Subtropical High Anomalies over the Western Pacific and Its Relations to the Asian Monsoon and SST Anomaly

Using the data of 500 hPa geopotential height from 1951 to 1995, SST roughly in the same period and OLR data from 1974 to 1994, the relation between the anomalies of subtropical high (STH for short) and the tropical circulations including the Asian monsoon as well as the convective activity are studied. In order to study the physical process of the air-sea interaction related to STH anomaly, the correlation of STH with SST at various sea areas, lagged and simultaneous, has been calculated. Comparing the difference of OLR, wind fields, vertical circulations and SST anomalies in the strong and weak STH, we investigate the characteristics of global circulations and the SST distributions related to the anomalous STH at the western Pacific both in winter and summer. Much attention has been paid to the study of the air-sea interaction and the relationship between the East Asian monsoon and the STH in the western Pacific. A special vertical circulation, related to the STH anomalies is found, which connects the monsoon current to the west and the vertical flow influenced by the SST anomaly in the tropical eastern Pacific.

A numerical study of the impact of SST anomaly in the warm pool area on atmospheric circulation in winter

In this paper the impacts of the anomalous SST in the warm pool area of the Western Equatorial Pacific on the winter time circulation and the East Asian monsoon are studied by using the NCAR CCM. It is found that the abnormal heating in the warm pool area will change the strength and the position of the Walker Cell in the Equatorial Pacific and the anti-Walker Cell in the equatorial Indian Ocean. Both the Walker and anti-Walker Cells are strengthened. The local Hadley Cells over two hemispheres near the warm pool are also strengthened. The subtropical highs in two hemispheres become stronger and move poleward slightly. The westerly jets in the extratropical regions have similar changes as the subtropical highs. The winter monsoon in South-East Asia is weakened by the abnormal heating in the warm pool. The experiment also show that there are wave trains emanating from surrounding areas of the warm pool to the high latitudes, causing various changes in circulations and local weather.

Understanding the role of SST anomaly in extreme rainfall of 2020 Meiyu season from an interdecadal perspective

Extreme Meiyu rainfall in 2020,starting from early June to the end of July,has occurred over the Yangtze River valley(YRV),with record-breaking accumulated precipitation amount since 1961.The present study aims to examine the possible effect of sea surface temperature(SST)on the YRV rainfall in Meiyu season from the interdecadal perspective.The results indicate that YRV rainfall in June exhibits more significant variability on interdecadal time scale than that in July.The interdecadal-filtered atmospheric circulation in June,compared with the counterpart in July,shows a more predominant and better-organized Western North Pacific Anticyclone(WNPAC)anomaly,which could transport abundant moisture to the YRV by anomalous southwesterly prevailing in northwestern flank of anomalous WNPAC.Both observation and numerical experiment indicate that the interdecadal change of the SST anomaly in tropical western Indian Ocean(TWI)from preceding May to June can significantly affect the anomalous WNPAC,leading to enhanced YRV rainfall in June.The TWI SST anomaly shifts from a cold phase to a warm phase around the early 2000s,with a magnitude of 0.7°C in 2020,which implies that such interdecadal warming might partly contribute to the heavy rainfall in June 2020 by providing a large-scale favorable background flow.

The Response of Anomalous Vertically Integrated Moisture Flux Patterns Related to Drought and Flood in Southern China to Sea Surface Temperature Anomaly

With the extreme drought(flood)event in southern China from July to August in 2022(1999)as the research object,based on the comprehensive diagnosis and composite analysis on the anomalous drought and flood years from July to August in 1961-2022,it is found that there are significant differences in the characteristics of the vertically integrated moisture flux(VIMF)anomaly circulation pattern and the VIMF convergence(VIMFC)anomaly in southern China in drought and flood years,and the VIMFC,a physical quantity,can be regarded as an indicative physical factor for the"strong signal"of drought and flood in southern China.Specifically,in drought years,the VIMF anomaly in southern China is an anticyclonic circulation pattern and the divergence characteristics of the VIMFC are prominent,while those are opposite in flood years.Based on the SST anomaly in the typical draught year of 2022 in southern China and the SST deviation distribution characteristics of abnormal draught and flood years from 1961 to 2022,five SST high impact areas(i.e.,the North Pacific Ocean,Northwest Pacific Ocean,Southwest Pacific Ocean,Indian Ocean,and East Pacific Ocean)are selected via the correlation analysis of VIMFC and the global SST in the preceding months(May and June)and in the study period(July and August)in 1961-2022,and their contributions to drought and flood in southern China are quantified.Our study reveals not only the persistent anomalous variation of SST in the Pacific and the Indian Ocean but also its impact on the pattern of moisture transport.Furthermore,it can be discovered from the positive and negative phase fitting of SST that the SST composite flow field in high impact areas can exhibit two types of anomalous moisture transport structures that are opposite to each other,namely an anticyclonic(cyclonic)circulation pattern anomaly in southern China and the coastal areas of east China.These two types of opposite anomalous moisture transport structures can not only drive the formation of drought(flood)in southern China but also exert its influence on the persistent development of the extreme weather.

NUMERICAL EXPERIMENT ON MID-LATITUDE SST ANOMALY AND VARIATION IN CIRCULATION PATTERN

By means of simulation experiments with a two-dimensional zonal-mean model,a study is made of the influence of the western North-Pacific midlatitude sea-surface temperature(SST)anomalies on the circulation pattern and wet-seasonal precipitation over the East-China Changjiang-Huaihe reaches and the North-China plain.The SST anomalies are divided into two types,one being“colder in the south and warmer in the north”and the other just opposite,depending on season.Results show that the occurrence of the anomalies is followed by considerable changes in the position of the subtropical high happening for 3-5 months to come.For instance,the spring“colder in the south and warmer in the north”anomalous type (i.e.,colder for 20—35°N,and warmer north of 35°N)leads to the intensification of the summer subtropical high,with the ridge line moved slightly northward,resulting in drought over Changjiang reaches for July-August and in excessive rainfall in the North-China plain,and vice versa.

Numerical Simulation of the Effect of the SST Anomalies in the Tropical Western Pacific on the Blocking Highs over the Northeastern Asia

The effects of the sea surface temperature (SST) anomalies in the tropical western Pacific on the atmospheric circulation anomalies over East Asia are simulated by the IAP-GCM with an observed and idealized distributions of the SST anomalies in the tropical western Pacific,respectively.Firstly,the atmospheric circulation anomalies during July and August,1980 are simulated by three anomalous experiments including the global SST anomaly experiment,the tropical SST anomaly experiment and the extratropical SST anomaly experiment,using the observed SST anomalies in 1980.It is shown that the SST anomalies in the tropical ocean greatly influence the formation and maintenance of the blocking high over the northeastern Asia,and may play a more important role than the SST anomalies in the extratropical ocean in the influence on the atmospheric circulation anomalies.Secondly,the effects of the SST anomalies in the tropical western Pacific on the atmospheric circulation anomalies over East Asia are also simulated with an idealized distribution of the SST anomalies in the tropical western Pacific.The simulated results show that the negative anomalies of SST in the tropical western Pacific have a significant effect on the formation and maintenance of the blocking high over the northeastern Asia.

Synergistic Interdecadal Evolution of Precipitation over Eastern China and the Pacific Decadal Oscillation during 1951-2015

By using the multi-taper method(MTM)of singular value decomposition(SVD),this study investigates the interdecadal evolution(10-to 30-year cycle)of precipitation over eastern China from 1951 to 2015 and its relationship with the North Pacific sea surface temperature(SST).Two significant interdecadal signals,one with an 11-year cycle and the other with a 23-year cycle,are identified in both the precipitation and SST fields.Results show that the North Pacific SST forcing modulates the precipitation distribution over China through the effects of the Pacific Decadal Oscillation(PDO)-related anomalous Aleutian low on the western Pacific subtropical high(WPSH)and Mongolia high(MH).During the development stage of the PDO cold phase associated with the 11-year cycle,a weakened WPSH and MH increased the precipitation over the Yangtze River Basin,whereas an intensified WPSH and MH caused the enhanced rain band to move northward to North China during the decay stage.During the development stage of the PDO cold phase associated with the 23-year cycle,a weakened WPSH and MH increased the precipitation over North China,whereas an intensified WPSH and the weakened MH increased the precipitation over South China during the decay stage.The 11-year and 23-year variabilities contribute differently to the precipitation variations in the different regions of China,as seen in the 1998flooding case.The 11-year cycle mainly accounts for precipitation increases over the Yangtze River Basin,while the 23-year cycle is responsible for the precipitation increase over Northeast China.These results have important implications for understanding how the PDO modulates the precipitation distribution over China,helping to improve interdecadal climate prediction.

Numerical Simulation of the Relationships between the 1998 Yangtze River Valley Floods and SST Anomalies

With the IAP/LASG GOALS model, the relationships between the floods in the Yangtze River valley arid sea surface temperature anomalies (SSTA) in the Pacific and Indian Oceans in 1998 have been studied. The results show that the model can reproduce the heavy rainfall over the Yangtze River valley in the sum-mer of 1998 forced by global observational sea surface temperatures (SST). The model can also reproduce the observed principal features of the subtropical high anomalies over the western Pacific. The experiments with the observed SST in different ocean areas and different periods have been made. By comparing the ef-fects of SSTA of different ocean areas on the floods, it is found that the SSTA in the Indian Ocean are a ma-jor contributor to the floods, and the results also show that the SSTA in the Indian Ocean and the western Pacific have a much closer relationship with the strong anomalies of the subtropical high over the western Pacific than the SSTA in other concerned areas. The study also indicates that the floods and subtropical high anomalies in the summer of 1998 are more controlled by the simultaneous summertime SSTA than by SSTA in the preceding winter and spring seasons.

Influences of two types of El Ni?o event on the Northwest Pacific and tropical Indian Ocean SST anomalies

Based on the Had ISST1 and NCEP datasets,we investigated the influences of the central Pacific El Ni?o event(CP-EL)and eastern Pacific El Ni?o event(EP-EL)on the Sea Surface Temperature(SST)anomalies of the Tropical Indian Ocean.Considering the remote ef fect of Indian Ocean warming,we also discussed the anticyclone anomalies over the Northwest Pacific,which is very important for the South China precipitation and East Asian climate.Results show that during the El Ni?o developing year of EP-EL,cold SST anomalies appear and intensify in the east of tropical Indian Ocean.At the end of that autumn,all the cold SST anomaly events lead to the Indian Ocean Dipole(IOD)events.Basin uniform warm SST anomalies exist in the Indian Ocean in the whole summer of EL decaying year for both CP-and EP-ELs.However,considering the statistical significance,more significant warm SST anomalies only appear in the North Indian Ocean among the June and August of EP-EL decaying year.For further research,EP-EL accompany with Indian Ocean Basin Warming(EPI-EL)and CP El Ni?o accompany with Indian Ocean Basin Warming(CPI-EL)events are classified.With the remote ef fects of Indian Ocean SST anomalies,the EPI-and CPI-ELs contribute quite differently to the Northwest Pacific.For the EPI-EL developing year,large-scale warm SST anomalies arise in the North Indian Ocean in May,and persist to the autumn of the El Ni?o decaying year.However,for the CPI-EL,weak warm SST anomalies in the North Indian Ocean maintain to the El Ni?o decaying spring.Because of these different SST anomalies in the North Indian Ocean,distinct zonal SST gradient,atmospheric anticyclone and precipitation anomalies emerge over the Northwest Pacific in the El Ni?o decaying years.Specifically,the large-scale North Indian Ocean warm SST anomalies during the EPI-EL decaying years,can persist to summer and force anomalous updrafts and rainfall over the North Indian Ocean.The atmospheric heating caused by this precipitation anomaly emulates atmospheric Kelvin waves accompanied by low level easterly anomalies over the Northwest Pacific.As a result,a zonal SST gradient with a warm anomaly in the west and a cold anomaly in the east of Northwest Pacific is generated locally.Furthermore,the atmospheric anticyclone and precipitation anomalies over the Northwest Pacific are strengthened again in the decaying summer of EPI-EL.Af fected by the local WindEvaporation-SST(WES)positive feedback,the suppressed East Asian summer rainfall then persists to the late autumn during EPI-EL decaying year,which is much longer than that of CPI-EL.

Numerical simulation of influence of the anomalies of theCentral-eastern Equatorial Pacific SST and Arctic seaice cover in summer on the atmospheric circulation

A series of numerical experiments have been conducted with a perpetual July, nine-level general circulation spectral model to determine the effect of variation of the Arctic sea ice cover extent and the joint effect of anomalies of both the Arctic sea ice cover and the Central-eastern Equatorial Pacific sea surface temperature on the summer general circulation. Results show that the two factors,anomalously large extent of the Arctic sea ice cover and anomalously warm sea surface temperature over the Central-eastern Equatorial Pacific Ocean, play substantially the equal role in the effect on the summer general circulation, and either of them can notably induce the atmospheric anomalies. The main dynamical processes determining the effect of the Arctic sea ice and the equatorial SST anomalies are associated with two leading teleconnection patterns, i. e. the Asia North/American and Eurasian patterns observed in atmosphere. The results presented in this paper again prove that the general circulation is fundamentally motivated by the non-uniform heating between the equator and the pole on the rotating earth.

Model Study on the Interannual Variability of Asian Winter Monsoon and Its Influence

The interannual variation of Asian winter (NE) monsoon and its influence is studied using the long-term integration of Max-Plank Institute ECHAM3(T42 L19) model. The simulation well reproduces the main features of the climatological mean Asian winter monsoon and shows pronounced difference of atmospheric circulation between strong and weak winter monsoon and for the consecutive seasons to follow. Most striking is the appearance and persistence of an anomalous cyclonic flow over the western Pacific and enhanced Walker circulation for strong winter monsoon in agreement with the observation. The contrast in summer rainfall patterns of both East China and India can also be discerned in the simulation. Comparison of three sets of experiments with different SST shows that the forcing from the anomalies of global SST makes a major contribution to the interannual variability of Asiao winter monsoon and, in particular, to the interseasonal persistence of the salient features of circulation. The SSTA over the tropical western Pacific also plays an important part of its own in modulating the Walker circulation and the extratropical flow patterns. The apparent effect of strong NE monsoon is to enhance the convection over the tropical western Pacific. This effect, on the one hand, leads to a strengthening of SE trades to the east and extra westerly flow to the west, thus favorable to maintaining a specific pattern of SSTA. On the other hand, the thermal forcing associated with the SSTA acts to strengthen the extratropical flow pattern which is, in turn, conducive to stronger monsoon activity. The result seems to suggest a certain self-sustained regime in the air-sea system, which is characterized by two related interactions, namely the air-sea and tropical-extratropical interactions with intermittent outburst of NE cold surge as linkage. There is a connection between the strength of the Asian winter monsoon and the precipitation over China in the following summer. Links between these two variabilities are mainly through SST anomalies but snow over Asia is a contributing factor as well.

Summer Persistence Barrier of Sea Surface Temperature Anomalies in the Central Western North Pacific

The persistence barrier of sea surface temperature anomalies （SSTAs） in the North Pacific was investigated and compared with the ENSO spring persistence barrier. The results show that SSTAs in the central western North Pacific （CWNP） have a persistence barrier in summer： the persistence of SSTAs in the CWNP shows a significant decline in summer regardless of the starting month. Mechanisms of the summer persistence barrier in the CWNP are different from those of the spring persistence barrier of SSTAs in the central and eastern equatorial Pacific. The phase locking of SSTAs to the annual cycle does not explain the CWNP summer persistence barrier. Remote ENSO forcing has little linear influence on the CWNP summer persistence barrier, compared with local upper-ocean process and atmospheric forcing in the North Pacific. Starting in wintertime, SSTAs extend down to the deep winter mixed layer then become sequestered beneath the shallow summer mixed layer, which is decoupled from the surface layer. Thus, wintertime SSTAs do not persist through the following summer. Starting in summertime, persistence of summer SSTAs until autumn can be explained by the atmospheric forcing through a positive SSTAs-cloud/radiation feedback mechanism because the shallow summertime mixed layer is decoupled from the temperature anomalies at depth, then the following autumnwinter-spring, SSTAs persist. Thus, summer SSTAs in the CWNP have a long persistence, showing a significant decline in the following summer. In this way, SSTAs in the CWNP show a persistence barrier in summer regardless of the starting month.

Causes of seasonal sea level anomalies in the coastal region of the East China Sea

Based on the analysis of sea level, air temperature, sea surface temperature（SST）, air pressure and wind data during 1980-2013, the causes of seasonal sea level anomalies in the coastal region of the East China Sea（ECS） are investigated. The research results show：（1） sea level along the coastal region of the ECS takes on strong seasonal variation. The annual range is 30-45 cm, larger in the north than in the south. From north to south, the phase of sea level changes from 140° to 231°, with a difference of nearly 3 months.（2） Monthly mean sea level（MSL）anomalies often occur from August to next February along the coast region of the ECS. The number of sea level anomalies is at most from January to February and from August to October, showing a growing trend in recent years.（3） Anomalous wind field is an important factor to affect the sea level variation in the coastal region of the ECS. Monthly MSL anomaly is closely related to wind field anomaly and air pressure field anomaly. Wind-driven current is essentially consistent with sea surface height. In August 2012, the sea surface heights at the coastal stations driven by wind field have contributed 50%-80% of MSL anomalies.（4） The annual variations for sea level,SST and air temperature along the coastal region of the ECS are mainly caused by solar radiation with a period of12 months. But the correlation coefficients of sea level anomalies with SST anomalies and air temperature anomalies are all less than 0.1.（5） Seasonal sea level variations contain the long-term trends and all kinds of periodic changes. Sea level oscillations vary in different seasons in the coastal region of the ECS. In winter and spring, the oscillation of 4-7 a related to El Ni？o is stronger and its amplitude exceeds 2 cm. In summer and autumn, the oscillations of 2-3 a and quasi 9 a are most significant, and their amplitudes also exceed 2 cm. The height of sea level is lifted up when the different oscillations superposed. On the other hand, the height of sea level is fallen down.

INTERDECADAL VARIABILITY IN A MODEL ATMOSPHERE

By using the simulation results of an AGCM, which had been run from 1945 to 1993 forced by COADS SST, the interdecadal variability of the model atmosphere was investigated and compared with that of NCEP reanalysis data. It was found that, interdecadal variability exists significantly in both the tropical Pacific wind fields and the mid-high latitude atmospheric circulation of the model atmosphere. The tendency of time variation and spatial distributions of the interdecadal variability of the model atmosphere are basically consistent with observation. Relative to the mid-high latitude atmospheric circulation, the simulation of tropical Pacific wind is more satisfying, which suggests that anomalous variation of SST is still the main factor for the interdecadal variability of tropical Pacific wind. It might have more significant influence on the tropical wind than on the mid-high latitude atmosphere. However, there is still obvious difference between the simulation and observation. They could be attributed to both the simulation capability of the model and absence of other factors in the model which are important for the interdecadal climate variation.

The Response of Geopotential Height Anomalies to El Niño and La Niña Conditions and Their Implications to Seasonal Rainfall Variability over the Horn of Africa

In this study, we unveil atmospheric circulation anomalies associated with the large-scale tropical teleconnections using National Center for Environmental Prediction (NCEP) reanalysis dataset. Composite analyses have been performed to know the impact of large-scale tropical circulations on the Horn of Africa. The composite analysis performed at the geopotential height of 850 Mb and 200 Mb, and precipitation rate (mm/day) during six strong El Niño and La Niña episodes revealed that the large-scale tropical variability induced climate anomalies in space and time. A substantial decrease in upper-level height (200 Mb) has been observed in the study area during El Niño composite years as compared to the La Niña years. During El Niño conditions, the upper-level divergence initiates low-level vertical motion, thereby enhancing convection, however, during La Niña composite years, nearly contrasting situations are noticed in Belg (February to May) season in Ethiopia. However, geopotential height anomalies at 850 Mb are above-normal during the strong El Niño years, suggesting suppressed convection due to vertical shrinking and enhancement of divergence at the lower level. Compared to the Belg (February to May), geopotential anomalies were generally positive during the Kiremt (June to September) season, thereby suppressing the rainfall, particularly in Southern Ethiopia and Northern Part of Kenya. In contrast, an increase in rainfall was observed during the Belg season (February to May).

Impacts of the SSTs over Equatorial Central–Eastern Pacific and Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in Southern China

Low temperature together with snow/freezing rain is disastrous in winter over southern China.Previous studies suggest that this is related to the sea surface temperature(SST)anomalies,especially La Nina conditions,over the equatorial central–eastern Pacific Ocean(EP).In reality,however,La Nina episodes are not always accompanied by rainy/snowy/icy(CRSI)days in southern China,such as the case in winter 2020/2021.Is there any other factor that works jointly with the EP SST to affect the winter CRSI weather in southern China?To address this question,CRSI days are defined and calculated based on station observation data,and the related SST anomalies and atmospheric circulations are examined based on the Hadley Centre SST data and the NCEP/NCAR reanalysis data for winters of1978/1979–2017/2018.The results indicate that the CRSI weather with more CRSI days is featured with both decreased temperature and increased winter precipitation over southern China.The SSTs over both the EP and the southeastern Indian Ocean(SIO)are closely related to the CRSI days in southern China with correlation coefficients of-0.29 and 0.39,significant at the 90%and 95%confidence levels,respectively.The SST over EP affects significantly air temperature,as revealed by previous studies,with cooler EP closely related to the deepened East Asian trough,which benefits stronger East Asian winter monsoon(EAWM)and lower air temperature in southern China.Nevertheless,this paper discovers that the SST over SIO affects precipitation of southern China,with a correlation coefficient of 0.42,significant at the 99%confidence level,with warmer SIO correlated with deepened southern branch trough(SBT)and strengthened western North Pacific anomalous anticyclone(WNPAC),favoring more water vapor convergence and enhanced precipitation in southern China.Given presence of La Ni?a in both winters,compared to the winter of 2020/2021,the winter of 2021/2022 witnessed more CRSI days,perhaps due to the warmer SIO.

Simulation of the Effects of the Preceding SST Anomalies over the Tropical Eastern Pacific on Precipitation to the South of the Yangtze River in June

Numerical experiments are performed to simulate the response of the atmospheric circulation and pre-cipitation over East China in June to the sea surface temperature（SST）anomalies over the tropical eastern Pacific（TEP）from preceding September to June by using an atmospheric general circulation model （AGCM）.We constructed composite positive/negative SST anomalies（P-SSTAs/N-SSTAs）based on the observational SST anomalies over the TEP from September 1997 to June 1998.The results show that：（1） the response of the precipitation in the Yangtze River basin and its southern area（YRBS）to El Nino with different durations varies with the maximum amplitude of the precipitation anomalies appearing when the imposed duration is from November to next June,and the minimum appearing when the SST anomalies is only imposed in June.The anomalies of the precipitation are reduced when the duration of the forcing SST anomalies over the TEP is shortened and the positive SST anomalies in the preceding autumn tend to cause significantly more rainfall in the YRBS.This is in agreement with previous diagnostic analysis results.（2）The simulated precipitation anomalies over the YRBS are always obviously positive under strong or weak positive SST anomalies over the TEP.The intensity of the precipitation anomalies increases with increasing intensity of the SST anomalies in the experiments.The simulation results are consistent with the observations during the warm SST events,suggesting reasonable modeling results.（3）When negative SST anomalies in the TEP are put into the model,the results are different from those of the diagnostic analysis of La Nina events.Negative precipitation anomalies in YRBS could be reproduced only when the negative SST anomalies are strong enough.

TROPICAL SEA SURFACE TEMPERATURE ANOMALY AND INDIAN SUMMER MONSOON

The time series of the sea surface temperature(SST)anomaly,covering the eastern (western)equatorial Pacific,central Indian Ocean,Arabian Sea.Bay of Bengal and South China Sea(SCS),have been analyzed by using wavelet transform.Results show that there exists same interdeeadal variability of SST in the tropical Pacific and tropical Indian Ocean,and also show that the last decadal abrupt change occurred in the 1970s.On the interannual time scale,there is a similar interannual variability among the equatorial central Indian Ocean and the adjacent three sea basins(Arabian Sea.Bay of Bengal and South China Sea).but the SST interannual changes of the Indian Ocean lagged 4—5 months behind that of the equatorial central-east Pacific.Meanwhile, the interannual variability and long-range change between SST anomaly and Indian summer monsoon rainfall in recent decades have been explained and analyzed.It indicates that there existed a wet(dry)period in India when the tropical SST was lower(higher)than normal,but there was a lag of phase between them.

The 2016 Summer Floods in China and Associated Physical Mechanisms:A Comparison with 1998

The characteristics of droughts and floods in China during the summers（May–August）of 2016 and 1998 were compared in great detail,together with the associated atmospheric circulations and external-forcing factors.Following results are obtained.（1）The precipitation was mostly above normal in China in summer 2016,with two main rainfall belts located in the Yangtze River valley（YRV）and North China.Compared with 1998,a similar rainfall belt was located over the YRV,with precipitation 100%and more above normal.However,the seasonal processes of Meiyu were different.A typical＂Secondary Meiyu＂occurred in 1998,whereas dry conditions dominated the YRV in2016.（2）During May–July 2016,the Ural high was weaker than normal,but it was stronger than normal in 1998.This difference resulted from fairly different distributions of sea surface temperature anomalies（SSTAs）over the North Atlantic Ocean during the preceding winter and spring of the two years.（3）Nonetheless,tropical and subtropical circulation systems were much more similar in May–July of 2016 and 1998.The circulation systems in both years were characterized by a stronger than normal and more westward-extending western Pacific subtropical high（WPSH）,a weaker than normal East Asian summer monsoon（EASM）,and anomalous convergence of moisture flux in the mid and lower reaches of the YRV.These similar circulation anomalies were attributed to the similar tropical SSTA pattern in the preceding seasons,i.e.,the super El Ni？o and strong warming in the tropical Indian Ocean.（4）Significant differences in the circulation pattern were observed in August between the two years.The WPSH broke up in August 2016,with its western part being combined with the continental high and persistently dominating eastern China.The EASM suddenly became stronger,and dry conditions prevailed in the YRV.On the contrary,the EASM was weaker in August 1998 and the＂Secondary Meiyu＂took place in the YRV.The Madden–Julian Oscillation（MJO）was extremely active in August 2016 and stayed in western Pacific for 25 days.It triggered frequent tropical cyclone activities and further influenced the significant turning of tropical and subtropical circulations in August2016.In contrast,the MJO was active over the tropical Indian Ocean in August 1998,conducive to the maintenance of a strong WPSH.Alongside the above oceanic factors and atmospheric circulation anomalies,the thermal effect of snow cover over the Qinghai–Tibetan Plateau from the preceding winter to spring in 2016 was much weaker than that in 1998.This may explain the relatively stronger EASM and more abundant precipitation in North China in 2016than those in 1998.

Detecting Primary Precursors of January Surface Air Temperature Anomalies in China

This study aims to detect the primary precursors and impact mechanisms for January surface temperature anomaly （JSTA） events in China against the background of global warming, by comparing the causes of two extreme JSTA events occurring in 2008 and 2011 with the common mechanisms inferred from all typical episodes during 1979- 2008. The results show that these two extreme events exhibit atmospheric circulation patterns in the mid-high latitudes of Eurasia, with a positive anomaly center over the Ural Mountains and a negative one to the south of Lake Baikal （UMLB）, which is a pattern quite similar to that for all the typical events. However, the Eurasian teleconnection patterns in the 2011 event, which are accompanied by a negative phase of the North Atlantic Oscillation, are different to those of the typical events and the 2008 event. We further find that a common anomalous signal appearing in early summer over the tropical Indian Ocean may be responsible for the following late-winter Eurasian teleeonnec- lions and the associated JSTA events in China. We show that sea surface temperature anomalies （SSTAs） in the preceding summer over the western Indian Ocean （WIO） are intimately related to the UMLB-like circulation pattern in the following January. Positive WIOSSTAs in early summer tend to induce strong UMLB-like circulation anomalies in January, which may result in anomalously or extremely cold events in China, which can also be successfully reproduced in model experiments. Our results suggest that the WIOSSTAs may be a useful precursor for predicting JSTA events in China.