VARIABILITY IN THE WESTERN PACIFIC SUBTROPICAL HIGH AND ITS RELATIONSHIP WITH SEA TEMPERATURE VARIATION CONSIDERING THE BACKGROUND OF CLIMATE WARMING OVER THE PAST 60 YEARS

By adopting characteristic index data for the Western Pacific Subtropical High(WPSH) from the National Climate Center of China, U.S. National Centers for Environmental Prediction-National Center for Atmospheric Research(NCEP/NCAR) reanalysis data, and the National Oceanic and Atmospheric Administration(NOAA) sea surface temperature(SST) data, we studied the WPSH variability considering the background of climate warming by using a Gaussian filter, moving averages, correlation analysis, and synthetic analysis. Our results show that with climate warming over the past 60 years, significant changes in the WPSH include its enlarged area, strengthened intensity,westward extended ridge point and southward expanded southern boundary, as well as enhanced interannual fluctuations in all these indices. The western ridge point of the WPSH consistently varies with temperature changes in the Northern Hemisphere, but the location of the ridgeline varies independently. The intensity and area of the WPSH were both significantly increased in the late 1980 s. Specifically, the western ridge point started to significantly extend westward in the early 1990 s, and the associated interannual variability had a significant increase in the late 1990 s; in addition, the ridgeline was swaying along the north-south-north direction, and the corresponding variability was also greatly enhanced in the late 1990 s. With climate warming, the SST increase becomes more weakly correlated with the WPSH intensity enhancement but more strongly correlated with the westward extension of the ridge point in the equatorial central and eastern Pacific Ocean in winter, corresponding to an expanding WPSH in space. In the northern Pacific in winter, the SST decrease has a weaker correlation with the southerly location of the ridgeline but also a stronger correlation with the westward extension of the ridge point. In the tropical western Pacific in winter, the correlations of the SST decrease with the WPSH intensity enhancement, and the westward extension of the ridge point is strengthened. These observations can be explained by strengthened Hadley circulations, the dominant effects of the southward shift, and additional effects of the weakened ascending branch of the Walker circulation during warm climatological periods,which consequently lead to strengthened intensities, increased areas, and southward expansions of the WPSH in summer.

THE POSITION VARIATION OF THE WEST PACIFIC SUBTROPICAL HIGH AND ITS POSSIBLE MECHANISM

Using NCEP/NCAR daily reanalysis data and SCSMEX data, an investigation is carried out ofthe relationship between the position variation of the west Pacific subtropical high (WPSH) and the apparentheating in June 1998 based on the complete vertical vorticity equation. It is found that the non-adiabatic heatingplays an important role in the position variation of WPSH. In comparison with climatic mean status, the verticalchange of non-adiabatic heating is stronger in the north side of WPSH in June 1998, but weaker in the south sideof WPSH. The anomalous non-uniform heating induces anomalous cyclonic vorticity in South China, areas to thesouth of the Yangtze and its mid-lower valleys, but anomalous anticyclonic vorticity in the Indo-China Peninsulaand South China Sea areas lead to the more southward position of WPSH than the mean.

IMPACT OF THE HEATING OVER SOUTH ASIA UPON THE SUBTROPICAL HIGH OVER WEST-PACIFIC

A case is reported, during which the Subtropical High over the Western Pacific (hereafter, SHWP in abbreviation) shifted northwestward and met-yu at Chaniiang River valley ended. Several numerical experiments onSHWP activity influenced by the heating over south Asia monsoon area are carried out, and the statistic significance of the results is checked. The results indicate that the enhancement of positive heating over South Asia willmotivate a wave-like series of anomaly centers, which propagate northeastward from the maximum heating center.so that a strong positive potential height anomaly center will set up from North China to Japan at Day X resultingin the enhancement of SHWP. Comparison of the influence upon SHWP by the heating over south Asia monsoonarea with that over ITCZ area south to SHWP is also carried out. It is pointed out that the heating over South Asiamonsoon area tends to favor SHWP north\vard movement while the heating over ITCZ area tends to thvor SHWPwestward stretching. As for the time to begin to influence on SHWP, the heating over south Asia monsoon areafavors the enhancement of SHWP atter Day 3 while that over ITCZ south to SHWP effects atter Day 5.

COMPARISONS OF THE WEST PACIFIC SUBTROPICAL HIGH AND THE SOUTH ASIA HIGH BETWEEN NCEP/NCAR AND ECMWF REANALYSIS DATASETS

Comparisons of the west Pacific subtropical high with the South Asia High are made using the NCEP/NCAR and ECMWF 500 hPa and 100 hPa monthly boreal geopotential height fields for the period 1961-2000. Discrepancies are found for the time prior to 1980. The west Pacific subtropical high in the NCEP/NCAR data is less intense than in ECMWF data before 1980. The range and strength of the west Pacific subtropical high variation described by the NCEP/NCAR data are larger than those depicted by ECMWF data. The same situation appears in the 100-hPa geopotential field. These discoveries suggest that the interdecadal variation of the two systems as shown by the NCEP/NCAR data may not be true. Besides, the South Asia High center in the NCEP/NCAR data is obviously stronger than in the ECMWF data during the periods 1969, 1979-1991 and 1992-1995. Furthermore, the range is larger from 1992 to 1995.

A CHARACTERISTIC CORRELATION ANALYSIS BETWEEN THE ASIA SUMMER MONSOON MEMBERS AND THE WEST PACIFIC SUBTROPICAL HIGH

In this paper, by using the pentad-mean NCAR/NCEP reanalysis data for the period of 1958-1997, some characteristic indices of describing the activity of Asian summer monsoon system members are defined and calculated. Based on the above works, a time-lag correlation analysis method is introduced for the correlation analysis between the Asian summer monsoon system and the west Pacific subtropical high (WPSH) area index, and some meaningful interaction processes and characteristic phenomena between them are revealed and discussed accordingly. It is shown that there exists some remarkable time-lag correlations in various degree between the Asian summer monsoon system members and the WPSH area index, and they interact and feedback with each other, which consists of the whole Asian summer monsoon system.

RELATIONSHIPS BETWEEN THE POSITION VARIATION OF THE WEST PACIFIC SUBTROPICAL HIGH AND THE DIABATIC HEATING DURING PERSISTENT INTENSE RAIN EVENTS IN YANGTZE-HUAIHE RIVERS BASIN

By using NCEP/NCAR daily reanalysis data and daily precipitation data of 740 stations in China, relationships between the position variation of the West Pacific subtropical high (WPSH) and the diabatic heating during persistent and intense rains in the Yangtze-Huaihe Rivers basin are studied. The results show that the position variation of WPSH is closely associated with the diabatic heating. There are strong apparent heating sources and moisture sinks in both the basin (to the north of WPSH) and the north of Bay of Bengal (to the west of WPSH) during persistent and intense rain events. In the basin, Q 1z begins to increase 3 days ahead of intense rainfall, maximizes 2 days later and then reduces gradually, but it changes little after precipitation ends, thus preventing the WPSH from moving northward. In the north of Bay of Bengal, 2 days ahead of strong rainfall over the basin, Q 1z starts to increase and peaks 1 day after the rain occurs, leading to the westward extension of WPSH. Afterwards, Q 1z begins declining and the WPSH makes its eastward retreat accordingly. Based on the complete vertical vorticity equation, in mid-troposphere, the vertical variation of heating in the basin is favorable to the increase of cyclonic vorticity north of WPSH, which counteracts the northward movement of WPSH and favors the persistence of rainbands over the basin. The vertical variation of heating in the north of Bay of Bengal is in favor of the increase of anti-cyclonic vorticity to the west of WPSH, which induces the westward extension of WPSH.

A WAVELET PACKET ENERGY DIAGNOSIS OF SOUTH ASIAN SUMMER MONSOON INFLUENCING ON THE WEST PACIFIC SUBTROPICAL HIGH

Based on the wavelet packet decomposition/reconstruction method and the NCEP/NCAR daily reanalysis data set, the relation between the south Asian summer monsoon and the west Pacific subtropical high seasonal variation was discussed, and a corresponding summer monsoon frequency-band energy criterion was defined and introduced for diagnosing the west Pacific subtropical high. Besides, some existing characteristics and rules about the west Pacific subtropical high were further argued and proofed, a few new phenomena and correlation between the south Asian summer monsoon and the west Pacific subtropical high were also revealed and presented.

ABRUPT CHANGES OF THE WESTERN PACIFIC SUBTROPICAL HIGH AND ITS INTERANNUAL VARIATION DURING LATE SPRING AND EARLY SUMMER

The 500-hPa geopotential height data used in this paper are from NCEP/NCAR data set for the period from 1979 to 1996 (from March to July). Using pentad average, we define the intensity, westernmost ridge point and mean latitude of the subtropical high ridge. Then the wavelet transform and EOF analysis are performed. It is found that there mainly exist three interseasonal abrupt change processes, which correspond to the onset time of the South China Sea Summer Monsoon (SCSSM), the beginning and the end of the Mei-yu respectively. The interannual variation of the subtropical high in late spring and early summer presents quasi-4-year and 8-year periods.

RELATIONSHIP BETWEEN WEST PACIFIC SUBTROPICAL HIGH AND ENSO AND ITS INFLUENCE ON RAINFALL DISTRIBUTION OF RAINY SEASON IN FUJIAN

Relationship between the variations of West Pacific subtropical high indices in the summer half of the year and preceding SST in North Pacific was examined based on a data set of 1951 2000. The correlation between the subtropical high indices and preceding SST in the equatorial East Pacific was the strongest among the others, and has great persistency from last autumn to spring. It is indicated that ENSO events appeared about six months earlier than the change of the subtropical high activities, and the subtropical high intensities enhanced (weakened) and western ridge point was westward (eastward) in the year of El Nino (La Nina) events. It was also observed that there were similar interdecadal oscillation and abrupt variations between Nino3 SST, subtropical high intensities and rainfall of rainy season in Fujian. Therefore, experiments were made on rainfall distribution of rainy season in Fujian. The results showed that the distribution was directly affected by the subtropical high activities, pronouncedly caused by ENSO effect.

SEASONAL PERSISTENCE OF THE WEST PACIFIC SUBTROPICAL HIGH AND ITS RELATIONSHIP WITH THE SURFACE HEAT FLUX ANOMALY

This paper investigates the interannual variation of the West Pacific Subtropical High(WPSH) intensity based on the data compiled by the Chinese National Climate Center.Monthly reanalysis data from National Centers for Environmental Prediction and National Center for Atmospheric Research(NCEP/NCAR) are also used to study the lead-lag relationship between WPSH intensity and surface heat flux anomalies.The three major findings are as follows:First,WPSH intensity presents good seasonal persistence,especially from winter to the ensuing summer.Persistence is more significant after 1977,especially from spring to summer,and from summer to autumn;persistence of anticyclonic anomalies are significantly better than cyclonic anomalies.Second,surface heat flux tends to present opposite anomalous patterns between the strong and weak years of the WPSH intensity,which is especially valid at the latent heat flux over the ocean.Simultaneous correlations between surface heat flux and WPSH intensity in each of the seasons are marked by similar key areas.Finally,surface heat flux from the preceding winter of a strong summer WPSH is quite similar to strong spring WPSH,but the positive anomalies over the northwest Pacific and south of Japan are notably stronger.The situations in the weak years are similar except for those over the northwest Pacific:winter surface heat flux shows negative anomalies for a weak spring WPSH,but positive anomalies for a weak summer WPSH.It is suggested that surface heat flux in the previous winter plays an important role in maintaining the WPSH intensity in the ensuing spring and summer.

MOVEMENT OF THE RIDGE LINE OF SUMMER WEST PACIFIC SUBTROPICAL HIGH AND ITS POSSIBLE MECHANISM

The analysis of the 40-year averaged daily data in this paper suggests that the vertical structure and movement of subtropical high (SH) ridge in summer evidently differ in different areas,which is close related with spatially nonuniform heating.The SH ridge over the West Pacific tilts northwards with height,while one over the Central Pacific tilts southwards.The northward movements of the Central/West Pacific SH ridges both show distinct low frequency oscillations of 10-20 days,and the movement over the East Asian monsoon area shows obvious oscillations of quasi-40-days as well.The analysis shows that the solar radiation drives the seasonal meridional movement of SH,while the spatially nonuniform heating modifies its movement speed and intensity,thus resulting in its anomalous motion.

Tropospheric biennial oscillation of the western Pacific subtropical high and its relationships with the tropical SST and atmospheric circulation anomalies

There is the significant period of tropospheric biennial Oscillation(TBO)over East Asian monsoon region at the interannual timescales,which has the important influences on East China climate.Based on a set of reconstructed indices which describes the western Pacific subtropical high(WPSH)objectively,this paper focuses on the TBO component of WPSH,one of the key members of the East Asian Monsoon system,and its relationships with the tropical SST and atmospheric circulation anomalies.It is found that(1)As an important interannual component of WPSH,the time series of TBO has the obvious transition in the late1970s,and the variability of the WPSH’s TBO component is more significant after the late 1970s.(2)The time-lag correlations between the WPSH’s TBO and the tropical sea surface temperature(SST)anomalies in several key ocean regions are more significant and have longer correlation duration than the raw data.The response of the western boundary index to ENSO is earlier than the intensity index,and the time-lag correlations of them are up to maximum when lagging ENSO by 3–5 months and 5–6months,respectively.(3)In the course of the WPSH’s TBO cycle,the occurrence of the El Ni o-like anomaly in the tropical central-eastern Pacific in winter is always coupled with the weak East Asian winter monsoon,with the most significant enhancing phase of the WPSH’TBO.In contrast,the La Ni a-like anomaly in the central-eastern Pacific in winter is coupled with the strong East Asian winter monsoon,with the most weakening phase of the WPSH’s TBO.(4)The distribution of the tropical SST and atmospheric circulations anomalies are asymmetric in the TBO cycle.The WPSH’s TBO is more significant in the period of the developing El Ni o-like anomaly in central-eastern Pacific than in the period of the developing La Ni a-like anomaly.Therefore,during the period of developing El Ni o-like anomaly,more attention should be paid to the interannual component of TBO signal in the short-term climate prediction.

CLIMATIC ANALYSIS OF IMPACTS OF THE TROPICAL CONVECTION IN WESTERN PACIFIC ON WESTERN PACIFIC SUBTROPICAL HIGH'S SHORT-TERM SCALES DURING BOREAL SUMMER

Based on the multi-year average NCEP/NCAR reanalysis data and NOAA's OLR data,the climatic characteristics of the tropical convection in tropical western Pacific and Indian Oceans as well as its relationship with western Pacific subtropical high (WPSH) is shown as follows:on short-term scales,the tropical convection that has significant influence on western Pacific high's latitudinal movement is located in the area of the South China Sea to the Philippines,which is the 2-day precursor prior to WPSH's latitudinal fluctuation,that is,WPSH is shifting to north 2 days after the tropical convection becomes more active,and vice versa.Moreover,the tropical convection has less effect on WPSH's longitudinal movement.

2022年夏季中国高温的环流异常特征

基于美国国家海洋和大气管理局(National Oceanic and Atmospheric Administration, NOAA)全球范围扩展重建海面温度资料第5版本(Extended Reconstructed Sea Surface Temperature version 5,ERSSTv5),以及美国国家环境预报中心和国家大气研究中心NCEP(National Centers for Environmental Prediction)/NCAR(National Center for Atmospheric Research)逐月全球再分析资料,采用相关、回归、合成及物理量诊断等方法,对2022年夏季中国大范围高温相关环流异常的可能成因进行了分析。结果表明:(1)2022年夏季南亚高压偏强并分别向东、西方向扩展,西太平洋副热带高压(以下简称“副高”)异常偏强西伸。2022夏季为拉尼娜(La Nina)年,但热带大西洋垂直上升环流相对西太平洋更强,且热带印度洋到西太平洋热带垂直上升环流异常也偏强。(2)2022年热带大西洋、印度洋到西太平洋上空垂直环流异常和La Nina共同作用,使得夏季南亚高压和西太平洋副高极端异常。La Nina和印度洋到西太平洋垂直环流异常有利于南亚高压和西太平洋副高的偏强西伸;热带大西洋环流异常则既有利于南亚高压的加强及东扩,也有利于西太平洋副高偏强西伸。(3)印度洋到西太平洋垂直环流主要通过局地经向哈得来(Hadley)环流影响青藏高原到中国东部的环流异常,表现为青藏高原到中国东部中低层为显著的辐散异常;热带大西洋则通过引起纬向风异常(急流异常),激发遥相关波列并向下游传播,进而影响青藏高原到中国东部地区的环流异常。

近160年西北太平洋副高的年代际变化及其对台风年际变化的影响

研究着眼于上百年尺度西太副高及台风活动变化特征,通过统计分析的方法对百年来台风活动特性及其与西太副高的相关性进行分析。结果表明:(1)近160a副热带高压的范围增大、强度增强,在1980年前后,副高平均北界明显北抬,西脊点明显西伸,这种变化与经向环流(Hadley环流)和纬向环流(Walker环流)的变化相关,Hadley环流下沉支(wh)对西太副高关键区的副高强度(h)存在正影响,Walker环流上升支(ww)则存在负影响;(2)各区域1930年后台风影响时长和年频数均增加。而1975—2020年时段西北太平洋台风年平均影响时长与1930—1975年时段相比有所下降,但发生年频率无明显变化;华南地区的年影响时长与年频数没有明显变化,但我国华东沿海地区台风的年影响平均时长与年频数均显著增加。Hadley环流下沉支、Walker环流上升支、副高强度与西北太平洋台风活动范围向西向北伸展及各区域发生频率显著相关,具有一定的指示意义。

2022年秋季我国气候异常特征及成因分析

2022年秋季,全国气候总体呈现暖干的特征,其中南方大部出现持续高温干旱。秋季平均气温为1961年以来历史同期最高。秋季降水季节内变率大,9月全国大部降水偏少,10月降水总体呈现南北少、中间多,11月我国中东部大部降水偏多而西部大部降水偏少。9—10月环流异常特征显示我国南方上空为偏北风距平,来自南海和西北太平洋的水汽输送条件极差,西北太平洋副热带高压偏强偏西,我国南方受下沉运动控制,有利于大部地区降水偏少、气温偏高,出现持续干旱。海温外强迫影响分析显示,2022年秋季印度-太平洋暖池异常偏暖,热带太平洋中东部偏冷,赤道印度洋西部偏冷,对应赤道印度洋上空纬向季风环流和太平洋上空Walker环流之间为显著的耦合特征。热带印度洋偶极子(TIOD)显著影响区域为江南西部和西南地区东南部,厄尔尼诺-南方涛动显著影响区域是江南大部和华南北部。即2022年秋季我国南方降水异常偏少受到TIOD负位相和拉尼娜状态的协同影响。

2022年“龙舟水”持续暴雨过程与夏季风关系初探

利用NCEP日平均再分析资料、广西降雨量资料,对2022年广西“龙舟水”期间持续强降雨的雨情、夏季风活动特征及其对持续性强降雨的影响进行分析。结果表明,2022年南海夏季风爆发偏早,在“龙舟水”期间强度偏强,倾向于向东传播,使广西强降雨区出现在桂东北地区。2022年“龙舟水”期间,西太平洋副热带高压西脊点位置偏东,西段脊线位置偏南,使水汽输送偏南偏多。另外,东亚大槽偏深,冷空气活动频繁等诸多因素使得夏季风前沿在华南地区长时间停滞,造成了广西的持续性强降雨。

2022年夏季中国高温环流特征及其与热带海面温度异常的关系

基于1979~2022年NCEP/DOE逐月再分析资料和NOAA海面温度资料,通过合成、相关分析等统计方法,以海面温度(Sea Surface Temperature,SST)异常影响为切入点,对比分析了2022年与La Niña年强迫作用下的我国夏季季节内环流差异,并在此基础上进一步探讨了异常高温与同期热带SST之间存在的可能联系。结果表明:1)2022年夏季,我国中东部高温区具有明显空间变化特征,6月位于华中地区、7月位于西南地区、8月影响整个长江流域。2)西太平洋副热带高压和南亚高压的同时异常加强,以及两者重叠打通并形成少见的北半球副热带高压带,是造成2022年夏季我国中东部异常高温过程的直接原因。3)在持续2年的较强La Niña背景下,2022年东亚夏季环流并未完全表现出对La Niña冷SST的响应,南海及菲律宾以东对流异常偏弱、东亚—太平洋遥相关型不显著均与La Niña年环流典型特征有较大出入。夏季同期热带SST异常对高温过程的形成具有一定贡献,热带西印度洋和热带中太平洋的冷SST异常分别有利于我国长江流域和华中地区出现高温酷暑,其中热带中太平洋冷SST异常可能是西太平洋副热带高压加强的重要原因,而热带西印度洋冷SST对南亚高压的增强有贡献作用。

2021年山东秋季降水异常偏多成因分析

利用1961—2021年山东123个国家级气象观测站逐日降水资料、ERA5逐月再分析资料和NOAA海温数据,对2021年山东秋季降水异常偏多成因进行分析。结果表明,500 hPa位势高度场上中高纬地区上空存在着“两脊一槽”双阻型的环流形势,贝加尔湖以西地区长波槽加深加强,有助于西路冷空气南下东传影响山东。西太平洋副热带高压(以下简称副高)较常年面积偏大,强度偏强,脊点偏西,脊线偏北,将外围充足的暖湿气流向北输送至黄淮地区,为山东地区提供了充足的水汽。冷空气与暖湿气流交汇于黄淮地区,导致降水异常偏多。进一步分析表明,在赤道中东太平洋冷水状态和印度洋海温持续暖位相的协同影响下,导致副高偏强偏西偏北,从而为暖湿气流输送提供有利的水汽条件。副高异常偏强偏北、南美东海岸和北太平洋海温异常偏暖、赤道中太平洋海温异常偏冷是造成山东9月降水异常偏多的主要原因。

基于自组织映射的梅雨期西太副高分型及降水特征研究

为了研究梅雨期西太副高(Western Pacific subtropical high,WPSH)的变化特征及其对梅雨降水的影响,利用1951—2016年西太副高监测指数,采用自组织映射(self-organizing map,SOM)方法对梅雨期西太副高进行聚类分析,得到9种SOM分型,并利用中国1 km分辨率逐月降水量数据集和中国雨季历年信息表进行不同西太副高分型下梅雨降水的特征分析。结果表明:9种梅雨期西太副高分型对应着多种不同的梅雨降水特征,纬向上,当西太副高偏东时,梅雨区降水强度偏低,反之则强度偏高。经向上,在西太副高主体位于中国陆地的前提下,脊线指数高于25°N时,梅雨区大部被副热带高压控制,整体降水偏少且有小幅度南少北多态势;脊线指数偏低时,副高控制梅雨区南部边缘或更南地区,水汽输送导致降水整体偏多。得到了不同类型副高与梅雨降水特征的对应关系,说明SOM方法能较好地对副热带高压等气象要素做出客观分型,并指导开展气候现象的相关性研究。