Asian Summer Climate Research Articles

Skilful predictions of the Summer North Atlantic Oscillation

The Summer North Atlantic Oscillation is the primary mode of atmospheric variability in the North Atlantic region and has a significant influence on regional European, North American and Asian summer climate. However, current dynamical seasonal prediction systems show no significant Summer North Atlantic Oscillation prediction skill, leaving society ill-prepared for extreme summers. Here we show an unexpected role for the stratosphere in driving the Summer North Atlantic Oscillation in both observations and climate prediction systems. The anomalous strength of the lower stratosphere polar vortex in late spring is found to propagate downwards and influence the Summer North Atlantic Oscillation. Windows of opportunity are identified for useful levels of Summer North Atlantic Oscillation prediction skill, both in the 50% of years when the late spring polar vortex is anomalously strong/weak and possibly earlier if a sudden stratospheric warming event occurs in late winter. However, we show that model dynamical signals are spuriously weak, requiring large ensembles to obtain robust signals and we identify a summer ‘signal-to-noise paradox’ as found in winter atmospheric circulation Our results open possibilities for a range of new summer climate services, including for agriculture, water management and health sectors.

Impact of the Atlantic multidecadal variability on East Asian summer climate in idealized simulations

The impact of the Atlantic multidecadal variability (AMV) on the East Asian summer climate is analyzed using a set of idealized simulations performed by CESM1. The results show that the AMV exerts a remote impact on the East Asian summer climate via two pathways: a midlatitude Rossby wave train and equatorial Kelvin waves. Specifically, the positive phase of the AMV can enhance the land–sea pressure contrast between the Asian continent and western North Pacific Ocean through the midlatitude Rossby wave train, and induce an anomalous anticyclone in the Northwest Pacific through exciting an equatorial Kelvin wave. These anomalies in atmospheric circulation in turn enhance the East Asian summer monsoon, leading to a northern-wet–southern-dry and warm summer climate in East Asia.

Decadal Shift in Summer Precipitation Variability over East Asia in the Mid-2000s and Wave Propagation toward North America

Abstract The East Asian summer climate displays a marked change after the late 1990s. This is principally due to a weakening of the Pacific–Japan (PJ) teleconnection pattern that was a dominant driver of precipitation variability over East Asia. Nevertheless, western Japan has frequently experienced heavy rainfall events over the past several years. Atmospheric reanalysis and observational datasets are used to investigate summer precipitation variability over East Asia in a view of interdecadal changes from 1979 to 2020. East Asian summer precipitation has increased the most in Japan, especially over and around the Southwest Islands in southern Japan, where cumulus convection has been more dominant for the characteristic of precipitation variability including mei-yu–baiu rainfall than frontal structure since the mid-2000s. Atmospheric analysis in vertical structures and convective instability indicates that moist instability is neutralized by cumulus convection over the southern East China Sea and off the Pacific coast of Japan, where recent warming in sea surface temperature (SST) along the Kuroshio exceeds the SST threshold for convection. This decadal shift in precipitation variability has a close relationship with the second precipitation mode over East Asia, which has taken the place of the PJ pattern as a leading driver of precipitation variability over western Japan in the past decade. The enhanced cumulus convection is enabled to act as a forcing mechanism for the Rossby wave train from East Asia toward North America along the westerly jet or for the eastward extension of the Silk Road pattern, which possibly favors a heatwave in the Pacific Northwest areas of North America.

Formation Mechanism of the ENSO-Independent Summer Western North Pacific Anomalous Anticyclone

Abstract The western North Pacific anomalous anticyclone (WNPAC) is the key circulation modulating the East Asian summer climate. In this study, the formation mechanism of the summer WNPAC that is independent of El Niño–Southern Oscillation (ENSO) is investigated. Although ENSO has a significant relationship with WNPAC, except for the super El Niño years, the WNPAC index remains almost unchanged after removing ENSO’s impact, suggesting the possibility of other origins of the WNPAC apart from ENSO. An Atlantic-to-Pacific two-step mechanism is proposed for the formation of ENSO-independent summer WNPAC. In boreal spring, diabatic heating induced by the positive sea surface temperature anomalies (SSTAs) over the tropical Atlantic could stimulate a stationary equivalent barotropic Rossby wave train that travels across the Eurasian continent and ends in the tropical North Pacific. At the end of the Rossby wave train, the lower-level anomalous anticyclone advects negative moist enthalpy into the equator, which suppresses the local convection over the tropical North Pacific and equatorial central Pacific, and thus triggers the lower-level equatorial easterly anomaly to its west. During boreal summer, the lower-level easterly anomaly leads to the zonal dipole SSTA pattern with a negative center in the tropical central Pacific and a positive one in the Maritime Continent. Then, this dipole SSTA pattern over the Pacific exerts a relaying effect that further reinforces and westward shifts the dipole convection anomaly pattern, generating the WNPAC as a Gill-type response. This study underpins the independent role of Atlantic oceanic forcing through the extratropical route in the formation of ENSO-independent summer WNPAC. Significance Statement By linearly removing ENSO’s impact on summer monthly sea surface temperature anomalies (SSTAs), this study explores the sources of summer western North Pacific anomalous anticyclone (WNPAC) apart from ENSO. It is found that the high correlation between WNPAC and ENSO largely depends on the super El Niño events, and the origin of the ENSO-independent WNPAC is rooted in the tropical Atlantic oceanic forcing. The positive tropical Atlantic SSTAs in spring can independently induce atmospheric teleconnection from the North Atlantic to the North Pacific through the extratropical route, which triggers anomalous easterlies over the equatorial Pacific via suppressed local convection caused by the dry air advection over the North Pacific. The equatorial easterlies enhance the dipole anomalous SST and convection pattern over the Maritime Continent and tropical western/central Pacific in summer, which finally stimulates the WNPAC. The result emphasizes the independent role of tropical Atlantic SSTAs in the formation of summer WNPAC, thus providing a new perspective for the seasonal prediction of East Asian summer climate.

The Influences of Atlantic Sea Surface Temperature Anomalies on the ENSO-Independent Interannual Variability of East Asian Summer Monsoon Rainfall

Abstract As the most dominant tropical climate mode on the interannual time scale, El Niño–Southern Oscillation (ENSO) is suggested to significantly influence the interannual variation of East Asian summer monsoon rainfall (IEASMR). However, the leading mode of IEASMR remains almost untouched when the impacts of preceding ENSO events are linearly removed, suggesting the existence of alternative impact factors and predictability sources of IEASMR. After removing the impact of ENSO, the sea surface temperature anomalies (SSTAs) over both the tropical Atlantic and extratropical North Atlantic are found to be related to IEASMR through atmospheric teleconnections. Positive SSTA over the tropical Atlantic could induce tropical diabatic heating, which triggers an equivalent barotropic Rossby wave train emanating from the Atlantic, going across the Eurasian continent, and ending with a cyclonic anomaly over northeast Asia. The tropical diabatic heating could also induce western North Pacific anomalous anticyclone via tropical routes. The dipole SSTA pattern with cooling in the west and warming in the east over the extratropical North Atlantic induces local circulation anomalies through heat flux exchange, which could further perturb a Rossby wave train with a cyclonic anomaly over northeast Asia, thus modulating IEASMR. Numerical experiments with prescribed atmospheric heating associated with Atlantic SSTAs could realistically reproduce these teleconnections toward IEASMR. By adding the predictability sources of Atlantic SSTAs, the seasonal hindcast skills of IEASMR could be significantly improved over both the tropical western North Pacific and subtropical land regions such as central China and Japan. Significance Statement The purpose of this article is to identify the alternative impact factors of the interannual variation of East Asian summer monsoon rainfall (IEASMR), after removing the impact of ENSO, considering the limited contribution of ENSO to the variances of IEASMR. Here, we find that the Atlantic sea surface temperature anomalies (SSTAs) play a considerable role in driving IEASMR. The impact of ENSO on IEASMR is mainly confined to the tropical western North Pacific, while the Atlantic SSTAs influence rainfall over subtropical East Asia and the tropical western North Pacific through both tropical and extratropical routes. The results unravel the important roles of Atlantic SSTAs in driving ENSO-independent IEASMR, which will have a large implication for the seasonal prediction of East Asian summer climate.

Drivers and characteristics of the Indo-western Pacific Ocean capacitor

Coherently coupled ocean-atmosphere variability of the tropical Indo-Pacific Oceans gives rise to the predictability of Asian summer climate. Recent advances in Indo-western Pacific Ocean capacitor (IPOC) theory and the relationship with El Niño-Southern Oscillation (ENSO) are reviewed. The IPOC features tropical Indian Ocean (TIO) warming and an anomalous anticyclonic circulation over the western North Pacific (WNPAAC), the latter driving water vapor transport to East Asia and causing extreme events, e.g., heavy rainfalls from central China to Japan during the boreal summer. IPOC events often occur in post-ENSO summers, but the significant TIO warming could sustain the WNPAAC without a strong El Niño, forced instead by a strong antecedent positive Indian Ocean Dipole (IOD). In latter cases, the Indian Ocean and WNP act as a self-sustaining system, independent of external forcings. El Niño or positive IOD induces the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to SST warming and a “C-shaped” wind anomaly during winter and early spring. Furthermore, the southwest TIO downwelling Rossby waves reflect as oceanic Kelvin waves on the African coast. In the early summer, the resultant southeast TIO SST warming induces a second “C-shaped” wind anomaly. Both southwest and southeast TIO warming contribute to the WNPAAC. The WNPAAC modulates the water vapor pathways to East Asia in the late spring and summer, which mostly converge over the South China Sea and adjacent regions before flowing further to the north. More water vapor is transported from the western Pacific warm pool and less from the southern hemisphere and the Indian Ocean. The enhanced Asian Summer monsoon and moisture content lead to extreme rainfalls in central China and Japan during the boreal summer.

Increased variability of the western Pacific subtropical high under greenhouse warming.

SignificanceThe western Pacific subtropical high (WPSH) channels moisture from the tropics that underpins the East Asian summer climate. Interannual variability of the WPSH dominates climate extremes in the densely populated countries of East Asia. In 2020, an anomalously strong WPSH led to catastrophic floods with hundreds of deaths, 28,000 homes destroyed, and tens of billions in economic damage in China alone. How the frequency of such strong WPSH events will change is of great societal concern. Our finding of an increase in future WPSH variability, translating into an increased frequency of climate extreme as seen in the 2020 episode, highlights the increased risks for the billions of people in the densely populated East Asia with profound socioeconomic consequences.

Late Holocene climate variability in central Korea indicated by vegetation, geochemistry, and fire records of the Yongneup moor

In many cases, proxy-based palaeoclimatic and palaeoenvironmental reconstructions in East Asia have shown mixed signatures of summer temperature and precipitation, driven by the predominant regional impact of the East Asian Summer Monsoon. Although these studies have contributed significantly to understanding the past variability in East Asian summer climate during the Holocene period, relatively little has been learned about climatic conditions in other seasons, such as winter and spring, and variations in the individual climatic elements of temperature and precipitation. To address these issues, in this study, we present a reconstruction of a 3000-year fire and vegetation history of the Yongneup moor, central Korea, during the late Holocene based on multi-proxy analyses of pollen, geochemistry, and macrocharcoal. Our data identify two fire episodes at Yongneup during ca. 2900–2700 and 2400–2300 cal yr B.P., possibly caused amid deteriorated climatic conditions not only in summer but also in winter–spring during contemporaneous grand solar minima. During the last two millennia, Yongneup likely experienced cold weather conditions at ca. 1420–1230 and 1000–590 cal yr B.P. and warm conditions at ca. 1850–1420 and 1230–1000 cal yr B.P., as well as wet conditions during ca. 1530–1400, 1220–1000, and 890–700 cal yr B.P. and drier conditions during ca. 1750–1530, 1400–1220, 1000–890, and 700–530 cal yr B.P. The dry periods were coincident with an increased El Niño–Southern Oscillation in the Tropical Pacific Ocean, which supports its role in controlling the hydroclimate of the Korean Peninsula. Our results also emphasise the potential value of further research on the “Millennium Eruption” of Mount Paektu/Changbaishan (946 CE), whose climatic consequences in the East Asian region remain poorly understood. • Late Holocene climate variability in Korea reconstructed from the Yongneup moor • Fire events coincident with solar minima at ca. 2900–2700 and 2400–2300 cal yr B.P. • Potential impact of climate deterioration on Mumun (Bronze Age) societies in Korea • Fluctuations in cold–warm and dry-wet conditions during the last two millennia • Implication on the volcanic impact of the “Millennium Eruption” of Mount Paektu

Centennial- to millennial-scale Asian summer monsoon changes during the MIS 5/4 transition revealed by high-resolution stalagmite records from southwestern China

During the Marine Isotope Stage (MIS) 5/4 transition, the global climate changed from an interglacial to a glacial state, and hence it was characterized by high-amplitude climate changes. High-resolution stalagmite records can potentially improve our understanding of climate change during this transition. Here we use a high-precision 230Th-dated, 50-yr-resolution stalagmite δ18O record from Yangkou Cave, in Chongqing, southwestern China, to reveal centennial- to millennial-scale changes in Asian summer monsoon (ASM) intensity during the interval of 98.8–59.3 kyr B.P. (thousands of years before 1950 CE). The record reveals five Chinese Interstadial events, namely CIS 18–22, and pronounced centennial-scale oscillations are evident and verified within these millennial-scale events. There are four centennial-scale events of monsoon strengthening in both CIS 21 and CIS 22, corresponding to Greenland Interstadial events GI 21 and GI 22. By contrast, CIS 18, CIS 19 and CIS 20 differ in both structure and onset time relative to the corresponding GI events during MIS 4. During MIS 5, reduced ice sheet and sea-ice cover and strong Atlantic Meridional Overturning Circulation (AMOC) forced the Intertropical Convergence Zone (ITCZ) towards its northern limit, which enhanced the teleconnection between the ASM and climate change in northern high latitudes. During MIS 5, the millennial-scale events (CIS 21 and 22) show a rapid atmospheric teleconnection between the ASM and the climate of northern high latitudes, but this coupling did not exist during MIS 4. The weakening of Northern Hemisphere summer insolation, the expansion of ice sheets and sea ice, combined with the increased influence of Antarctica, may have led to the decoupling of the Asian summer monsoon and climate change in high northern latitudes.

Prediction of the Western North Pacific Subtropical High in Summer without Strong ENSO Forcing

The western North Pacific subtropical high (WNPSH) is one of the deterministic predictors of the East Asian summer climate, and a better prediction of the WNPSH favors more reasonable forecast of the East Asian summer climate. This study focuses on seasonal prediction of the WNPSH during neutral summers without strong El Niño-Southern Oscillation (ENSO) forcing, and explores the associated predictable sources, using the one-month lead time retrospective forecasts from the Ensembles-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) project during 1960–2005. The results indicate that the ENSEMBLES atmosphere-ocean-land coupled models exhibit considerable prediction skill for the WNPSH during neutral summers, with successful reproduction of the WNPSH in the majority of neutral summers. The anomalous WNPSH in neutral summers, which corresponds to cyclonic/anticyclonic anomalies in the lower troposphere, is highly correlated with an east-west dipole local sea surface temperature (SST) distribution over the tropical WNP, suggesting an intimate local air-sea coupling. Further diagnosis of the local SST-rainfall relationship and surface heat flux indicates that the anomalous local SST plays an active role in modulating the variation of the WNPSH during neutral summers, rather than passively responding to the atmospheric change. The local SST anomalies and relevant air-sea coupling over the tropical WNP are reasonably well reproduced in the model predictions, and could act as primary predictable sources of the WNPSH in neutral summers. This could aid in forecasting of the East Asian rainband and associated disaster mitigation planning.

Dynamical downscaling simulation of the East Asian summer monsoon in a regional Climate‐Weather Research and Forecasting model

AbstractA regional Climate‐Weather Research and Forecasting (CWRF) model with a 30‐km horizontal resolution was applied to simulate the East Asian summer monsoon (EASM) and climate in China from 1980 to 2016. As compared with observations and reanalysis data, the model can reasonably reproduce the spatial distributions of the climatological mean atmospheric circulation and water vapour transport in East Asia, as well as the seasonal advance and retreat of EASM and rain bands. The correlation coefficient between the EASM circulation index in the simulation and reanalysis is .97. The model can well represent the geographic distributions of summer mean temperature and precipitation over most of China. However, model biases still exist, in particular the skill in the Yangtze–Huaihe River basin is relatively low. The simulated climatological mean temperature and precipitation deviations from observations may be related to the model's systematic circulation biases, with a thicker lower troposphere (i.e., higher temperature between 500 and 1,000 hPa) over most of China (except for the south and southwest) and a thinner one over the coastal oceans in summertime. Compared with the reanalysis, the model overestimates the land‐ocean thermal contrast, which causes the South Asian High shifted to the north and east, as well as the subtropical high and tropical convective activity located further north and persisted longer. Consequently, more water vapour transports northward, leading to more precipitation over North China–Northeast China and less precipitation over the Yangtze and Huaihe River basin. Therefore, the enhanced land‐ocean thermal contrast may be a major factor for the stronger EASM in the model, and corresponding precipitation and temperature biases in China. This analysis provides important information for further improving model performance in EASM simulation.

The Influence of Atlantic Variability on Asian Summer Climate Is Sensitive to the Pattern of the Sea Surface Temperature Anomaly

AbstractWe simulate the response of Asian summer climate to Atlantic multidecadal oscillation (AMO)-like sea surface temperature (SST) anomalies using an intermediate-complexity general circulation model (IGCM4). Experiments are performed with seven individual AMO SST anomalies obtained from CMIP5/PMIP3 global climate models as well as their multimodel mean, globally and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive (warm) AMO phase, a Rossby wave train propagates eastward, causing a high pressure and warm and dry surface anomalies over eastern China and Japan. During the negative (cool) phase of the AMO, the midlatitude Rossby wave train is less robust, but the model does simulate a warm and dry South Asian monsoon, associated with the movement of the intertropical convergence zone in the tropical Atlantic. The circulation response and associated temperature and precipitation anomalies are sensitive to the choice of AMO SST anomaly pattern. A comparison between global SST and North Atlantic SST perturbation experiments indicates that East Asian climate anomalies are forced from the North Atlantic region, whereas South Asian climate anomalies are more strongly affected by the AMO-related SST anomalies outside the North Atlantic region. Experiments conducted with different amplitudes of negative and positive AMO anomalies show that the temperature response is linear with respect to SST anomaly but the precipitation response is nonlinear.

Evaluation of East Asian Summer Climate Prediction from the CESM Large-Ensemble Initialized Decadal Prediction Project

Based on surface air temperature and precipitation observation data and NCEP/NCAR atmospheric reanalysis data, this study evaluates the prediction of East Asian summer climate during 1959–2016 undertaken by the CESM (Community Earth System Model) large-ensemble initialized decadal prediction (CESM-DPLE) project. The results demonstrate that CESM-DPLE can reasonably capture the basic features of the East Asian summer climate and associated main atmospheric circulation patterns. In general, the prediction skill is quite high for surface air temperature, but less so for precipitation, on the interannual timescale. CESM-DPLE reproduces the anomalies of mid- and high-latitude atmospheric circulation and the East Asian monsoon and climate reasonably well, all of which are attributed to the teleconnection wave train driven by the Atlantic Multidecadal Oscillation (AMO). A transition into the warm phase of the AMO after the late 1990s decreased the geopotential height and enhanced the strength of the monsoon in East Asia via the teleconnection wave train during summer, leading to excessive precipitation and warming over East Asia. Altogether, CESM-DPLE is capable of predicting the summer temperature in East Asia on the interannual timescale, as well as the interdecadal variations of East Asian summer climate associated with the transition of AMO phases in the late 1990s, albeit with certain inadequacies remaining. The CESM-DPLE project provides an important resource for investigating and predicting the East Asian climate on the interannual and decadal timescales.

Recent Shift in the State of the Western Pacific Subtropical High due to ENSO Change

AbstractThe boreal summer western Pacific subtropical high (WPSH) exhibits a remarkable decadal shift in its spatial pattern and periodicity around the late 1990s. In the former period, the WPSH is primarily characterized by a large-scale uniform pattern over Asia and its surrounding area with an oscillating period of ~4–5 yr. However, the WPSH-related atmospheric circulations shift to a dipole structure and oscillate at ~2–3 yr in the recent period. We found that this decadal shift is largely contributed by the ENSO regime change. During the former period, the tropical Pacific was dominated by the conventional eastern Pacific (EP) El Niño–Southern Oscillation (ENSO) with an oscillating period of ~4–5 yr. Strong anticyclone anomalies usually are maintained over the western North Pacific (WNP) during the EP El Niño decaying summer, accounting for most of the WPSH temporal and spatial variability. In contrast, the recent period features much more frequent occurrence of central Pacific (CP) El Niño events in the tropical Pacific with a ~2–3-yr oscillating period. A dipole structure in the WNP and Indian Ocean is evident during both developing and decaying summers of CP El Niño, consistent with the WPSH leading mode after the late 1990s. The results have important implications for seasonal prediction of the WPSH and associated Asian summer climate anomalies.

Projected changes in the western North Pacific subtropical high under six global warming targets

The summer western North Pacific subtropical high (WNPSH) has large influences on the East Asian summer climate. Many studies have focused on the projected changes in the WNPSH, but little is known...

Interdecadal variation of Indian Ocean basin mode and the impact on Asian summer climate

AbstractThis study documents an interdecadal variation in the pattern of Indian Ocean basin mode (IOBM) of sea surface temperature (SST) during boreal summer, where the center of action in the IOBM shifted towards the Arabian Sea after the late 1970s. Correspondingly, the influences of an interannual warming in the tropical Indian Ocean on the Asian summer climate became more significant after the late 1970s due to the Arabian Sea center of action, which are characterized by anomalous subsidence and anticyclones over the Bay of Bengal and the South China Sea. These atmospheric anomalies may result in surface temperature and precipitation anomalies in a wide area of Asia. Numerical experiments demonstrate the different influences of different IOBM patterns on the Asian summer climate. Statistical analysis and numerical experiments indicate a minor role of the different tropical Pacific SSTs concurrent with the different IOBMs in causing the different Asian summer climate anomalies.

The non-linear relationship between the western North Pacific anticyclonic circulation and Korean summer precipitation on subseasonal timescales

It has been widely known that the pulse of the western North Pacific anticyclonic circulation (WNPAC) plays a key role on East Asian summer climate variability in subseasonal to interannual time scales. Yet, the relation between the WNPAC and summer precipitation over Korea is not robust on seasonal and subseasonal timescales for the recent few decades. Here, we show that the low correlation between WNPAC and Korean precipitation is attributable to their distinctive non-linear relationship and investigate detailed features in their four different phase relationships on subseasonal time scales. First, the positive Korean precipitation anomaly occurs as the part of zonally elongated precipitation band along with the positive WNPAC anomaly closely connected with the decaying phase of El Nino and Indian Ocean warming. The second case is enhanced Korean precipitation accompanying with negative WNPAC anomaly. In this case, the negative WNPAC anomaly constitutes a part of atmospheric wave train across the North Pacific and this feature is associated with the decaying phase of La Nina. The typhoon-related precipitation event also contributes to this case. Third, the negative Korean precipitation anomaly arises when the low-level easterly anomaly along with the northern flank of negative WNPAC anomaly leads to suppressed convective activity in Korea. Lastly, the negative Korean precipitation anomaly happens with the positive WNPAC anomaly when anomalous high covers entire East Asia and the WNP mainly during the decaying phase of central Pacific El Nino. The understanding of the non-linear relationship between WNPAC and Korean precipitation in this study provides better insight into the potential impacts of El Nino-Southern Oscillation and circulation/convection anomaly over the WNP on summer Korean precipitation.

Solar cycle modulation of the relationship between the boreal spring Northern Atlantic Oscillation and the East and Southeast Asian summer climate

Previous studies have suggested that boreal spring Northern Atlantic Oscillations (NAOs) exert a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship between the spring NAO and the following EASM varies with the 11-year solar cycle. The analysis of the time–lag relationship between the NAO in May and the following summer rainfall indicates that during the low solar activity (LS) summers, a more robust relationship is established, with below-normal rainfall anomalies in South China and the Sunda Islands in relation to a positive NAO. In addition, the May NAO-related atmospheric circulation anomalies during boreal summer over East and Southeast Asia may well explain the distinct rainfall difference. The possible mechanism for this solar modulation is attributed to the changes in structure of the spring NAO and associated tripole SST anomalies in the North Atlantic. Specifically, in the LS phase, the North Atlantic tripole SST anomalies associated with the May NAO can strongly develop and persist into the early summer. These SST anomalies tend to induce a remarkable subpolar wave train across the northern Eurasian continent and an enhanced East Asian jet in the summer. Our results indicate that, for the summer climate prediction over East and Southeast Asia, both the spring NAO and the 11-year solar cycle should be considered.

Northwest Pacific Anticyclonic Anomalies during Post–El Niño Summers Determined by the Pace of El Niño Decay

Abstract This study investigates the characteristics and maintaining mechanisms of the anomalous northwest Pacific anticyclone (NWPAC) following different El Niño decaying paces. In fast decaying El Niño summers, the positive SST anomalies in the tropical central-eastern Pacific (TCEP) have transformed to negative, and positive SST anomalies appear around the Maritime Continent (MC), whereas in slow decaying El Niño summers, positive SST anomalies are present in the TCEP and in the tropical Indian Ocean (TIO). During fast decaying El Niño summers, the cold Rossby wave in response to the negative TCEP SST anomalies has a primary contribution to maintaining the NWPAC anomalies. The warm Kelvin wave response and enhanced Hadley circulation anomalies forced by the positive MC SST anomalies also facilitate developing the NWPAC anomalies. During slow decaying El Niño summers, the warm Kelvin wave anchored over the TIO plays a crucial role in sustaining the NWPAC anomalies, while the warm Rossby wave triggered by the positive TCEP SST anomalies weakens the western part of the NWPAC anomalies. The southwesterly anomalies of the NWPAC anomalies during fast decaying El Niño summers can reach to higher latitudes than those during slow decaying El Niño summers. Correspondingly, positive rainfall anomalies appear in northern China and the Yangtze River basin in fast decaying El Niño summers but are only distributed in the Yangtze River basin in slow decaying El Niño summers. This study implies that the El Niño decaying pace is a key factor in East Asian summer climate.

Structural Changes in the Pacific–Japan Pattern in the Late 1990s

AbstractThe Pacific–Japan (PJ) pattern, also known as the East Asia–Pacific pattern, is a teleconnection that significantly influences the East Asian summer climate on various time scales. Based on several reanalysis and observational datasets, this study suggests that the PJ pattern has experienced a distinct three-dimensional structural change in the late 1990s. Compared with those during 1979–98, the PJ pattern shifts eastward by approximately 20° during 1999–2015, and the intensity of its barotropic structure in the extratropics weakens significantly. As a result, its influences on the summer rainfall along the mei-yu band are weakened after the late 1990s. These observed changes can be attributed to three reasons. First, the location where the PJ pattern is excited shifts eastward. Second, the easterly shear of the background wind is very weak around the source region of the PJ pattern after the late 1990s, which prevents the convection-induced baroclinic mode from converting into barotropic mode and thereby from propagating into the extratropics. Third, the PJ pattern–induced rainfall anomalies are weak along the mei-yu band after the late 1990s. As a result, their feedbacks to the PJ pattern become weak and play a considerably reduced role in maintaining the structure of the PJ pattern in the midlatitudes. In contrast, the eddy energy conversion from the basic flow efficiently maintains the PJ pattern before and after the late 1990s and thereby contributes little to the observed change.