. The formation of the anomalous cyclone is attributed to feedback of the synoptic-scale eddy-to-mean-flow energy flux and associated vorticity transportation. Specifically, easterly wind anomalies over the midlatitudes related to the weakened ALI are accompanied by a decrease in synoptic-scale eddy activity, which forces an anomalous cyclone to its southern flank. The accompanying westerly wind anomalies over the tropical west-central Pacific induce SST warming in the equatorial central-eastern Pacific during the following summer–autumn via triggering eastward-propagating warm Kelvin waves, which may sustain and develop into an El Niño event during the following winter via positive air–sea feedback. The relation of March ALI with the following winter ENSO is independent of the preceding tropical Pacific SST, the preceding-winter North Pacific Oscillation, and the spring Arctic Oscillation. The results of this analysis may provide an additional source for the prediction of ENSO. [ABSTRACT FROM AUTHOR]

Previous studies indicated that sea surface temperature (SST) anomalies in the Tropical Northern Atlantic (TNAT) have significant impacts on the August rainfall over the monsoon transitional belt (MTB) in China through inducing an atmospheric wave train over Eurasia and an anomalous anticyclone over the subtropical western North Pacific (STWNP). This study reveals that the MTB rainfall-TNAT SST linkage experiences a clear interdecadal shift around the late 1990s. The relationship was strong and significant before the late 1990s, whereas it became fairly weak after the late 1990s. Our result shows that the shift in spatial structure of Eurasian wave train induced by the TNAT SST plays an important role in the changes of the TNAT SST-MTB rainfall relationship. Before late 1990s, positive TNAT SST anomalies induces an atmospheric wave train emanating from the North Atlantic and propagating eastward to East Asia, with negative geopotential height anomalies over the MTB. In addition, a clear anticyclonic anomaly is induced over the STWNP. As such, TNAT SST anomalies have a significant impact on the MTB rainfall. After late 1990s, TNAT SST anomalies can also induce an anticyclonic anomaly over the STWNP and an atmospheric wave train over the Eurasia. However, spatial structure of the Eurasian wave train shows a significant difference. In particular, the negative geopotential height anomaly locates more northward after the late 1990s. Thus, the MTB rainfall-TNAT SST relation is weak. Further analysis also indicates that change in the MTB rainfall-TNAT SST relation is not related to change in the El Niño Southern Oscillation/North Atlantic Oscillation-TNAT SST relation. [ABSTRACT FROM AUTHOR]

Observational evidences have shown that the increased water vapour and accompanying enhancement of downward infrared radiation (IR) can explain the abrupt warming of surface air temperature (SAT) over the Barents–Kara Seas during boreal winter. This study investigates anomalous moisture transport associated with the abrupt change of SAT since the mid‐2000s over the Barents–Kara Seas, since moisture transport contributes to the anomalous water vapour and downward IR. Results show that anomalous stationary moisture flux induces anomalous moisture convergence over the Barents–Kara Seas region, whereas the anomalous transient flux opposes the anomalous moisture convergence there. Hence, the anomalous stationary moisture transport is suggested to be responsible to the increased water vapour over the Barents–Kara Seas region. Further decomposition of the stationary moisture flux indicates that the anomalous moisture convergence associated with the change in atmospheric mean flow plays an important role in the increased water vapour on both interannual and decadal time scales. In addition, decrease in the synoptic‐scale eddy activity observed over northern Siberia may contribute to generation of the anomalous atmospheric mean flow via eddy feedback processes. [ABSTRACT FROM AUTHOR]

This study examines projected changes in surface air temperature (SAT) and precipitation over mid–high latitudes of Eurasia during boreal spring in a 1.5/2°C warmer world (relative to pre‐industrial level) using historical, RCP4.5 and RCP8.5 simulations from 16 climate models of the Coupled Model Intercomparison Project Phase 5. Results show that spatial patterns of spring SAT and precipitation changes over Eurasia in the 1.5/2°C warmer world relative to the current climate (1979–2005) are similar between the two RCPs, but with a slightly larger amplitude in the RCP8.5. Spring SAT warmings over Eurasia are faster than the global annual mean. Amplitudes of the spring Eurasian SAT warmings are larger over higher latitudes. An additional 0.5°C global warming leads to extra spring Eurasian SAT warmings, with the magnitude larger than 0.5°C. Changes in the spring maximum temperature are comparable to the mean temperature changes both in the 1.5 and 2°C warming targets. Significant increases in the mean and extreme precipitation are only seen over parts of Eurasia. An additional 0.5°C global warming does not lead to obvious changes in the mean and extreme spring precipitation. Further analyses show that there are large uncertainties in the projected changes in the inter‐annual variability of the Eurasian spring SAT and precipitation. [ABSTRACT FROM AUTHOR]

Based on multiple data sets and methods, this study investigates the impacts of intra‐seasonal oscillations (ISOs) on the South China Sea summer monsoon (SCSSM) withdrawal. A daily SCSSM withdrawal date is established, which can capture reasonably the consistent transition of low‐level zonal wind from westerly to easterly over the South China Sea (SCS). The bandpass‐filtered outgoing longwave radiation and low‐level winds are then composited with respect to the monsoon withdrawal date. It is found that a 30–60‐day oscillation originating from the equatorial Indian Ocean and a quasi‐biweekly oscillation (QBWO) propagating from the equatorial western Pacific collectively contribute to the SCSSM withdrawal. Under the background of slow annual cycle (weak zonal wind during late September), the local convectively inactive phases of these ISOs induce anomalous easterly winds, which could trigger monsoon withdrawal over the SCS. The boreal summer ISO indices also confirm that both the 30–60‐day oscillation and the QBWO could modulate SCSSM withdrawal, which is more likely to occur when the suppressed convection caused by these ISOs is either encroaching on or occupying the SCS. [ABSTRACT FROM AUTHOR]

This study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic. [ABSTRACT FROM AUTHOR]

Previous studies suggested that sea surface temperature (SST) anomalies in the tropical central‐eastern Pacific (TCEP) and tropical Northern Atlantic (TNA) both have significant impacts on the inter‐annual variation of precipitation over the monsoon transitional zone (MTZ) in China during August–September. This study further reveals that the relationship between TCEP (TNA) SST and MTZ precipitation during August–September is strongly modulated by the sign of the TNA (TCEP) SST. When TCEP and TNA SST anomalies have the same sign, connections of the TCEP and TNA SST with the MTZ precipitation are unclear. In contrast, TCEP and TNA SST changes both have significant relation with the MTZ precipitation when they have the opposite sign. During the same‐sign years, the anticyclonic (cyclonic) anomaly over the western North Pacific generated by the TCEP SST cooling (warming) is weakened by the cyclonic (anticyclonic) anomaly triggered by the TNA SST cooling (warming). Thus, connections of the MTZ precipitation with the TCEP and TNA SST are weak. However, during the opposite‐sign years, the anticyclonic anomalies over the western North Pacific generated by the TCEP SST anomalies have a constructive superposition on those induced by the TNA SST changes. As such, connections of the TCEP and TNA SST with the MTZ precipitation variation are significant. Further analysis shows that the prediction skill of precipitation over the MTZ is enhanced if taking both the TCEP and TNA SST signals into account. [ABSTRACT FROM AUTHOR]

Previous studies indicated that the meridional (northerly or southerly) wind anomalies over East China play an important role in modulating interannual variation of the winter haze pollution in the North China Plain (NCP) mainly via changing surface wind speed and humidity. Here, we report that the factors for the formation of the meridional wind anomalies over East China related to interannual variation of winter haze pollution experienced a significant interdecadal change around the mid‐1990s. Before the mid‐1990s, two upstream atmospheric wave trains contribute to generation of the meridional wind anomalies over East China via inducing significant geopotential height anomalies over northeast Asia. The first occurred over mid‐latitude Eurasia and propagated eastward into East Asia, resembling the East Atlantic‐west Russia (EAWR) pattern. The second propagated eastward along the subtropical Asian jet. Furthermore, during this period, the change in the intensity of East Asian trough (EAT) was closely linked with interannual variation of the winter haze variation in the NCP. By contrast, after the mid‐1990s, the atmospheric wave train along the Asian subtropical jet was not observed. Furthermore, the connection between the EAT intensity and the winter time NCP haze variation was weak. The mid‐latitude EAWR‐like pattern and the El Niño‐Southern Oscillation‐related sea surface temperature anomalies in the tropical Pacific were possible factors that explain the meridional wind anomalies over East China. Understanding the change in the atmospheric anomalies contributing to interannual variation of the haze is essential for the prediction of haze in the NCP. [ABSTRACT FROM AUTHOR]

Interdecadal change in the correlation coefficient between two variables is an important scientific issue since it can affect climate predictions. A new statistical method is proposed for the quantitative decomposition of the difference in the correlation coefficient, and its derivation is based only on the first-order Taylor expansion. Particularly, the difference in the correlation of two time series between two specific periods is divided into three linear terms, which are only related to the changes in the covariance and variances. This method is illustrated in detail with an example of the recently recovered relationship between El Niño-Southern Oscillation (ENSO) and the South Asian summer monsoon (SASM). For the two periods of 1979–2000 and 2001–2022, the correlation between ENSO and the SASM is − 0.29 and − 0.60. The difference in the correlation coefficient (− 0.31) is mainly resulted from the enhanced covariance of ENSO and SASM (− 0.26) and decreased variance of ENSO (− 0.15), and is partly offset by the increased variance of SASM (+ 0.11). These three terms contribute to 84%, 48%, and − 36% of the interdecadal change in the correlation coefficient, respectively. The summation of these three terms accounts for 96% of the difference in the correlation, which indicates a nearly neglecting contribution from the high-order term. A similar decomposition has been applied to the correlation maps, such as the correlation between ENSO and the velocity potential at the upper troposphere. This statistical method is suggested to be a useful tool, which can potentially promote our understanding of the interdecadal changes in the correlation coefficients. [ABSTRACT FROM AUTHOR]