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The onset of the South China Sea summer monsoon (SCSSM) implies the transition of East Asian monsoon circulation from winter to summer, which affects not only the local weather and climate near the South China Sea (SCS) but also other regions through remote teleconnection. In this paper, the interdecadal variability characteristics of the SCSSM onset are analyzed by using various datasets as well as its possible mechanisms. The results show that there is a significant interdecadal delay in the onset date of the SCSSM after 2010. The onset is early around May 13th during 1994–2009. However, the onset is significantly later during 2010–2020 with an average of May 30th. Besides, the air and ocean conditions in SCS in May also took a significant decadal shift around 2010 and manifested itself in atmospheric forcing on the ocean, thus the SCSSM onset is mainly remotely influenced by the tropical Ocean instead of local SST. It is found that the interdecadal variation of the SCSSM onset is mainly caused by the decadal warming/cooling of the tropical central Pacific SST. The SCSSM onset's relationship with the tropical central Pacific SST seems more binding and stable relative to that with the tropical eastern Pacific SST. Their correlation is stable and still reaches the significant test in recent years. Therefore, the warming of the tropical central Pacific SST during 2010–2020 tends to postpone the SCSSM onset by means of related anomalous anticyclones and weakened activity of low-frequency Oscillation and tropical cyclones. On the contrary, its cooling led to early SCSSM onset during 1994–2009. Therefore, more and special attention should be paid to the tropical central Pacific SST anomaly, it will help us to monitor and predict the SCSSM onset effectively, and improve the seasonal prediction of East Asia summer monsoon greatly. [ABSTRACT FROM AUTHOR]

Recent observational and modeling studies have demonstrated the substantial influence of the Tibetan Plateau (TP) spring land surface temperature (LST) and subsurface temperature (SUBT) on downstream summer droughts/floods events in East Asia, highlighting the potential application of LST/SUBT on sub-seasonal to seasonal prediction (S2S). In this study, we employ the National Centers for Environment Prediction—Global Forecast System/Simplified Simple Biosphere model version 2 (GFS/SSiB2) to investigate the potential role of the late spring warm LST anomaly over the TP on the extraordinary June 1998 flood in the south of the Yangtze River region. Numerical experiments indicate that the warmer (above normal) May LST over the TP may contribute to the extreme flood of 1998 over the south of the Yangtze River region, with the LST reproducing about 57% and 64% of observed above-normal rainfall anomaly over the south of the Yangtze River region and southeastern China, respectively. Further analyses reveal a possible effect of springtime TP's LST on summer southern and eastern Asian rainfall and identify some hot spots, suggesting that the TP's spring LST effect is not only limited to the Yangtze River region, but to a much larger scale. The imposed warm LST/SUBT over the TP triggers a strong wave activities propagating eastward along the upper-level westerly jet, associated with an increase of the atmospheric baroclinic instability as well as a strengthening and southeastward movement of the South Asian high, leading to intensified moisture convergence and convective instability favorable to the excessive rainfall in the downstream region of East Asia. The results of the 1998 case have also been compared with the results from year of 2003, which had a very cold spring LST anomaly over the TP and a severe downstream June 2003 drought (flood) in southern (northern) of the Yangtze River Basin area. Simulation results provide further evidence of the great importance of the TP spring land surface temperature anomaly in regulating summer extreme hydroclimatic events (e.g. droughts and floods) in South and East Asia. The present study suggests that consideration of LST/SUBT anomalies has a strong potential for more skillful S2S prediction of extreme hydroclimatic events such as floods, droughts and heatwaves over both Southern and Eastern Asia. [ABSTRACT FROM AUTHOR]

In early summer 2020, eastern China was hit by record-breaking Meiyu rainfall. The anomalous western Pacific subtropical high (WPSH) played a crucial role in this event. This paper exposes the abnormal features of the WPSH and its related vertical circulations and reveals its dynamical causes in this period, from the perspective of the three-pattern decomposition of global atmospheric circulation (3P-DGAC). Analyzing the anomalous spatial patterns of the three stream functions R, H, and W of the horizontal, meridional, and zonal circulations defined by the 3P-DGAC, we discovered that the WPSH described by R at 850 hPa was intensified and westward from the horizontal circulation. For the vertical circulations, the local meridional circulation along East Asia expressed by H manifested an abnormal downdraft at 20° N, which was beneficial to strengthening the WPSH. The zonal circulation in the tropical Pacific represented by W was anomalously far to the west, which was related to the westward extension of the WPSH. Then, to uncover the causes of the anomalous WPSH from the dynamics, we used the vorticity equation based on the 3P-DGAC and found that the negative relative vorticity advection anomaly over the western Pacific played a key role in this event. Due to the positive sea surface temperature (SST) anomaly and enhanced convection over the Maritime Continent (MC), local vertical circulations at low latitudes were significantly activated. The intense local meridional circulation along East Asia carried the relative vorticity from the MC to the western Pacific and strengthened the WPSH. The ascending branch of the zonal circulation in the tropical Pacific moved westward, which may favor the westward extension of the WPSH. This study represents a promising step in revealing the three-dimensional (3D) structure and internal dynamics of anomalous WPSH. [ABSTRACT FROM AUTHOR]

In this paper, the synergistic effect of the preceding winter positive Northern Hemisphere annular mode (pNAM) and spring negative tropical North Atlantic (nTNA) sea surface temperature anomaly (SSTA) on spring extreme cold events in the mid-high latitudes of East Asia (MHEA) is investigated. The results show that the co-occurrence of the two factors is unfavorable for extreme cold events during spring in the MHEA via the snow cover and atmospheric bridges. Over the Atlantic, the spring nTNA SSTA can lead to an atmospheric response that is similar to the North Atlantic Oscillation, which enhances the persistence of the pNAM and in turn amplifies the negative spring Eurasian snow cover extent (EASCE) anomaly caused by the preceding winter pNAM. Meanwhile, the spring EASCE is closely related to the spring MHEA anomalous anticyclone. In addition to storing its signal in the spring EASCE, the spring nTNA SSTA can also lead to the spring MHEA anomalous anticyclone via the eastward Rossby wave train. The evidence shows that the Rossby wave energy can propagate eastward to the MHEA as a result of the enhanced negative spring EASCE anomaly and Rossby wave induced by the spring nTNA SSTA, and the two factors have an obvious synergistic effect on the spring MHEA anomalous anticyclone. This anomalous MHEA anticyclone becomes a barrier that can hinder the intrusion of cold air from the polar region and can increase the thickness of the atmospheric layer. The anomalous sinking motion of the spring MHEA anomalous anticyclone can also lead to an increase in net radiation received at the surface and increase the air temperature through the vertical motion of air. The southerly wind over the west side of the spring MHEA anomalous anticyclone leads to horizontal warm advection. All of the above processes favor an increase in air temperature and dampen extreme cold events, implying the synergistic effect of the preceding winter pNAM and spring nTNA SSTA on spring extreme cold events in the MHEA. [ABSTRACT FROM AUTHOR]

Heat waves, droughts, and compound drought and heat waves (CDHWs) have received extensive attention because of their disastrous impacts on agriculture, ecosystems, human health, and society. Here, we computed the heat wave magnitude index (HWMI), drought magnitude index (DMI), and compound drought and heat wave magnitude index (CDHMI) for Yangtze River Valley (YRV) from July to August during 1961–2022. We compared the large-scale atmospheric circulation characteristics of different extreme events based on these indexes. The results show that the positive center with sink motion in East Asia provides a favorable circulation background for heat wave events. Drought events are mainly affected by the zonal wave train dominated by a significant negative anomaly in Siberia and a high-pressure anomaly upstream, and a anticyclonic water vapor with strong divergence over the Yangtze River basin. During CDHW events, both anomalous systems that affect heat waves and droughts appear and strengthen simultaneously. Specifically, in the middle and upper troposphere, the positive height anomaly center in YRV expands abnormally, and the "+–+" wave train over the northern 50° N region of East Asia becomes more obvious. Therefore, the positive anomaly and water vapor anomaly brought by the two circulation patterns at different latitudes are superimposed over the YRV, leading to severe CDHWs. At the same time, the warm positive eddy center and cold negative eddy center in high latitudes exhibit more stable positive pressure features, which are conducive to the persistent development and strengthening of CDHWs. In addition, the anomalous warm sea surface temperature in western Pacific moderating the favorable circulation patterns may also promote the occurrence of CDHWs in the YRV during the same period. [ABSTRACT FROM AUTHOR]

Present study investigates the characteristics and factors of summer drought variations over Asia during 1950–2020 using 3-month Standardized Precipitation Evapotranspiration Index (SPEI03) in July. The leading mode of Asian summer drought variations exhibits a roughly north–south contrasting distribution across 30° N with dominant interannual variability. The interannual variation of the leading mode is associated with both precipitation and temperature. In comparison, the contribution of precipitation is larger in the eastern part of the mid-latitude Asia. The Asian summer drought variations subject to both individual and combined impacts of preceding winter equatorial central-eastern Pacific (ECEP) sea surface temperature (SST) anomalies and concurrent North Atlantic (NA) tripole SST anomalies. Preceding ECEP SST anomalies induce anomalous vertical motion and precipitation over the mid-latitude Asia through large-scale divergence and convergence in previous winter and spring. The resulted soil moisture anomalies persist to summer and are conducive to anomalous summer precipitation through evaporation. The accompanying changes in cloud-radiation, surface longwave radiation and sensible heat flux induce temperature and evapotranspiration anomalies. The NA tripole SST anomalies induce anomalous vorticity forcing over the western mid-latitude NA. This contributes to a wave train that propagates from the NA to East Asia and affects wind and vertical motion over the mid-latitude Asia. [ABSTRACT FROM AUTHOR]

This paper reports findings from a diagnostic and modeling analysis that investigates the impacts of the boreal late-spring soil moisture anomalies over North Eurasia on the summer rainfall over northern East Asia (NEA). The soil moisture in May from the Kara-Laptev Sea coasts to Central Siberian Plateau is found to be negatively correlated with the summer rainfall from Mongolia to Northeast China. The atmospheric circulation anomalies associated with the anomalously dry soil are characterized by a pressure dipole with the high-pressure center located over North Eurasia and low-pressure center over NEA, where an anomalous water vapor convergence occurs, favoring rainfall formation. Diagnoses and modeling experiments demonstrate that the effects of the spring lower soil moisture over North Eurasia may persist into the following summer, then increase the low-level air temperature at higher latitudes through modulating local surface latent and sensible heat fluxes, and effectively reduce the meridional temperature gradient north of NEA. The weakened temperature gradient could induce the decreased vertical shear of zonal wind and generate an anomalously cyclonic center over NEA by affecting the baroclinicity around 60° N, associated with a favorable condition of local synoptic activity to increase rainfall. The above relationships and mechanisms are vice versa for the prior wetter North Eurasian soil and decreased NEA rainfall. These findings suggest that the soil moisture anomalies at higher latitudes may act as a new precursor providing an additional predictability source for better predicting the summer rainfall in NEA. [ABSTRACT FROM AUTHOR]

The East Asian winter monsoon (EAWM) intensifies in the early twenty-first century and links with frequent impacts of large-scale persistent extreme cold events in winter in East Asia in recent years. We found that there has been a significant positive correlation between the EAWM and Interhemispheric Oscillation (IHO). However, conspicuous interdecadal variations have occurred in the relationship between the EAWM and IHO. The relationship between the IHO and EAWM was most significant during 1979–2020, but this relationship was weak and insignificant during 1962–1978. During 1979–2020, the atmospheric mass (surface pressure) difference between the Northern Hemisphere (NH) and Southern Hemisphere (SH) during 1979–2020 was significantly reduced by 7.85% (0.75 × 1015 kg) compared with that during 1962–1978. Such interhemispheric redistribution of atmospheric mass (AM) has had a distinct impact on the land-sea pressure contrast in East Asia and has intensified the connection between the EAWM and IHO. A strengthened EAWM has resulted in notable cooling and more severe winters in China. The apparent exportation of AM in the Antarctic region is an important driving factor for this interhemispheric change. The accompanying anomalous accumulation of AM in the NH is linked with an increase in the pressure difference between land and sea in East Asia, resulting in intensifying correlation between IHO and the EAWM. The decadal enhancement of the IHO during 1979–2020 was closely connected with conspicuous warming in the tropical troposphere/lower stratosphere (UTLS). A seesaw pattern of anomalous air temperature and ozone between tropical and Antarctic UTLS has induced a decrease in Antarctic ozone masses and air temperature and has strengthened the polar vortex, corresponding to a decadal enhancement of interhemispheric AM imbalance. [ABSTRACT FROM AUTHOR]

Extreme heat events happen frequently in East Asia, and may cause great damage to the ecosystem and society. The synergistic effect of the summer positive North Atlantic Oscillation (pNAO) and positive northwest Pacific (pNWP) sea surface temperature anomaly (SSTA) on the interannual variability of the extreme heat events in the central–eastern China (CEC) is investigated in this study. The two factors act synergistically in strengthening the extreme heat events in the CEC via a series of atmospheric bridges, and the CEC is likely to experience a hotter summer when both the summer pNAO and pNWP SSTA occur. The pNWP SSTA increases the strength of pNAO via the eastward propagating Rossby wave from the western Pacific. The enhanced pNAO induces a stronger eastward Rossby wave propagation across the Eurasian continent, and leads to a strong anomalous anticyclone over the CEC. The significantly increased atmosphere thickness increases the air temperature of the layer, and favors the extreme heat events in the CEC. Besides, the pNWP SSTA also has adjacent effect on the atmospheric circulation over the CEC, which could lead to a positive geopotential height anomaly. Therefore, the summer pNAO and pNWP SSTA act synergistically in influencing the atmospheric circulation over the CEC, and thereby significantly increase the extreme heat events in the CEC. [ABSTRACT FROM AUTHOR]

The predictability and variability of extratropical climate are largely associated with tropical forcing through various teleconnections. The East Asia–Pacific/Pacific-Japan (EAP/PJ) teleconnection is one of the most important factors affecting the summer climate in East Asia. In this work, we confirm the weakening relationship between the El Niño-Southern Oscillation (ENSO) and the following summer PJ teleconnection after the late 1990s and diagnose the possible causes. The results show that compared with that of in 1981–1998, ENSO is less conducive to the formation of anomalous anticyclones in the following summer in the Northwest Pacific (NWP) and the northward propagating Rossby wave train from latitudes of NWP in 2000–2017, resulting in a weaker summer PJ teleconnection. Meanwhile, variation of ENSO feature since 2000 with a suppressed variability leads to a weakened response in the tropical Indian Ocean in the spring and summer. The reduced sea surface temperature response in the tropical Indian Ocean suppresses the excitation of eastward-propagating Kelvin waves and declines the capacitor effect of the Indian Ocean, which is unfavorable for generating anomalies in the NWP and motivating strong PJ teleconnection in meridional direction. Thus, the weakening relationship of ENSO-PJ after late 1990s is the result of the weakened ENSO's impact on the formation of anticyclones in the following summer in NWP and the declined Indian Ocean capacitor effect on motivating meridionals wave train over East Asia and NWP region. [ABSTRACT FROM AUTHOR]

Owing to the East Asian summer monsoon, extreme precipitation occurred frequently over the Yangtze River Basin (YRB), leading to flooding and secondary disasters. Therefore, understanding the physical mechanism and seeking predictability sources of extreme precipitation in the YRB are of scientific and practical importance. The present study examines the independent and compound impacts of leading sea surface temperature (SST) modes in the Indian Ocean and the North Atlantic Ocean on summer extreme precipitation days (EPDs) over the YRB. The Indian Ocean basin-wide uniform SST mode (the Indian Ocean Basin Mode) influences the EPDs over the YRB by inducing a Kelvin wave response and the Pacific–Japan pattern, whereas the two leading SST modes of the North Atlantic Ocean show different meridional tripole patterns with different climate impacts on East Asia. The North Atlantic SST tripole southern mode (NATS) induces quasi-stationary Rossby wave trains over mid-latitude Eurasia and the tropical waves that influence the EPDs in the YRB. The North Atlantic SST tripole northern mode (NATN) impacts the circulation anomaly over Northeast Asia through inducing different Eurasian quasi-stationary Rossby wave trains. When the IOBM and the NATS are both in the positive phase, enhanced EPDs occur over the YRB. On the one hand, the IOBM induces a Kelvin wave response, which strengthens the western North Pacific anomalous anticyclone (WNPAC). On the other hand, the NATS stimulates the mid-latitude quasi-stationary Rossby waves and results in the Northeast Asia anomalous cyclone (NEAC). The warm, moist air over the northwestern flank of the WNPAC and the cold, dry air over the southern flank of the NEAC converge in the YRB, leading to more EPDs in the region. When the IOBM and the NATN are out of phase, the Kelvin wave response in terms of the WNPAC induced by the IOBM warming is modulated by the negative phase of the NATN via quasi-stationary Rossby wave trains over mid-latitude Eurasia, resulting in more EPDs in the YRB. Based on the compound effect of different SST modes in the two ocean basins, the year-to-year EPDs over the YRB can be reconstructed reasonably well, which provides useful predictability sources for the seasonal prediction. [ABSTRACT FROM AUTHOR]

Vegetation is one of the most important parts of the Earth's land surface system. To investigate the impact and uncertainty of multi-source vegetation data on climate simulations in East Asia, we have compared two Fractional Vegetation Cover (FVC) and five Leaf Area Index (LAI) products and then performed seven experiments from 1999 to 2018 with the WRF model. The results suggest that the sensitivity of vegetation climatology is large especially in south East Asia, with a maximum difference of 4 m2/m2 in LAI and 16% in FVC. For the mean temperature, the responses in East Asia are between ± 0.4 K, and the uncertainty over the Chinese Mainland gradually enhances with the rapid vegetation growth. The feedback of precipitation is highly heterogeneous with sensitivities ranging between ± 1 mm. For interannual variability, vegetation has a greater impact in summer and autumn. The uncertainty of temperature and precipitation in summer over the Chinese Mainland is 118 and 1754 times that in winter. In addition, the linear trend results also indicate that the maximum trend difference in temperature is around 0.04 K/a in eastern China, while the precipitation has a larger range of 0.04–0.10 mm/a in southern East Asia. Besides, the biophysical mechanism analysis suggests that vegetation impact on surface energy budget is mainly a competition between albedo feedback and moisture feedback, and the large differences in multi-source FVC and LAI are sufficient to simulate different or even opposite feedbacks. [ABSTRACT FROM AUTHOR]

The northernmost margin of the East Asian summer monsoon (NMEASM) is the northernmost position that the East Asia summer monsoon (EASM) can reach. NMEASM has obvious multi-scale variability, and well reflects the wet/dry climate variability in northern China. Predicting the location change of the NMEASM is important for understanding future East Asian climate change. However, the variability of the NMEASM has not been studied extensively, and its underlying mechanisms have not been clarified. To explore the movement of the NMEASM and its causes, we use reanalysis datasets to evaluate the NMEASM index from 1979 to 2018. The NMEASM indicates a decreasing trend over 40 years and a significant abrupt point in 2000, which is positively correlated with the Tibetan Plateau snow cover before 2000 and the Siberian snow cover after 2000 in spring. The decreased Siberian snow cover increases the soil temperature and decreases the atmospheric baroclinicity over Mongolia and northern China after 2000. The decreased atmospheric baroclinicity induces the dipole mode of anticyclonic anomaly over Mongolia and northern China and the cyclonic anomaly over the Sea of Japan by modulating the wave activity flux (WAF). The WAF's southeastward propagation strengthens the anticyclonic anomaly over Mongolia and northern China and the cyclonic anomaly over the Sea of Japan, which weakens the upward movement and water vapor transport, respectively. Hence, the decreased Siberian snow cover in spring modulates the precipitation over Mongolia and northern China and the southward movement of NMEASM by turbulent westerly circulation. [ABSTRACT FROM AUTHOR]

This study develops a skill evaluation metric for an individual forecast by applying a Taylor expansion to the commonly-used temporal correlation skill. In contrast to other individual forecast evaluation metrics, which depend on the amplitude of forecasted and observed anomalies, the so-called "association strength (AS) skill" is less affected by the anomaly amplitude and mainly depends on the degree of similarity between the forecasted and the observed values. Based on this newly developed index, the forecast skill is evaluated for an individual case, then, a group is categorized with respect to the AS skill. The cases with the highest AS skill exhibit the highest correlation skill than any group randomly selected, indicating that the AS skill is a powerful metric to evaluate the non-dimensionalized forecast skill. This strategy is adopted for the subseasonal East Asian summer precipitation forecasts produced by the UK Met Office's ensemble Global Seasonal forecast system version 5 (GloSea5). In the group with the highest AS skill of the East Asian summer precipitation index (i.e., highest AS cases), the geopotential height anomalies showed quasi-stationary Rossby waves from the North Atlantic to East Asia. The spatial distribution of the dominant subseasonal anomalies for cases with the highest AS is distinct from the cases or groups with the lowest AS skill. Furthermore, the dominant pattern with the highest AS is not solely explained by any well-known typical subseasonal climate patterns, such as the Madden–Julian Oscillation, circumglobal teleconnection pattern, Pacific-Japan pattern, or the Summer North Atlantic Oscillation. This implies that the excitation of well-known climate patterns only partly contributes to increasing the mid-latitude climate predictability in the GloSea5. [ABSTRACT FROM AUTHOR]

In 2021, the Yangtze–Huaihe River Basin (YHRB) of China underwent normal precipitation during the classical Meiyu period in June–July but suffered extreme precipitation and severe floods in August. The present study revealed that the extreme heavy precipitation anomalies over the YHRB region were closely related to the combined effect of the strong western North Pacific anomalous anticyclone (WNPAC) and East Asian middle–high latitude circulation anomalies in August 2021. Moreover, the background of sea surface temperature (SST) anomalies was a strong eastern-Pacific La Niña event that rapidly decayed in spring and a second La Niña emerged in late summer, accompanied by a weak tropical Indian Ocean warming and a strong tropical Atlantic warming in summer. The rapidly decaying La Niña event and the weak tropical Indian Ocean warming alone seem to be insufficient to induce the strong WNPAC in August 2021. Observational evidence and numerical experiments suggest that the rapid tropical Atlantic warming from late spring to summer observably contributed to the enhancement and westward shift of the WNPAC, and the cold SST anomalies in the eastern tropical Pacific associated with the second La Niña event also provided favorable conditions for a strong WNPAC in August 2021. Overall, rapid tropical Atlantic warming and the development of the second La Niña contributed to the strong WNPAC in August 2021, while the maintenance mechanism of the East Asian middle–high latitude circulation anomalies that controlled cold air activity remains unclear. [ABSTRACT FROM AUTHOR]

The multi-year simulation of tropical cyclones (TCs) over the Western North Pacific (WNP) in the variable resolution (VR) CAM-MPAS model is studied. Experiments with the global quasi-uniform low resolution of 120 km (MPAS-UR) and the variable resolution mesh of 30–120 km refined over East Asia (MPAS-VR) are integrated from 1980 to 2005 following the Atmospheric Model Intercomparison Project protocol. By utilizing an objective detection method, TCs in ERA5 reanalysis and model simulations are tracked and compared against observations. MPAS-VR shows significant advantages over MPAS-UR as indicated by more realistic TC counts, intensities, lifetime distribution, and seasonal variation. The large-scale circulation and precipitation patterns associated with TCs are also improved in MPAS-VR relative to MPAS-UR. Based on the theory of Dynamic Genesis Potential Index, the multi-year TC records are further used to quantify the dependence of TC genesis on various dynamical environmental factors from the perspective of seasonal variation. We find that in ERA5, the relative contribution of the 500 hPa vertical pressure velocity term to TC genesis exceeds that of the 200–850 hPa vertical wind shear term, which is responsible for the August peak and strong seasonal variation of TC genesis. MPAS-UR fails to capture such relationship while MPAS-VR performs much better in this regard, suggesting that the higher skills in simulating the relative contributions from different dynamical environmental factors to the simulated seasonal cycle of TC genesis may explain the improvements from MPAS-UR to MPAS-VR. [ABSTRACT FROM AUTHOR]

Different evolutionary patterns of the East Asian trough (EAT) anomaly could cause different climate patterns over East Asia. In this study, the leading mode of month-to-month variation in winter (December–January–February) EAT during 1979/80–2018/19 is investigated using the extended empirical orthogonal function (EEOF). The first EEOF mode (EEOF1) shows a persistent anomalous EAT pattern throughout the three winter months, which results in persistent air temperature anomalies over East Asia. Mechanism analyses indicate that the El Niño-Southern Oscillation (ENSO) negatively relates to the EEOF1, but its significant influence on the EAT is only in December. In contrast, a dipole pattern of sea surface temperature (SST) anomalies over the Eastern North Atlantic-Eastern Mediterranean Sea closely links with the EAT EEOF1. Further analyses indicate that the connection of the dipole SST pattern with the EAT EEOF1 depends on its persistence. When there is positive feedback between the local atmosphere and sea, the dipole SST pattern is well-sustained in the three winter months and further leads to a persistent EAT anomaly by exciting the Rossby wave train over mid-latitude Eurasia. Numerical simulations by the linear baroclinic model further validate the role of dipole SST pattern-related diabatic and vorticity forcing in air-sea interactions. In contrast, in the SST poorly-sustained years, the dipole SST pattern has a weak connection with the EAT EEOF1. Based on the aforementioned analysis, we propose a combined index using the dipole SST pattern and anomalous Northeast Atlantic cyclone in December, which could provide valuable information to predict the dipole SST pattern persistence and EAT EEOF1 in winter. [ABSTRACT FROM AUTHOR]

In the summer (July and August) of 2022, unprecedented heat wave occurred along the Yangtze River Valley (YRV) over East Asia while unprecedented flood occurred over western South Asia (WSA), which are located on the eastern and western sides of Tibetan Plateau (TP). Here, by analyzing the interannual variability based on observational and reanalysis data, we show evidences that the anomalous zonal flow over subtropical Tibetan Plateau (TP) explains a major fraction the extreme events occurred in 2022. As isentropic surfaces incline eastward (westward) with altitude on the eastern (western) side of the warm center over TP in summer, anomalous easterly (westerly) flow in upper troposphere generates anomalous descent (ascent) on the eastern side of TP and anomalous ascent (descent) on the western side of TP via isentropic gliding. The anomalous easterly flow is extremely strong to reverse the climatological westerly flow over subtropical TP in 1994, 2006, 2013 and 2022. The easterly flow in 2022 is the strongest since 1979, and it generates unprecedented descent (ascent) anomaly on the eastern (western) side of TP, leading to extreme heat wave over YRV and extreme flood over WSA in 2022. The anomalously strong easterly flow over subtropical TP in 2022 is dominated by atmospheric internal variability related to mid-latitude wave train, while the cold sea surface temperature anomaly over the tropical Indian Ocean increases the probability of a reversed zonal flow over TP by reducing the meridional gradient of tropospheric temperature. [ABSTRACT FROM AUTHOR]

The effect of model resolution on the simulation of tropical cyclone (TC) landfall frequency in East Asia [including the South China Sea (SCS), Taiwan and coastal areas of East China (TWCN) and Japan (JP)] was investigated by comparing Atmospheric Model Intercomparison Project (AMIP) type simulations on the basis of 50-km High Resolution Atmospheric Models (HiRAMs) and 25-km HiRAM. The number of TC landfalls in the TWCN region was realistically simulated by the 50-km HiRAM ensemble model. However, fewer (more) TCs were steered westward (northward) toward the SCS (JP) because of an overestimation of the monsoon trough in the western North Pacific (WNP). The overestimation created a low-level cyclonic circulation anomaly in the WNP, which substantially modified steering flow. Consequently, more (less) TC made landfall in JP (SCS). The overestimation of the monsoon trough in model was primarily resulted from compounding factors, including the AMIP type simulation, upscale feedback of TCs to mean flow and the monsoon flow–topography interaction in the Indochina Peninsula Mountains and Philippine. First, the SST was negatively correlated with precipitation in the WNP during the typhoon season for the observation. Conversely, the SST–precipitation relationship was positive in the AMIP run. Second, the upscale feedback of TCs to mean flow (monsoon trough) was overestimated, which in term contributed to the overestimation of monsoon trough. Third, the model underestimated the mountain lifting effect in the Indochina Peninsula and Philippine. Overall, the aforementioned biases were substantially improved by increasing model's horizontal resolution from 50-km to 25-km HiRAM. [ABSTRACT FROM AUTHOR]

Pacific decadal oscillation (PDO) has been regarded as one of the most important interdecadal factors to the interdecadal summer precipitation in eastern China, which, however, cannot keep a consistent pattern during the same positive or negative PDO phase. Because sea surface temperature (SST) anomalies vary along with PDO evolvement, the PDO impacts are investigated in terms of developing and decaying stages of its positive and negative phases, respectively. It is found that the SST cooling in mid-latitude North Pacific and SST warming in off-equator central-eastern Pacific are generally intensified from the developing stage to the decaying stage for positive PDO phase, and vice versa. Particularly, during the decaying stage of positive (negative) PDO phase, the enhanced SST warming (cooling) in northeastern tropical Pacific triggers a stronger (weaker) large-scale Walker circulation and suppresses (enhances) the convective activities in western tropical Pacific, which favors a meridional wave train like East Asia–Pacific (EAP) teleconnection over East Asia. Meanwhile, the westward extended SST cooling (warming) in mid-latitude North Pacific together with the SST warming (cooling) to its south significantly increases (decreases) the meridional SST gradient and low-level atmospheric baroclinicity, conductive to the strengthened (weakened) and southward (northward) shifted East Asian subtropical westerly jet as well as an equivalent barotropic "cold trough" in western North Pacific. Because of such tropical and extratropical dynamical processes, a meridional tripole pattern of precipitation anomalies occurs in eastern China which dominates the PDO impact in the same phase and distinctly different from the dipole pattern during the developing stage. [ABSTRACT FROM AUTHOR]

Northern Hemispheric summer monsoons were more intense during the mid-Holocene (MH ~ 6000 years ago) and coincided with a northward shift of the Intertropical Convergence Zone (ITCZ) compared to the pre-industrial (PI) climate. Ancient civilizations in the Indus valley, Mesopotamia, and Egypt appear to have flourished during this period, thanks to abundant water availability. This study exploits a high-resolution variable grid global atmosphere model to understand the role of orbital forcing and ocean surface conditions in strengthening the monsoons and shifting the ITCZ northward over Africa, India, and East Asia during the MH. The combined impact of orbital forcing and sea surface temperature (SST) boundary conditions led to a change in monsoon rainfall of around 42, 30, 21, and 41% over Africa, East Asia, India, and northwest India (NWI) relative to the PI conditions. Changes in orbital parameters alone account for more than 36 and 26% of total rainfall increases in Africa and East Asia. Over the Indian subcontinent, the strengthening of monsoon was primarily a combined effect of SST and orbital forcing. In contrast, the SST boundary condition alone could explain the 39% of rainfall increase over NWI, where the Indus valley civilization once existed. Through moisture budget analysis, the study further illustrates the role of dynamic and thermodynamic factors responsible for the changes in monsoon precipitation. The enhanced monsoon resulted in a northward shift of ITCZ by around 3°N, 1.9°N, and 2.5°N over Africa, East Asia, and India, respectively, compared to its PI position. Analogous to the precipitation changes, orbital forcing mostly mediated ITCZ changes across Africa and East Asia, but the combined impact of orbital forcing and SST was responsible for the changes over India. [ABSTRACT FROM AUTHOR]

Several of the best track datasets reveal the earlier end of the tropical cyclone (TC) season over the main WNP (the east of the Philippine Islands) from 1980 to 2019, especially by the date of the upper decile of the storm days (constructed by adding one to each date if any TC exists). The enhanced vertical wind shear (VWS) during the late season (October and November) is consistent with this earlier end of the TC season. In October, the enhanced VWS is the result of the increased zonal and meridional shears, which are affected by the earlier circulation transition of the East Asian summer monsoon recessed and winter monsoon onset during late October. During November, the zonal wind shear dominates the enhanced VWS, which vertically is the opposite trend of air temperature below and above the tropopause and the meridionally opposite trend of air temperature in the north and south of 30°N. The opposite air temperature trends induce the thermal wind balance, which causes enhanced westerlies in the mid-latitudes and strong zonal shear. Three pathways increase VWS in the late season over the WNP, leading to the earlier end of the TC season. The strong impact of VWS limits TC formation, although the ambient warming underlying the surface (supports a broader and stronger potential intensity). The effect of the thermodynamical parameter, depending on relative humidity at the low and middle troposphere, is uncertain because of the inconsistent and weak trends in October between ERA5 and MERRA-2, but they both become unfavorable to TC genesis during November. The practically important meaning of the VWS for the TC season provides a possibility for future projections of the TC season. [ABSTRACT FROM AUTHOR]

Transitional climate zone (TCZ) over East Asia is located between humid and arid regions, which is a highly sensitive and disaster-prone region especially under global climate change. Due to limited water resources, the atmospheric moisture availability has a dominant control on the precipitation variability. Hence, this study is motivated to reveal the key teleconnection patterns and associated moisture processes that govern the interannual variability of the summer precipitation over TCZ. In order to better diagnose moisture budget, the Lagrangian particle dispersion model FLEXPART is employed for quantifying contribution from moisture sources. Above all, the observational analysis highlights two critical modes, one is Eurasian teleconnection (EU) and the other is Circumglobal Teleconnection (CGT). As regards EU pattern, positive EU phase corresponds to ample precipitation in TCZ. In the presence of positive phase, it underlines a "+-+-" pattern of geopotential height anomalies stretching from western Europe to Mongolia plateau. In the context, the cyclonic flow and low pressure over Mongolia plateau act to enhance moisture flux from the west and the south and to prompt upward motions. Further moisture diagnoses illuminate largest increase of moisture uptake in monsoon dominated region, followed by the westerlies dominated region. However, the eventual contribution of summer monsoon is a little bit less than that of westerlies, due to the grand loss en route. In addition, the local evaporation exerts little impact. CGT propagates along the mid-latitude westerly jet, which is positively coupled with the precipitation in TCZ. Under the positive phase, there is an ascending motion over TCZ, which bears great resemblance to the EU case. However, unlike the result of moisture attribution in EU case, the southerly monsoon has the largest contribution followed by local effect, while the westerlies have little impact due to the cancellation of wetting and drying regimes along the pathway. [ABSTRACT FROM AUTHOR]

Understanding the cloud feedback (CFB) process remains a critical challenge for climate projection. While previous studies have mainly focused on the globe, the tropics, or the Southern Ocean, this study analyzes the long-term CFB over East Asia (EA) using data from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and radiative kernels newly developed. We find that the overall CFBs over EA act to amplify warming in an annual scale, with mean values of 0.66 ± 0.47 and 0.63 ± 0.39 W m−2 K−1 for the SSP2-4.5 and SSP5-8.5 scenarios, respectively, although remarkable seasonal discrepancies. There are two distinct subregions: the positive center over the East Asian monsoon region, primarily due to the significant reduction in low cloud cover; the negative center over the Tibet Plateau, largely owing to the reduced high cloud cover and the ice-to-liquid conversion of cloud phase. Furthermore, the CFB over EA changes from negative to positive in the early 1980s, and then enhance at a rate faster than the global average until the early years of the twenty-first century, which may be linked with the weakening and subsequent recovery of the East Asian summer monsoon. Generally, inter-kernel uncertainty is markedly smaller than inter-model uncertainty; however, in regional scale, inter-kernel uncertainty is larger and inter-model uncertainty is smaller than those in the global scale. This feature implies that ignoring inter-kernel uncertainty may bring additional errors to regional CFB quantification. Notably, these results are comparable between the SSP2-4.5 and SSP5-8.5 scenarios, suggesting that CFB processes may be insensitive to emission scenarios. [ABSTRACT FROM AUTHOR]

This study reveals a marked enhancement in the relationship between the variations in location of the winter East Asian Polar Front Jet (EAPJ) and the surface air temperature (SAT) in Eurasia since the mid-1990s. Before the mid-1990s, an evident wave train related to the meridional location of the EAPJ exhibited an anticyclonic anomaly over northern Europe and a cyclonic anomaly in northwestern Asia. With an equatorward shift of the EAPJ after the mid-1990s, the wave train experiences a notable adjustment that is conducive to East Asian cooling, displaying an anticyclonic anomaly around the Kara-Laptev Seas and a cyclonic anomaly near northeastern Asia. Arctic warming anomalies and sea ice loss contributed significantly to these decadal changes. Simulation experiments forced by observed Arctic sea-ice variability further confirm this result. Since the mid-1990s, Arctic sea ice loss (or Arctic warming anomaly) has contributed to a reduction in westerly winds in high latitudes by modulating the meridional temperature gradient. The deaccelerated winds intensify the Arctic cold air propagating to the south, enhancing the atmospheric baroclinicity and the westerly flow in the upper level at the south side of the EAPJ, favoring the southward shift of the EAPJ. With the equatorward shift of the EAPJ, the corresponding SAT anomalies in East Asia are more salient. [ABSTRACT FROM AUTHOR]

Few studies have investigated the retreat of the East Asian summer monsoon (EASM) in comparison to its seasonal advances. This study examined the climatological characteristics of the EASM retreat based on the Japanese 55-year Reanalysis data and multiple observation data. First, the retreat date of EASM is defined based on the reversal of 850 hPa meridional winds over coastal East Asia. Then, the climatological characteristics of the EASM retreat are investigated based on the retreat date of the EASM. At lower troposphere, the differences of the 850-hPa winds between after and before the EASM retreat display an anticyclone (cyclone) over the East Asian continent (the subtropical western Pacific), resulting in the strengthening of northerly winds over the coastal region of East Asia. Such northerly winds lead to decrease of moisture transport over East Asia and bring colder air from higher latitudes to the coastal East Asia, both favoring the retreat of the EASM. At middle troposphere, the WPSH moves eastward, and a mid-latitude trough appears over northeast Asia. In addition, the difference fields show descending (ascending) motion and cold (warm) advection in the rear (front) of the mid-latitude trough. At upper troposphere, a divergent center exhibits a southeastward movement, which shifts from the Philippine Islands to the western North Pacific. As such, a convergence center appears over the coastal East Asia in the difference map. The retreat of the EASM is also associated with the decreased rainfall, the demise of the wet season, and the broadscale cooling over East Asia. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) downstream of the Tibetan Plateau (TP) exhibit unique precipitation features. These MCSs can have damaging impacts and there is a critical need for improving the representation of MCSs in numerical models. However, most global climate models are typically run at resolutions that are too coarse to reasonably resolve MCSs, and it is still unclear how well higher-resolution global models can reproduce the precipitation characteristics of MCSs. In this study, the sensitivity of MCSs simulated by a global high resolution (~ 10 km), atmosphere-only climate model to different treatments of convection (with and without parametrized convection, and a hybrid representation of convection) have been investigated. The results show that explicit convection (i.e., non-parameterized) can better reproduce the observed pattern of MCS precipitation over the East Asian Summer Monsoon region. In general, explicit convection better simulates the diurnal variability of MCSs over the eastern China, and is able to represent the distinctive diurnal variations of MCS precipitation over complex terrain particularly well, such as the eastern TP and the complex terrain of central-northern China. It is shown that explicit convection is better at simulating the timing of initiation and subsequent propagating features of the MCS, resulting in better diurnal variations and further a better spatial pattern of summer mean MCS precipitation. All three experiments simulate MCS rainfall areas which are notably smaller than those in observations, but with much stronger rainfall intensities, implying that these biases in simulated MCS morphological characteristics are not sensitive to the different treatment of convection. [ABSTRACT FROM AUTHOR]

A comprehensive model assessment on a 6-hourly scale from multiple dimensions is critical for model selection of dynamic downscaling to reduce uncertainties. In this study, we evaluated the performance of 13 models involved in the Coupled Model Intercomparison Project-6 (CMIP6) by comparing the simulated meteorological variables at the pressure levels of 850, 500, and 250 hPa, including 6-hourly air temperature, specific humidity, zonal wind, meridional wind, and geopotential height, to those from the ERA5 reanalysis data for the 1979–2014 period. The results indicated that the CMIP6 models could mostly reproduce the spatial pattern of the climatology. Most models underestimated air temperature and geopotential height while overestimating specific humidity and zonal and meridional wind speeds in the upper troposphere. Additionally, the interannual variability of zonal and meridional wind exhibited a relatively better performance but limited ability for specific humidity at 850 hPa. Regarding the annual and diurnal cycles, CMIP6 models reasonably captured the annual cycle shape, while an overestimation was detected in simulating the diurnal amplitude, notably at 250 hPa. Based on a comprehensive rating index and overall rankings, our findings showed that no single model was identified as suitable for the simulation of any variables and regions. The models performed well for five variables, including MPI-ESM1-2-LR, MPI-ESM1-2-HR, TaiESM1, and UKESM1-0-LL, over East Asia. Then, according to the overall performance of the five variables and model accessibility, the optimal model varied by region and shared socioeconomic pathway (SSP) scenario. The multi-model ensemble mean outperformed individual models over almost all regions when it comes to comprehensive performance. The MPI-ESM1-2-LR and MRI-ESM2-0 models were the best two out of 13 models as lateral boundary conditions applied to seven climate scenarios over East Asia. This study provides valuable scientific references for selecting the optimal CMIP6 models for the projection of dynamic downscaling over East Asia and eight subregions. [ABSTRACT FROM AUTHOR]

The East Asian subtropical westerly jet (EASWJ) is one of the most crucial subtropical circulation systems affecting the precipitation over East Asia. Based on the ERA5 dataset, the dominant modes of the intraseasonal variability of the EASWJ in early (May and June) and late (July and August) summer are investigated, respectively, through the empirical orthogonal function (EOF) analysis. The EOF1 in early summer is characterized by the anomalous westerlies centered over the North China and anomalous easterlies centered over the south of Japan. This mode is led by the south-eastward propagating wave train initiating from the Barents Sea, where the ridge of the wave train coincides with an anomalous warm advection in the low level, due to the negative phase of Arctic Dipole (AD). Since lag − 4 days, the East Asia/Western Ruassia (EAWR) teleconnection contributes to the wave trains. The EOF1 in late summer is characterized by the anomalous westerlies centered over the south of Baikal and anomalous easterlies centered over the Central China, which is affected by the two wave trains along different directions. One wave train propagate zonally across Eurasia initiated from North Atlantic, where significant signal of East Atlantic (EA) teleconnection is found as a precursor. When the wave train disperses downstream to Eurasia, the EAWR play a dominant role on the growth and persistence of the EASWJ variability. The other one is similar to the East Asia–Pacific (EAP) teleconnection propagating poleward from the Southern Asia and Western North Pacific, where the active convection anomalies may be a key driver. The intraseasonal wave trains that influence the EASWJ are different between early and late summer probably due to the discrepancies of background status, such as the background temperature gradient and the curviness of the jet climatology. [ABSTRACT FROM AUTHOR]

This study estimates short-term climate feedbacks by using a new set of radiative kernels applied to observations and the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. The new kernels are generated based on multiyear satellite observations, and they can well reproduce the top-of-atmosphere (TOA) radiation budget. The choice of radiative kernels influences the feedback estimation, especially the surface albedo feedback and cloud feedback in the Arctic and the Southern Ocean. Observational estimates show that tropospheric water vapor feedback makes the largest contribution to global warming, while lapse rate feedback is the largest contributor to local warming over East Asia. Compared to the observations, biases occur but differ when simulating global and East Asian local climate feedbacks. CMIP6 models overestimate global mean Planck, lapse rate, stratospheric temperature and water vapor, and cloud feedbacks, but underestimate global mean tropospheric water vapor and surface albedo feedbacks. Over East Asia, local Planck and lapse rate feedbacks are underestimated, while tropospheric water vapor, stratospheric temperature, and cloud feedbacks are overestimated. The simulation biases in local longwave (LW) and shortwave (SW) cloud feedbacks over East Asia are considerable, probably due to the failure in simulating cloud fraction response of marine cirrostratus, deep convective cloud, and stratus. The intermodel spread of cloud feedback is the largest for both global and East Asian local feedback processes. Our results suggest that contemporary climate models are still difficult to accurately simulate global and local climate feedback processes. [ABSTRACT FROM AUTHOR]

We found an unintentional error in the calculation of monthly anomalies used in our previous paper (Arai and Kimoto 2008). [ABSTRACT FROM AUTHOR]

This paper studies the impacts of El Niño-Southern Oscillation (ENSO) on wintertime extreme precipitation in China from 1961 to 2017, and possibly different influences of two El Niño types are also examined. We find that ENSO poses profound influences on extreme precipitation in many portions of China. El Niño (La Niña) intensifies (weakens) the winter precipitation extremes in southeastern China (SEC) and slightly weakens (intensifies) those in central-north China, mainly by changing the frequency rather than the intensity of extreme precipitation. Further analysis suggests that the East Asian winter monsoon tends to be weaker during El Niño winters, suppressing the southward invasion of cold dry air. Via the weakened Walker circulation, El Niño also triggers anomalously descending motion and anticyclone over the western North Pacific (WNP), which exhibits southwesterly anomalies over SEC and is thus conducive to the transport of sufficient moisture into that area. The changes over WNP are also accompanied by ascending motions over East Asia via a local meridional circulation alike the Pacific–Japan pattern. Meanwhile, El Niño induces a southward-displaced East Asian jet stream, as characterized by intensified westerly over southern China, corresponding to anomalously ascending motion there. Together, these changes facilitate the updrafts of circulations and the condensation of water vapor, thus increasing the occurrence of extreme precipitation events in SEC. Moreover, we find that different Niño types exhibit distinct subregional influences. In particular, the canonical eastern Pacific El Niño significantly increases the frequency of extreme precipitation in South China, whereas the Modoki central Pacific El Niño mainly impacts that in East China. [ABSTRACT FROM AUTHOR]

The mid-Holocene (MH; 6 ka) is one of the benchmark periods for the Paleoclimate Modeling Intercomparison Project (PMIP) and provides a unique opportunity to study monsoon dynamics and orbital forcing (i.e., mostly precession) that differ significantly from the present day. We conducted a data–model comparison along with a dynamic analysis to investigate monsoonal (i.e., East Asian summer monsoon; EASM) precipitation changes over East Asia during the MH. We used the three phases of the PMIP simulations for the MH, and quantitatively compare model results with pollen-based climate records. The data–model comparison shows an overall increase in the summer monsoon precipitation, except a local decrease during the MH. Decomposition of the moisture budget into thermodynamic and dynamic components allows us to assess their relative role in controlling EASM precipitation during the MH, and to investigate the precipitation changes obtained from pollen records in terms of physical processes. We show that the dynamic effect, rather than the thermodynamic effect, is the dominant control in increased EASM precipitation during the MH in both the proxy records and models. The dynamic increase in precipitation results mainly from the enhancement of horizontal monsoonal moisture transport that is caused by intensified stationary eddy horizontal circulation over East Asia. In addition, a cloud-related cooling effect reduced the thermodynamic contribution to the increase in EASM precipitation during the MH. [ABSTRACT FROM AUTHOR]

Transitional Climate Zone (TCZ) over East Asia, characterized by semi-arid climate, is ecologically fragile environment with limited water resources, making atmospheric moisture supply being the key influential factor. This study investigates the moisture sources of summer (JJA) rainfall in the TCZ over East Asia and associated impact during 1979–2010 with the Lagrangian particle dispersion model. Seven moisture source regions and associated contribution are quantified: Eurasia continent to northwest of the TCZ (EC, 32.14%), central-eastern China (CEC, 16.62%), western Pacific Ocean (WPO, 8.58%), South China Sea and Indonesia (SCSI, 2.99%), Bay of Bengal (BOB, 1.4%), Arabian Sea (AS, 0.71%) and local evaporation (TCZ, 32.58%). The direct moisture contribution from ocean (13.67%) is much less than those from the continent (81.34%), due to the great loss en-route. In particular, the local evaporation not only contributes the most moisture among 7 selected source regions, but also exerts the greatest influence in summer precipitation variability in TCZ. Furthermore, the moisture related to summer rainfall over TCZ conveyed by westerlies and monsoon are discriminated according to the dominant system of moisture source regions. It is found that moisture contributions from summer monsoon system (30.3%) and the mid-latitude westerlies (32.14%) dominant areas are quite close. However, further analysis shows that summer monsoon system takes more responsibility for inter-annual fluctuation of summer precipitation in TCZ from the perspective of moisture supply, followed by local evaporation and mid-latitude westerlies. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) play an important role in the climate of East Asia due to their close linkage to precipitation extremes. In this study, long-term trends in ARs over East Asia for the period 1951–2015 are investigated using long-term records of historical climate, including the ERA5 climate reanalysis and the APHRODITE precipitation dataset. These datasets are produced at a relatively high spatial resolution of 0.25° × 0.25°, which allows for evaluation of the long-term trends in the fine-scale characteristics of ARs. The results indicate a significant decreasing trend in ARs and the associated precipitation over the north of East Asia. These dynamical changes dominate the decreasing trend of total summer precipitation amounts in parts of northern China. The decreasing trend in ARs over the north is principally related to the intensification and southward displacement of the southwesterly monsoon flow in boreal summer. In contrast, increasing AR activity and the associated precipitation and heavy rain events over the south of East Asia are observed. These changes are associated with a warmer and more humid environment along AR axes, as well as the southward shift of ARs driven by the dynamical responses of the large-scale environments in the context of climate warming. These responses include the intensification of the upper-level westerly jet accompanied with the strengthening of the South Asian Anticyclone during summer season. Moreover, in contrast to the general decreasing trends in boreal summer, AR activity during boreal winter-spring exhibits significant increasing trends, implying a potential weakening of the seasonality of ARs. This study shows that ARs are important synoptic mechanisms within observed precipitation trends over East Asia, such that understanding their response to a warming climate is a prerequisite to characterizing the nature of future precipitation changes in this region. [ABSTRACT FROM AUTHOR]

The Taiwan Strait has the largest temporal variability in the wintertime sea surface temperature (SST) along the China coast. The warming and cooling trends reach about ± 1 °C per decade in winter during 1982–1999 and 1999–2014, respectively, which are about 4 times larger than neighboring coastal area and open ocean. Previous studies have noted these opposite trends, but the cause remains unclear due to insufficient study on teleconnections of this local signals to large-scale climate signals (e.g., Pacific Decadal Oscillation, PDO; El Niño–Southern Oscillation, ENSO). Using different period filters, wintertime SST anomaly in the Taiwan Strait (TS-SSTa) of different timescales were separated and connected to the large-scale climate signals. Besides the impact of global warming, we also found that the interdecadal signal of PDO contribute significantly to the warming and cooling trends of the wintertime TS-SSTa during 1982–2014 (mostly the positive PDO phase). During the positive PDO phase, a sea level pressure (SLP) dipole develops at the North Pacific, leading to a northeasterly wind jet along the Eurasian eastern coast, and affecting the interdecadal wintertime TS-SSTa through the East Asia Winter Monsoon (EAWM) and North Pacific Subtropical Gyre. During the negative PDO phase, the influence from North Pacific weakens, and the tropical Pacific has a greater influence on the interannual wintertime TS-SSTa through the northward movement of the ENSO-related cyclone/anticyclone anomaly over East Asia. Thus, the influence from North Pacific (PDO) and tropical Pacific (ENSO) alternately control the TS-SSTa variation during the positive and negative PDO phases. [ABSTRACT FROM AUTHOR]

A record-breaking Meiyu-Baiu rainfall hit East Asia in June–July 2020. The warm Indian Ocean (IO) has been identified as a primary cause, but it cannot explain the heavy rainfall in July, a striking characteristic of the 2020 Meiyu-Baiu rainfall. A remarkable retreat of Arctic sea ice in the late spring and early summer of 2020 also promoted Meiyu-Baiu rainfall by favoring North Asian blockings and cold air outbreaks. However, its importance compared with IO warming is unclear. Our result shows that the abundant moisture supply to the 2020 Meiyu-Baiu rainfall mainly stems from anomalous meridional wind convergence, while the excessive ascending motions are due to warm advection tied to enhanced mid-troposphere westerlies. AGCM experiments are used to examine the relative importance of IO warming and Arctic sea ice anomalies. In June, IO warming is responsible for the atmospheric circulation anomalies around the Meiyu-Baiu region and accounts for ~ 75% of the Meiyu-Baiu rainfall anomalies, despite the Arctic sea-ice loss explaining most circulation anomalies over Eurasian high latitudes. In July, both IO warming and Arctic sea-ice loss are necessary for meridional convergence, enhanced westerlies, and thus the heavy rainfall over the Meiyu-Baiu region. Their effects are interdependent rather than additive. Strong IO warming is rarely observed alongside severe Arctic sea-ice loss before 2020 because of their discordant interannual variations. In the future, the combined effects of IO warming and Arctic sea-ice loss on the Meiyu-Baiu rainfall may become more pronounced as their long-term trends continue. [ABSTRACT FROM AUTHOR]

This study identified three distinct convection structures over the Indo-western Pacific that caused high-temperature extreme (HTE) events in Korea during the boreal autumn season based on k-means clustering analysis. These three clusters commonly involve barotropic high-pressure anomalies over East Asia that act as essential components for driving HTE events. However, the dynamical atmospheric teleconnection pathways from the Indo-western Pacific to East Asia for the three clusters were distinguishable. The convection structure of the first cluster exhibited a meridional tripole pattern from the eastern Indian Ocean to East Asia and was strongly associated with phase 5 of the Madden Julian Oscillation (MJO). In response to these convective heating anomalies, high-pressure anomalies over East Asia are induced by the northeastward propagating low-level atmospheric wave train and are also affected by the zonally elongated high-pressure system within the upper atmosphere. In contrast, the HTE events in Korea for the second cluster are derived from enhanced convection over the northeastern Indian Ocean that acts as the atmospheric heat source that generates an eastward atmospheric wave train, thus forming a high-pressure anomaly over East Asia. Finally, the third cluster represents the response of El Niño and positive Indian Ocean dipole, showing a sharp contrast of convection between the western Indian Ocean and the eastern Indian Ocean/western Pacific. The impacts of the third cluster on the Korean HTE primarily stem from Rossby wave trains forced by convective heating anomalies around the tropical Indo-western Pacific. Through analysis of the dynamical variables, detailed descriptions of the physical characteristics of the three distinct clusters of Indo-western Pacific convection are presented in this study, and possible implications for autumn climate variability over East Asia and Korea are also provided. [ABSTRACT FROM AUTHOR]

Previous studies have revealed that the Madden–Julian oscillation (MJO) can exert a profound impact on the surface air temperature (SAT) anomalies over East Asia during winter (December–February). In this study, it is found that such MJO-related winter SAT anomalies can be modulated by the interannual variability of the Tibetan Plateau snow cover (TPSC). During the excessive TPSC (ETPSC) winters, the MJO-related East Asian SAT anomalies in phase 3 are significantly colder than normal. However, during the reduced TPSC (RTPSC) winters, such SAT anomalies are close to normal. A linear baroclinic model is used to examine the possible physical mechanisms. During the ETPSC winters, the more energetic MJO can excite stronger poleward Rossby waves and intensify the upper level cyclonic anomalies over East Asia and lead to the significantly colder SAT anomalies. While during the RTPSC winters, the suppressed MJO convection excites weaker poleward Rossby waves and cannot make colder SAT anomalies over East Asia. The numerical evidences also show that the variation of the mean state could affect the teleconnections but it does not benefit a stronger Rossby wave train over East Asia in ETPSC winters. These results confirm that the TPSC can exert a profound modulation effect on the MJO teleconnection and further impact on the winter SAT anomalies over East Asia. [ABSTRACT FROM AUTHOR]

Pliocene warmth has been used as an analogue for predicting climate response to the rapid atmospheric CO2 increase in the future. Pedogenic carbonates provide an essential archive to reconstruct terrestrial paleoenvironment in a warm world. The stable carbon isotopic compositions of pedogenic carbonate ( δ 13 C carb ) are principally used to reconstruct the history of C4 plants along with coexisting organic matter δ 13 C SOM . However, a growing body of evidence shows spatial and temporal decoupling between δ 13 C carb and δ 13 C SOM especially in arid regions, raising concerns when using δ 13 C carb as a proxy for photosynthetic pathway. Here, we report paired δ 13 C carb and δ 13 C SOM data from Pliocene to early Pleistocene loess-paleosols in Chinese Loess Plateau (CLP), middle latitude East Asia. The model sensitivity analysis suggests the dominant control of soil respiration flux (SRF) on δ 13 C carb values in this region. SRF is then reconstructed using the Monte Carlo simulation method. Our results show that the SRF was enhanced during Pliocene warmth, associated with increased vegetation density and East Asian Summer Monsoon (EASM) precipitation. An overall higher SRF across the CLP during the mid-Pliocene, compared with the Holocene and Last Glacial Maximum (LGM) period, suggests there was a northwestward shift of the EASM rain belt and vegetation under warmer climate. Our results suggest that global temperature has dominated the middle latitude ecosystem and soil variations through its influence on the hydrological cycle, providing insights into future ecological and environmental response to global warming in East Asia. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO) is one of the leading modes of tropical intra-seasonal variability, which exerts significant impacts on the weather and climate across the globe, particularly in the tropics. MJO affects the Asian monsoon by producing enhanced and suppressed convection during the active and break periods, respectively. In the recent decades, the heat content of Indo-western Pacific Ocean has increased significantly, which strengthened the MJO activity. Previous studies also have shown that the expansion of Indo-western Pacific warm pool led to the warping of MJO life cycle, which decreases its residence time over the Indian Ocean (IO) and increases over the Pacific Ocean. Here we show that in the boreal summer months, MJO amplitude has strengthened during the global warming hiatus or rapid IO warming period (1999–2015) compared to the previous period (1982–1998). In the later period, MJO exhibits a faster regeneration over the western IO, and its residence time has increased in the western hemisphere and western IO but decreased in the eastern IO and eastern Pacific Ocean. The strengthening of MJO and the readjustment in its residence time are due to the local MJO feedback on the IO and the La Nina like sea surface temperature pattern in the Pacific Ocean. The prolonged MJO activity leads to bursts of rainfall over the Indian subcontinent in Phase 3 and Phase 4, influencing the active spells of the Indian summer monsoon and causing heavy rainfall over central India and East Asia. [ABSTRACT FROM AUTHOR]

Heat waves, can be exacerbated by the co-occurrence of daytime heat waves and nighttime heat waves. Over China, the Yangtze–Huaihe River basin (YHRB) is the core region of the occurrence of such "compound heat waves", which exert profound impacts on the society and ecosystems. However, the physical mechanisms responsible for the variability of the YHRB compound heat waves remain unclear. In this study, the interannual variability of YHRB compound heat waves in peak summer (July–August) and its possible causes are investigated based on station observations across China and global reanalysis datasets. A strong link is found between the previous winter Arctic Oscillation (AO) and these peak-summer compound heat waves. During a negative AO in winter, an anomalous tripolar pattern of sea surface temperature (SST) in the North Atlantic is induced by the AO-related atmospheric circulation. Such tripolar SST pattern can persist until the following summer. By that time, oceanic forcing dominates, and positive SST anomalies over the tropical North Atlantic can excite a Rossby wave train propagating eastward from the southwest coast of North America to East Asia. This results in a northwest-southeast tilted high pressure system over the YHRB, favoring the occurrence of peak-summer compound heat waves. Such a delayed influence of the winter AO, together with the North Atlantic capacitor effect, is clearly seen in a case study of the YHBR compound heat waves in 2010. The proposed mechanism is further verified based on numerical experiments with an atmospheric general circulation model. [ABSTRACT FROM AUTHOR]

Atmospheric blockings and their impacts on wintertime surface air temperature in East Asia are investigated by using three blocking indices. Blockings in two specific regions that indirectly or directly modulate East Asian climate variability are analyzed: Ural (UR) blocking over East Asia's northwest and Okhotsk (OK) blocking over East Asia's northeast. Both lead to cold anomalies in East Asia but through slightly different mechanisms. In the typical case of UR blocking, cold air associated with a mid-level trough develops to the east of the quasi-stationary blocking and slowly moves southeastward as the Siberian high intensifies. Four days after the blocking onset, East Asia experiences significant cooling due to the temperature advection associated with anomalous temperature gradient and adiabatic cooling. During the typical OK blocking event, a preexisting cold anomaly over northern Eurasia is pushed southeastward by the westward expansion of the blocking. The anomalous meridional flow due to blocking-induced dipolar circulation over the North Pacific further reinforces the cold advection over East Asia, with a minor contribution of vertical processes. In both types of blocking, the cold anomaly persists even in the decaying phase, due to an offset between the temperature advection and the diabatic heating. [ABSTRACT FROM AUTHOR]

The impacts of global warming on Meiyu–Baiu extreme rainfall and the associated mid-latitude synoptic-scale weather systems over the Eastern China (EC) and the Baiu rainband (Bu) regions in East Asia have been examined, based on simulations from the 20-km Meteorological Research Institute atmospheric general circulation model (MRI-AGCM3.2S). This model was demonstrated to give realistic Asian extreme rainfall, when compared with data from the Tropical Rainfall Measuring Mission (TRMM). Here we used a novel wave-selection algorithm based on the 300 hPa wind, in order to identify upper-level propagating wave signals in conjunction with the occurrence of extreme precipitation in either EC or Bu. The same algorithm was applied for both the present (1979–2003) and future (2075–2099) climate simulations from the AGCM, so as to infer the impacts of global warming on the behavior of these systems. Results show robust decrease of intensity of systems influencing both Bu and EC in the future warmer climate. Their corresponding low-to-mid level circulation, as revealed by vertical velocity, temperature advection and sea-level pressure composites, was also found to be weakened. This is likely related to changes in the background circulation in future over the East Asian mid-latitude zone, such as the widespread increment of the seasonal mean static stability at 500 hPa. However, the wave-associated precipitation over these regions was enhanced in the future climate simulations. This can be attributed to more strong intensity rainfall, which increases as the background temperature in these regions warms, largely following the Clausius–Clapeyron relation. Therefore, changes of wave-related extreme precipitation in EC and Bu are mainly controlled by the thermodynamic effect; the latter appears to be much stronger than the potential impacts due to the slight weakening of these weather systems. [ABSTRACT FROM AUTHOR]

Previous studies have projected an increase in future summer precipitation across East Asia (EA). This study investigates the relative contributions of thermodynamic and dynamic components to future precipitation changes in three key sub-regions of EA where the maximum centers of the historical precipitation are located (the tropical region, East China, and the Japan and Korea sector), and analyzes the causes of the changes in thermodynamic and dynamic components. Outputs from 30 climate models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used. From these, the five best-performing models for historical summer precipitation climatology for EA are selected. The future summer precipitations in the three sub-regions over the near- to mid-term (2020–2069) and the long-term (2070–2095) are then examined using the multi-model ensemble mean of the five models selected (MMM05). The projections were driven by four combined scenarios of the Shared Socioeconomic Pathways (SSPs) and forcing levels of the Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). The results show that long-term precipitations under SSP5-8.5 are greater than those under the other scenarios across all sub-regions. After the 2070s under SSP5-8.5, a marked precipitation intensification is identified in all three sub-regions, but with different rates of increase. The projected precipitation increase is primarily attributed to the thermodynamic component, while the dynamic component related to circulation changes is relatively weak. Further analysis indicates that the pattern of the thermodynamic component in the three sub-regions is dominated by the climatological upward motion, mediated by an increase in moisture. [ABSTRACT FROM AUTHOR]

This paper identifies the seasonal relationships between monsoon activities over South China (SC) in winter and summer, which can help improve seasonal predictability. A predefined unified monsoon index that can well represent monsoon characteristics in both winter and summer over SC is employed. A time series of this unified monsoon index in summer and winter, and a lead-lag correlation analysis of the unified monsoon index, found that from summer to winter, monsoon activities tend to have an out-of-phase relationship (weak summer with strong winter monsoons or strong summer with weak winter), while from winter to summer, they tend to be in-phase (weak winter with weak summer or strong winter with strong summer). The composite difference between strong and weak winter monsoons shows that in the preceding summer of strong winter monsoons, monsoon activities tend to be weak, due to the influence of developing La Niña-like events; in the ensuing summer, monsoon activities tend to be strong, modulated by an anomalous cyclone over the western North Pacific (WNP), which is triggered by that La Niña-like pattern over the East Pacific. From summer to winter, the Indian Ocean dipole (IOD) mode is evident over the Indian Ocean (IO); from winter to summer, the Indian Ocean Basin (IOB) mode is dominant, which confirms the important role of the IO in these seasonal relationships of monsoon activities. A negative IOD pattern acts together with La Niña-like forcing, enhances northerly anomalies over East Asia, strengthens winter monsoon winds, and thus enhances the out-of-phase relationship. IOB cooling can capture the anomalous signal of El Niño–Southern Oscillation (ENSO) and favor the persistence of an anomalous cyclone over the WNP until the following summer ("capacitor effect"), which is important for the in-phase relationship. These roles are also further verified by numerical experiments that prescribe ENSO-like, IOD-like, and IOB-like heat source anomalies over the Pacific and IO in an anomalous atmospheric general circulation model. [ABSTRACT FROM AUTHOR]

This paper evaluates multi-decadal simulations of the UCLA version of Climate Forecast System version 2, in which the default Noah land surface model has been replaced with the Simplified Simple Biosphere Model version-2. To examine the influence of the atmosphere-ocean (AO) interaction on the variability, two different simulations were conducted: one with interactive ocean component, and the other constrained by the prescribed sea surface temperature. We evaluate the mean seasonal climatology of precipitation and temperature, along with the model's ability to reproduce atmospheric variability at different scales over the globe, including extratropical modes of atmospheric variability, and long-term trends of global and hemispheric temperature and regional precipitation. Here, we particularly selected two monsoon regions, East Asia and West Africa, where the simulation of multi-decadal variations which has heretofore been a challenging task, to examine decadal variation of monsoon precipitation. In general, temperature anomaly trends were better captured than those of precipitation in both simulations. Results suggest that the AO interaction, represented as latent heat flux, contributes to improve reproducibility of global-wide climatology, extratropical modes of atmospheric variability, and variability in the multi-decadal climate simulation, as well as for inter-decadal variability of the East Asian summer monsoon. [ABSTRACT FROM AUTHOR]

Southern China (SC) has been increasingly affected by extreme high-temperature events during summer (July and August, JA) in recent decades. However, to what extent the anomaly pattern of the extreme high temperature days (EHDs) over SC is predictable remains largely unexplored. To address this issue, we build physics-based empirical model to predict the spatial distribution of JA EHDs over SC and examine its predictability. Two major modes of EHDs over SC are identified by empirical orthogonal function analysis based on the observational data during period of 1980–2009. The first mode is a homogeneous EHDs mode related to a prevailing low-level high-pressure anomaly over SC. The second mode of EHDs is a meridional dipole pattern associated with the Pacific-Japan pattern over East Asia/Western Pacific sector and Pacific Meridional Mode-like SSTA over North Pacific. Via tracking the boundary anomalies forcing of EHDs over SC, physically meaningful predictors are selected to establish a set of physics-based empirical (P-E) models for predicting the first two leading principal components (PCs). The two modes can be predicted reasonably well by the P-E models. Thus, they can be regarded as the predictable modes. Assuming that these two modes can be predicted perfectly, 66.1% of the total variance for the distribution of summer EHDs over SC are potentially predictable. Using the first two predicted PCs and corresponding observed spatial pattern of EOF modes, the prediction of anomalies pattern of EHDs over SC can be obtained. The independent forecast skill (2010–2019) of the domain-averaged temporal correlation coefficient is 0.37, which is close to that of 0.44 for maximum potential attainable prediction skill. [ABSTRACT FROM AUTHOR]

The leading mode of the atmospheric background flow over the North Atlantic–Arctic Ocean (i.e., 300-hPa meridional wind; V300-EOF1) exhibited notable decadal variations during 1979–2019, with intensified V300-EOF1 since the early-2000s. The modulation of the interdecadal change in the East Asian winter monsoon (EAWM) by the V300-EOF1 mode and its link with the concurrently warmer North Atlantic and low Arctic sea ice was explored through observational analysis and numerical simulations. The results indicated that the intensified V300-EOF1 mode enhanced the modulation of Rossby wave trains propagating from the North Atlantic–Arctic region to East Asia, which interfered with the climatological quasi-stationary waves to induce a stronger EAWM. The widespread North Atlantic–Arctic warming tended to modulate and interact with this V300-EOF1 background flow, which further enhanced the quasi-stationary Rossby waves along the midlatitude and sub-polar waveguides, resulting in a strengthened EAWM and cold East Asia. This finding was further verified by numerical experiments conducted with an atmospheric model. The model responses generally captured the observed physical processes, albeit weaker in magnitude, suggestive of a modulating role played by external forcings in the enhancement of the V300-EOF1 mode and the EAWM. Noting also the large internal variability in the atmospheric responses, with approximately half of the model years capturing a stronger-than-average East Asian cooling along with deep and strong Arctic warming. This further indicates the importance of atmospheric internal variability in the development of the V300-EOF1 mode. These results have implications for future changes in the EAWM and concomitant East Asian cooling on interdecadal timescales. [ABSTRACT FROM AUTHOR]

Eastern China is regularly exposed to extreme precipitation with significant socio-economical consequences. Following an observational analysis in a first part of this study, here the ability of the Coupled Model Intercomparison Project Phase 6 models to reproduce the main modes of interannual variability of 5-day summer extreme precipitation over eastern China is evaluated, using an empirical orthogonal teleconnection (EOT) method. These models capture the main patterns and magnitudes of the different EOT patterns, although the North China mode is less well represented. Models also reproduce the dynamical features associated with each mode. There is no systematic improvement in the ability of models to simulate either the pattern or the 5-day intensity when using higher resolution models compared to coarser resolution ones. Instead, multi-member or multi-model ensembles lead to results closer to observations. Using a low mitigation projection pathway (SSP-370), it is shown that the risk of the most extreme 5-day precipitation events by about 40%, 80% and more than 150% for global-mean warming levels, relative to 1850–1900, of + 1.5, + 2 and + 3 ∘ C respectively. This increase is found to be more significant for 5-days events than for seasonal scale precipitation, consistent with previous studies. [ABSTRACT FROM AUTHOR]