South Asian High and Asian-Pacific-American Climate Teleconnection

Growing evidence indicates that the Asian monsoon plays an important role in affecting the weather and climate outside of Asia. However, this active role of the monsoon has not been demonstrated as thoroughly as has the variability of the monsoon caused by various impacting factors such as sea surface temperature and land surface. This study investigates the relationship between the Asian monsoon and the climate anomalies in the Asian-Pacific-American （APA） sector. A hypothesis is tested that the variability of the upper-tropospheric South Asian high （SAH）, which is closely associated with the overall heating of the large-scale Asian monsoon, is linked to changes in the subtropical western Pacific high （SWPH）, the midPacific trough, and the Mexican high. The changes in these circulation systems cause variability in surface temperature and precipitation in the APA region. A stronger SAH is accompanied by a stronger and more extensive SWPH. The enlargement of the SWPH weakens the mid-Pacific trough. As a result, the southern portion of the Mexican high becomes stronger. These changes are associated with changes in atmospheric teleconnections, precipitation, and surface temperature throughout the APA region. When the SAH is stronger, precipitation increases in southern Asia, decreases over the Pacific Ocean, and increases over the Central America. Precipitation also increases over Australia and central Africa and decreases in the Mediterranean region. While the signals in surface temperature are weak over the tropical land portion, they are apparent in the mid latitudes and over the eastern Pacific Ocean.

Simulations of the 100-hPa South Asian High and Precipitation over East Asia with IPCC Coupled GCMs

The South Asian High （SAH） and precipitation over East Asia simulated by 11 coupled GCMs associated with the forthcoming Intergovernmental Panel on Climate Change＇s （IPCC） 4th Assessment Report are evaluated. The seasonal behavior of the SAH is presented for each model. Analyses of the results show that all models are able to reproduce the seasonal cycle of the SAH. Locations of the SAH center are also basically reproduced by these models. All models underestimate the intensity and the extension of coverage in summer. The anomalous SAH can be divided into east and west modes according to its longitudinal position in summer on the interannual timescale, and the composite anomalies of the observed precipitation for these two modes tend to have opposite signs over East Asia. However, only several coupled GCMs can simulate the relationship between rainfall and SAH similar to the observed one, which may be associated with the bias in simulation of the subtropical anticyclone over the West Pacific （SAWP） at 500 hPa. In fact, it is found that any coupled GCM, that can reproduce the reasonable summer mean state of SAWP and the southward （northward） withdrawal （extension） for the east （west） mode of SAH as compared to the observed, will also simulate similar rainfall anomaly patterns for the east and west SAH modes over East Asia. Further analysis indicates that the observed variations in the SAH, SAWP and rainfall are closely related to the sea surface temperature （SST） over the equatorial tropical Pacific. Particularly, some models cannot simulate the SAWP extending northward in the west mode and withdrawing southward in the east mode, which may be related to weak major E1 Nifio or La Nifia events. The abilities of the coupled GCMs to simulate the SAWP and ENSO events are associated partly with their ability to reproduce the observed relationship between SAH and the rainfall anomaly over East Asia.

Influence of Late Springtime Surface Sensible Heat Flux Anomalies over the Tibetan and Iranian Plateaus on the Location of the South Asian High in Early Summer

Variation in the location of the South Asian High （SAH） in early boreal summer is strongly influenced by elevated surface heating from the Tibetan Plateau （TP） and the Iranian Plateau （IP）. Based on observational and ERA-Interim data, diagnostic analyses reveal that the interannual northwestward-southeastwaxd （NW-SE） shift of the SAH in June is more closely correlated with the synergistic effect of concurrent surface thermal anomalies over the TP and IP than with each single surface thermal anomaly over either plateau from the preceding May. Concurrent surface thermal anomalies over these two plateaus in May are characterized by a negative correlation between sensible heat flux over most parts of the TP （TPSH） and IP （IPSH）. This anomaly pattern can persist till June and influences the NW-SE shift of the SAH in June through the release of latent heat （LH） over northeastern India. When the IPSH is stronger （weaker） and the TPSH is weaker （stronger） than normal in May, an anomalous cyclone （anticyclone） appears over northern India at 850 hPa, which is accompanied by the ascent （descent） of air and anomalous convergence （divergence） of moisture flux in May and June. Therefore, the LH release over northeastern India is strengthened （weakened） and the vertical gradient of apparent heat source is decreased （increased） in the upper troposphere, which is responsible for the northwestward （southeastward） shift of the SAH in June.

Establishment of the South Asian High over the Indo-China Peninsula During Late Spring to Summer

The establishment of the upper-level South Asian high （SAH） over the Indo-China Peninsula （ICP） during late boreal spring and its possible causes are investigated using long-term NCEP-NCAR and ERA-40 reanalysis and satellite-observed OLR data. Results show that, from early March to mid-April, deep convection stays south of -6°N over the northern Sumatran islands. As the maximum solar radiation moves over the latitudes of the ICP （10-20°N） in late April, the air over the ICP becomes unstable. It ascends over the ICP and descends over the adjacent waters to the east and west. This triggers deep convection over the ICP that induces large latent heating and strong updrafts and upper-level divergence, leading to the formation of an upper-level anticyclonic circulation and the SAH over the ICE During early to mid-May, deep convection over the ICP intensifies and extends northwards to the adjacent waters. Strong latent heating from deep convection enhances and maintains the strong updrafts and upper-level divergence, and the SAH is fully established by mid-May. Thus, the seasonal maximum solar heating and the land-sea contrast around the ICP provide the basic conditions for deep convection to occur preferentially over the ICP, which leads to the formation of the SAH over the ICP from late April to mid-May. Simulations using RegCM4 also indicate that the diabatic heating over the ICP is conducive to the generation and development of upper-level anticyclonic circulation, which leads to an earlier establishment of the SAH.

THE INTERDECADAL VARIATION OF THE SOUTH ASIAN HIGH AND ITS ASSOCIATION WITH THE SEA SURFACE TEMPERATURE OF TROPICAL AND SUBTROPICAL REGIONS

This study aims to explore the interdecadal variation of South Asian High(SAH) and its relationship with SST(sea surface temperature) of the tropical and subtropical regions by using the NCEP/NCAR monthly reanalysis data from 1948 to 2012, based on the NCAR CAM 3.0 general circulation model. The results show that: 1) the intensity of SAH represents a remarkable interdecadal variation characteristic, the intensity of SAH experienced from weak to strong at the late 1970 s, and after the late 1970 s, its strength is enhanced and the area is expanded in the east-west direction.The expansion degree is greater westward than eastward, while it is opposite in summer. 2) Corresponding to the interdecadal variation of SAH intensity, after the late 1970 s, the divergent component of wind field has two ascending and three descending areas. Of the two ascending areas, one is located in the East Pacific, the other location varies with the season from the Indian Ocean in winter to the South China Sea and West Pacific in summer. Three descending areas are located in the north-central Africa, the East Asia and the Middle Pacific region respectively. 3) Corresponding to the interdecadal variation of SAH intensity, the rotational component of wind field at the lower level is an anomalous cyclone over the South China Sea and West Pacific in summer, while in winter, it is an anomalous cyclone over the Indian Ocean, and an anomalous anticyclone over the equatorial Middle Pacific. 4) Numerical simulations show that the interdecadal variation of SAH is closely related to the SST of the tropical and subtropical regions. The SST of Indian Ocean plays an important role in winter, while in summer, the SST of the South China Sea and West Pacific plays an important role, and the SST of the East Pacific also plays a certain role.

Intercomparison of the South Asian High in NCEP1, NCEP2, and ERA-40 Reanalyses and in Station Observations

The South Asian Highs (SAHs) at 100 hPa over China in the three reanalysis datasets NCEP1, NCEP2, and ERA-40 are evaluated by using station observation data. The results demonstrate a substantial discrepancy even between the reanalyses. First, the data of the three reanalyses generally underestimate the intensity of the SAH in the China domain. Second, there are interdecadal changes in the SAH, with highs in the 1960s and 1980s and lows in the 1970s, 1990s, and 2000s. This interdecadal variation of the SAH can be well depicted with NCEP1 data, but the high in the 1980s is missed by ERA-40. The NCEP2 corresponds well with NCEP 1 and captures the decreasing trend after 1979. Furthermore, the NCEP1 reanalysis overestimates the interdecadal changes of SAH, while ERA-40 underestimates the interdecadal changes. This work suggests that much caution should be exerted when the reanalysis datasets are adopted to study the interdecadal variability of SAH.

INTERDECADAL VARIATION OF THE INTENSITY OF SOUTH ASIAN HIGH

The characteristics and possible physical mechanism of interdecadal variation of the intensity of the South Asian High(SAH) in summer are analyzed using the NCEP/NCAR reanalysis data and NOAA extended reconstructed sea surface temperature(SST) data.The results indicate that a remarkable interdecadal transition occurred in the late1970 s that increased the intensity of SAH,or,an abrupt climate change was around 1978.A comparative analysis between the weak and strong period of the SAH intensity shows that the related anomalous patterns of the atmospheric circulation(including wind field,air temperature field and vertical velocity field) are nearly opposite to each other.The surface latent heat flux anomalies over the plateau(especially in the northwest of the plateau) in summer exert great influence on the interdecadal variation of the SAH intensity and the surface sensible heat flux anomalies play a more important role.Consistent with the interdecadal variation of the SAH intensity,the monopole mode of the tropical Indian Ocean SST in summer also experienced a low to high transition in the late 1970 s.To some extent,this can reveal the impact of the anomalous monopole mode of the tropical Indian Ocean SST in summer on interdecadal variation of the SAH.

Comparison of the seasonal evolution of the South Asian high associated with two types of El Ni?o event

EI Nino （EN） episodes can be classified based on their time of onset as spring onset EN （SPEN） events and summer onset EN （SUEN） events. To evaluate the different influences of SPEN and SUEN events on the South Asian high （SAH）, this study compared the seasonal evolution of the SAH （SESAH） associated with SPEN and SUEN events through analysis of geopotential height and zonal wind data derived from NCEP-NCAR Reanalysis-1 and sea surface temperature data obtained from the Hadley Center. The main features of the SESAH during an EN event are similar to its climatological characteristics. Climatologically, the SAH forms in May, strengthens, and moves northwestward in June and July. It does not change much in August, but then it returns south and weakens during September and October. However, its lifespan is shorter and its intensity weaker during EN periods. Furthermore, there are significant differences between the SESAH during SPEN and SUEN events. During a SPEN episode, the movement of the SAH to the northwest during May and June is slower than during a SUEN event, i.e. the SPEN SAH has a shorter lifespan. In comparison with the SUEN SAH, the SPEN SAH in July and September tends more towards the Tibetan high mode rather than the Iranian high mode. The SPEN SAH in October moves southeastward faster than the SUEN SAH, which also indicates that the SAH has a shorter lifespan during a SPEN event than during a SUEN episode.

Recent Change of the South Asian High

This paper investigates the variability of the summer （May-September） South Asian Iligh （SAIl） for the period 1979-2012. Results show that the intensity and the area of the summer SAH decreased around 2002 at the decadal scale; and the East Asian westerly jet suppressed at the north edge of the SAH, which is consistent with the SAH variation. The precipitation pattern over eastern China also shifted during the same periods, with increased rainfall in the Huang-Huai River region and South China and decreased rainfall in the Yangtze River region. The relationship between the two variations is evidently strengthened via changes in moisture flux.

Assessment of the Summer South Asian High in Eighteen CMIP5 Models

The South Asian High(SAH) is one of the most important components of the Asian summer monsoon system. To understand the ability of state-of-the-art general circulation models(GCMs) to capture the major characteristics of the SAH, the authors evaluate 18 atmospheric models that participated in the Coupled Model Intercomparison Project Phase 5/Atmospheric Model Intercomparison Project(CMIP5/AMIP). Results show that the multi-model ensemble(MME) mean is able to capture the climatological pattern of the SAH, although its intensity is slightly underestimated. For the interannual variability of the SAH, the MME exhibits good correlation with the reanalysis for the area and intensity index, but poor skill in capturing the east-west oscillation of the SAH. For the interdecadal trend, the MME shows pronounced increasing trends from 1985 to 2008 for the area and intensity indexes, which is consistent with the reanalysis, but fails to capture the westward shift of the SAH center. The individual models show different capacities for capturing climatological patterns, interannual variability, and interdecadal trends of the SAH. Several models fail to capture the climatological pattern, while one model overestimates the intensity of the SAH. Most of the models show good correlations for interannual variability, but nearly half exhibit high root-mean-square difference(RMSD) values. Six models successfully capture the westward shift of the SAH center in the interdecadal trends, while other models fail. The possible causes of the systematic biases involved in several models are also discussed.

Synoptic Verification of Medium-Extended-Range Forecasts of the Northwest Pacific Subtropical High and South Asian High Based on Multi-Center TIGGE Data

Synoptic verification of medium-extended-range forecasts of the Northwest Pacific subtropical high （NWPSH） and South Asian high （SAH） is performed for the summers of 2010-2012 using TIGGE data from four operational centers at the CMA,ECMWF,JMA,and NCEP.The overall activities of the NW-PSH and SAH are examined along with their local characteristics such as the spatial coverage of each high in the East Asian key area （10°-40°N,105°-130°E）,the mean position of the ridge of each high over 110°-122.5°E,the westward extent of the NWPSH ridge,and the eastward extent of the SAH ridge.Focus on the NWPSH and SAH is justified because these two systems have pronounced influences on the summertime persistent heavy rainfall in China.Although the overall activities of both highs are reproduced reasonably well in the TIGGE data,their spatial coverages are reduced in the East Asian key area and both of them are weaker compared with observations.On average,their ridges shift more northward relative to observations.The NWPSH ridge is less westward while the SAH ridge is generally more eastward early in the forecast but too westward later in the forecast.The JMA ensemble prediction system （EPS） produces the best mediumrange （1-10 days） forecasts of the NWPSH based on these metrics,while the ECMWF EPS produces the best medium-range forecasts of the SAH and the most reliable extended-range （11-15 days） forecasts of both highs.Forecasts of the spatial coverage of both highs in the East Asian key area and the mean positions of the ridges are generally valid out to lead times of 7-12 days.By contrast,forecasts of the longitudinal extent of the ridges are typically only valid to lead times of 5-7 days.All the four operational centers＇ models produce excellent forecasts of the mean zonal position of the SAH ridge.The ensemble mean forecast is more reliable than the control forecast over the areas where the NWPSH （20°-30°N,135°-165°E） and SAH （23°-30°N,70°-100°E） are most active.Forecasts of both highs have advantages and disadvantages in the peripheral areas away from their respective center of high activity.

Dynamic Effects of the South Asian High on the Ozone Valley over the Tibetan Plateau

In this study,the TOMS/SBUV（Total Ozone Mapping Spectrometer/Solar Backscatter Ultraviolet Radiometer） data and SAGE（Stratospheric Aerosol and Gas Experiment） II data were employed to calculate the monthly total zonal ozone deviations over the Tibetan Plateau and the 150-50-hPa zonal ozone variations.The results show that there is a significant correlation between the two,with a correlation coefficient of 0.977.From 150 to 50 hPa,the ozone valley over the Tibetan Plateau（OVTP） becomes the strongest based on the SAGE II data,and the South Asian high（SAH） is the most active according to the 40-yr reanalysis data of the European Centre for Medium-Range Weather Forecasts（ERA40）,so a correlation between the SAH and the OVTP may exist.The WACCM3（Whole Atmosphere Community Climate Model version 3） simulation results show that both SAH and OVTP could still present within 150-50 hPa with reduced strength even when the height of the Tibetan Plateau was cut down to 1500 m.It is also shown that the seasonal variation of SAH would result in a matched seasonal variation of the OVTP,which suggests a meaningful effect of SAH on the OVTP.Meanwhile,it is found that the atmospheric circulation would impose different effects on the OVTP,depending on the SAH＇s evolution stages and movement directions.At 150-50 hPa,as the SAH approaches the plateau,the SAH zonal（meridional） transport would make the OVTP deeper（shallower）,while the vertical transport of ozone produces a deeper（shallower） OVTP at the lower（higher） level;the combined dynamic effects lead to a weakened OVTP.When the SAH stabilizes over the plateau,the zonal（meridional） transport results in a shallower（deeper） OVTP while the vertical transport would create a deeper（shallower） OVTP at the middle（bottom and top） levels;the combined dynamic effects produce a deeper OVTP.As the SAH retreats from the plateau,the OVTP becomes deeper（shallower） under the zonal（meridional） effect or shallower under the vertical effect;the combined dynamic effects contribute to a deeper（shallower） OVTP at the middle（bottom and top） levels.The SAH would have a weak effect on the OVTP over the plateau when positioned over the tropical Pacific.

THE TIMING OF SOUTH-ASIAN HIGH ESTABLISHMENT AND ITS RELATION TO TROPICAL ASIAN SUMMER MONSOON AND PRECIPITATION OVER EAST-CENTRAL CHINA IN SUMMER

The timing of the South Asian High(SAH) establishment over the Indochina Peninsula(IP) from April to May and its relations to the setup of the subsequent tropical Asian summer monsoon and precipitation over eastern-central China in summer are investigated by using NCEP/NCAR daily reanalysis data,outgoing longwave radiation(OLR)data and the daily precipitation data from 753 weather stations in China.It is found that the transitions of the zonal wind vertical shear and convection establishment over tropical Asia are earlier(later) in the years of early(late) establishment of SAH.In the lower troposphere,anti-cyclonic(cyclonic) anomaly circulation dominates the equatorial Indian Ocean.Correspondingly,the tropical Asian summer monsoon establishes earlier(later).Furthermore,the atmospheric circulation and the water vapor transport in the years of advanced SAH establishment are significantly different from the delayed years in Asia in summer.Out-of-phase distribution of precipitation in eastern-central China will appear with a weak(strong) SAH and western Pacific subtropical high,strong(weak) ascending motion in the area south of Yangtze River but weak(strong) ascending motion in the area north of it,and cyclonic(anti-cyclonic) water vapor flux anomaly circulation from the eastern-central China to western Pacific.Accordingly,the timing of the SAH establishment at the upper levels of IP is indicative of the subsequent onset of the tropical Asian summer monsoon and the flood-drought pattern over eastern-central China in summer.

Bimodality of the South Asia High Simulated by Coupled Models

The observed South Asia High （SAH） center is characterized by two distinctive equilibrium modes during boreal midsummer, namely the center of SAH is located between 82.5°-92.5°E for the Tibetan Plateau mode and between 55°-65°E for the Iranian Plateau mode respectively. The present study describes the ability of 15 coupled general circulation models （CGCM） used in the Intergovernmental Panel on Climate Change＇s （IPCC） 4th Assessment Report to reproduce the observed bimodality of the SAH. These models reveal a wide range of skill in simulating this bimodality. Nearly half of the models reproduced the bimodality, while the other half of the models did not simulate well these two modes whereas usually preferring one mode, The models that reproduced the bimodality of the SAH present similar horizontal and vertical circulations as those features from the NCEP reanalysis data. The results from these models identify the warm characteristics of the SAH and indicate that these two modes have different dynamic and thermodynamic properties. Different characteristics of the simulated sensible heat and latent heat related to precipitation partly contribute to the difference in the simulations of the SAH bimodality. The majority of these models that prefer to simulate the Tibetan Plateau mode produce a small sensible heat flux difference between the Iranian Plateau and the Tibetan Plateau, and also generally simulate a very strong false precipitation center over the east of the Tibetan Plateau, which indicates strong latent release and thereby contributes to the preference of the SAH center on the Tibetan Plateau. Whereas, the models that reproduce the bimodality of the SAH tend to simulate large precipitation over the southern Himalayas and no obviously false precipitation is produced over the east of the Tibetan Plateau. In addition, the model＇s resolution may also have important impacts on the simulations of precipitation.

2023年麦收期河南省连阴雨的气候特征和可能成因

2023年麦收期河南省出现了罕见的连阴雨天气,降水和阴雨日数异常偏多,分别为自1961年以来的历史第五位和第二位。分析连阴雨的可能气候成因,主要结论如下:(1)造成连阴雨的环流形势为欧亚中高纬度西高东低,乌拉尔山高压脊持续发展并出现阶段性阻塞,东北冷涡活跃,冷空气能持续南下,西太平洋副热带高压(以下简称副高)偏北、偏强且异常偏西,来自副高西北侧边缘的暖湿气流和冷空气在河南省交汇。(2)副高偏强、偏北、偏西是由偏强、偏北、偏大、偏东的南亚高压和偏强、偏北的东亚副热带西风急流(以下简称西风急流)的引导所致,而东南亚对流层中高层的暖中心偏大、偏北和偏东且呈东西向带状分布,使南亚高压形成了上述的特征。暖中心和东北冷涡的共同作用,使东亚中高层的气温梯度增大,进而加强了西风急流。(3)2023年前期的La Ni1a事件有利于麦收期副高偏北,但不利于副高偏强和明显偏西。麦收期由于台风扰动由泰国湾至菲律宾群岛以东的热带西太平洋对流活跃释放的凝结潜热较多,以及麦收期前期的春季和麦收期伊朗高原至青藏高原的地面感热和潜热较强,使暖中心形成了上述的特征,进而决定了南亚高压和副高的特征。(4)2023年麦收期北大西洋海温偏高,以及北大西洋三极子为负位相,导致了乌拉尔山高压脊持续发展并出现阶段性阻塞,麦收期前期的春季和麦收期伏尔加河至贝加尔湖西北侧地面感热和潜热偏强,而下游贝加尔湖东南侧至鄂霍次克海地面感热和潜热偏弱一定程度上导致上述两区域分别出现了大范围的位势高度正距平和负距平。

CMA-CPSv3对夏季南亚高压和西太副高的预测能力评估

为了评估CMA-CPSv3模式对影响我国夏季降水的两个高压系统-南亚高压和西太平洋副热带高压的预测能力,利用该模式2001-2020年3月、5月起报的回报结果和ERA5再分析数据,首先评估了该模式对两个高压系统特征指数的预测能力,然后分析了不同起报时间对东亚地区夏季环流系统的预测能力,最后对比了两个高压系统预测同好年和同差年夏季环流系统、水汽输送特征的差异以及对我国夏季降水的影响,并探讨了两个高压系统预测偏差的可能原因。研究表明:(1)CMA-CPSv3模式对西太副高和南亚高压的平均脊线位置的预测效果最好,预测的强度指数和面积指数明显偏强、偏大;与3月起报相比,5月起报的预测结果对西太副高的预测有一定的提高。(2)CMA-CPSv3模式较好地预测了100 hPa和500hPa环流形势,其中温度场和风场的预测效果较好,但预测的南亚高压和西太副高的范围整体偏强。(3)预测的两个高压系统同好年和同差年,两个高压脊线位置预测较好,预测的面积和强度均明显偏强,同好年的预测偏差较小;能够较好地预测出东亚地区水汽通量的空间分布和我国夏季降水分布格局。(4)预测的两个高压系统所在经度区域的上升运动较ERA5结果偏弱,可能是引起预测的南亚高压和西太副高范围偏大、强度偏强的原因之一。

夏季南亚高压东-西振荡过程中青藏高原及周边上对流层水汽的分布和传输特征

夏季南亚高压的“双模态”分布对应着其中心位置在10~20天准双周时间尺度上的东—西振荡,对青藏高原及周边上对流层的水汽分布和传输有显著影响。本文利用夏季7~8月逐日的ERAI再分析资料,通过基于南亚高压东—西振荡指数的位相合成分析发现,当南亚高压呈青藏高原模态时,青藏高原(伊朗高原)地区上对流层水汽含量异常偏高(低),伊朗高原模态时则相反;伴随南亚高压中心位置由青藏高原向西移至伊朗高原上空,上对流层水汽含量正异常中心亦自青藏高原东侧向西逐渐传播到伊朗高原以西地区。进一步诊断表明,除了在青藏高原北侧和南侧水汽经向绝热输送异常有抵消作用外,两高原地区上对流层水汽倾向异常主要由水汽纬向绝热输送异常及其辐合辐散异常所贡献,而青藏高原地区对流活动引起的垂直非绝热输送异常在上对流层则主要与剩余项(水汽的凝结和蒸发)相抵消。因此,青藏高原(伊朗高原)上对流层为水汽含量正异常时对应着青藏高原上空的对流活动异常偏弱(强)。而南亚高压中心位置和上对流层水汽含量正异常中心自青藏高原向伊朗高原移动的过程,对应着青藏高原地区的对流活动异常和垂直向上的水汽非绝热输送异常不断增强,同时上对流层水汽凝结异常也不断增强。此外,南亚高压向西移动过程中,上对流层水汽绝热辐合(辐散)异常主要发生在其西(东)侧,这是造成水汽含量异常中心纬向传播的主要原因。

Interannual Meridional Displacement of the East Asian Upper-tropospheric Jet Stream in Summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS's meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

甘肃东西区夏季不同强度降水事件天气成因分析

基于1961—2018年甘肃省逐日降水格点资料和NECP/NCAR再分析资料,依据逐年降水变化“东西反向”型分布模态,将甘肃以104°E为界分为东西两区开展不同强度的降水事件变化研究及成因分析。结果表明:中降水事件在两区夏季降水的占比均超46%,占主导作用;西区强、中降水事件发生频次呈上升趋势,东区各强度降水事件频次均呈下降趋势。天气成因分析表明,甘肃东西区降水偏强时,南亚高压异常增强,提供了较好的高空辐散条件,西太平洋副热带高压偏强、偏北或偏西,中纬度地区“+-+”的位势高度异常促使冷暖空气增强并交汇于甘肃地区,西风带波动小槽的发展,配合近地层高原东侧低涡将低纬度水汽接力向西北输送,导致降水发生;相较于东区,西区降水夏季风系统异常更强,西风带波动显著,位势高度负异常位置偏北。

2023年夏季新疆高温的阶段性特征及环流演变

气候变暖背景下,2023年夏季新疆气温异常偏高,平均气温、高温日数、高温过程持续时间均居1961年以来同期第一位,且高温覆盖面积最广。针对本次高温的阶段性和极端性特征,利用新疆105个国家级气象站及NCEP/NCAR再分析等资料,分析其不同阶段对流层中高层环流演变规律。结果表明:100 hPa南亚高压偏东偏北且强度偏强,500 hPa伊朗副高持续加强东伸,西太副高加强西伸北扩且异常偏西偏强,两个层面的系统相互呼应,形成垂直高度的稳定正压结构,有利于气温偏高并长期维持,后期随着100 hPa南亚高压主体进入青藏高原上空,500 hPa伊朗副高西退,副热带高压带断裂,中亚地区对流层中高层的稳定结构瓦解,在新疆上游出现低压槽区,有利于冷空气自北方进入,造成阶段性降温天气,高温减弱。通过对极端高温过程的环流特征分析可知,新疆区域性极端高温过程与100 hPa南亚高压系统密切相关,尤其跟中心纬度位置显著正相关,当南亚高压主体在55°~90°E且中心面积偏大时,中心位置偏北有利于新疆区域性极端高温过程的发生。