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1.
The relationships between ENSO and the East Asian-western North Pacific monsoon simulated by the Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 (FGOALS-s2), a state-of-the-art coupled general circulation model (CGCM), are evaluated. For El Nio developing summers, FGOALS-s2 reproduces the anomalous cyclone over the western North Pacific (WNP) and associated negative precipitation anomalies in situ. In the observation, the anomalous cyclone is transformed to an anomalous anticyclone over the WNP (WNPAC) during El Nio mature winters. The model reproduces the WNPAC and associated positive precipitation anomalies over southeastern China during winter. However, the model fails to simulate the asymmetry of the wintertime circulation anomalies over the WNP between El Nio and La Nia. The simulated anomalous cyclone over the WNP (WNPC) associated with La Nia is generally symmetric about the WNPAC associated with El Nio, rather than shifted westward as that in the observation. The discrepancy can partially explain why simulated La Nin a events decay much faster than observed. In the observation, the WNPAC maintains throughout the El Nio decaying summer under the combined effects of local forcing of the WNP cold sea surface temperature anomaly (SSTA) and remote forcing from basinwide warming in the tropical Indian Ocean. FGOALS-s2 captures the two mechanisms and reproduces the WNPAC throughout the summer. However, owing to biases in the mean state, the precipitation anomalies over East Asia, especially those of the Meiyu rain belt, are much weaker than that in the observation.  相似文献   

2.
El Ni?o–Southern Oscillation(ENSO) events significantly affect the year-by-year variations of the East Asian winter monsoon(EAWM). However, the effect of La Ni?a events on the EAWM is not a mirror image of that of El Ni?o events. Although the EAWM becomes generally weaker during El Ni?o events and stronger during La Ni?a winters, the enhanced precipitation over the southeastern China and warmer surface air temperature along the East Asian coastline during El Ni?o years are more significant. These asymmetric effects are caused by the asymmetric longitudinal positions of the western North Pacific(WNP) anticyclone during El Ni?o events and the WNP cyclone during La Ni?a events; specifically, the center of the WNP cyclone during La Ni?a events is westward-shifted relative to its El Ni?o counterpart. This central-position shift results from the longitudinal shift of remote El Ni?o and La Ni?a anomalous heating, and asymmetry in the amplitude of local sea surface temperature anomalies over the WNP.However, such asymmetric effects of ENSO on the EAWM are barely reproduced by the atmospheric models of Phase 5 of the Coupled Model Intercomparison Project(CMIP5), although the spatial patterns of anomalous circulations are reasonably reproduced. The major limitation of the CMIP5 models is an overestimation of the anomalous WNP anticyclone/cyclone, which leads to stronger EAWM rainfall responses. The overestimated latent heat flux anomalies near the South China Sea and the northern WNP might be a key factor behind the overestimated anomalous circulations.  相似文献   

3.
Bin WANG  Juan LI  Qiong HE 《大气科学进展》2017,34(10):1235-1248
Severe flooding occurred in southern and northern China during the summer of 2016 when the 2015 super El Nio decayed to a normal condition. However, the mean precipitation during summer(June–July-August) 2016 does not show significant anomalies, suggesting that — over East Asia(EA) — seasonal mean anomalies have limited value in representing hydrological hazards. Scrutinizing season-evolving precipitation anomalies associated with 16 El Nio episodes during 1957–2016 reveals that, over EA, the spatiotemporal patterns among the four categories of El Nio events are quite variable, due to a large range of variability in the intensity and evolution of El Nio events and remarkable subseasonal migration of the rainfall anomalies. The only robust seasonal signal is the dry anomalies over central North China during the El Nio developing summer. Distinguishing strong and weak El Nio impacts is important. Only strong El Nio events can persistently enhance EA subtropical frontal precipitation from the peak season of El Nio to the ensuing summer, by stimulating intense interaction between the anomalous western Pacific anticyclone(WPAC) and underlying dipolar sea surface temperature anomalies in the Indo-Pacific warm pool, thereby maintaining the WPAC and leading to a prolonged El Nio impact on EA. A weak El Nio may also enhance the post-El Nio summer rainfall over EA, but through a different physical process: the WPAC re-emerges as a forced response to the rapid cooling in the eastern Pacific. The results suggest that the skillful prediction of rainfall over continental EA requires the accurate prediction of not only the strength and evolution of El Nio, but also the subseasonal migration of EA rainfall anomalies.  相似文献   

4.
The winters of 1997/1998 and 1998/1999,corresponding to El Ni(?)o and La Ni(?)a episodes,respectively, were two typical rain-abundant and-scarce seasons for the southern China.In order to understand the cause of the anomalous precipitation during the two winters,a comparative analysis technique has been employed to investigate the differences in general circulation and moisture transportation between the two seasons. The results show that the abundant rainfall during the winter of 1997/1998 was associated with the ENSO warm episode event,eastward shifted weak westerly trough/ridge,weakened East Asian winter monsoon (EAWM),strengthened subtropical high,and presented two anti-cyclonic circulations over Hokkaido and the Philippine Sea,respectively,as well as one cyclonic circulation over the Yangtze River Basin in the anomalous wind fields of the lower troposphere.During the rain-scarce winter,however,the patterns of equatorial sea surface temperature anomalies and the circulation systems both in upper and lower levels were nearly the opposite of those during the rain-abundant winter.It has also been discovered that the water vapor over southern China during the winters came mainly from the southwesterly flow ahead of troughs in the southern branch of westerlies and the turning flow over the South China Sea-Indo-China Peninsula area;and the moisture transportation channels varied significantly with regard to height.The intensified flow in the southern branch of westerlies and the anti-cyclonic circulation anomaly over the Philippine Sea during the winter of 1997/1998 were favorable for moisture transportation to mainland China,however the two moisture transportation streams were dramatically weakened during the winter of 1998/1999 due to weak westerly flow and the dominance of a cold high system in the lower level over the southeast coast of China.Such a significant inter-annual change of moisture transportation is a key factor resulting in the obvious difference in precipitation between the two winters.  相似文献   

5.
Five sets of model sensitivity experiments are conducted to investigate the influence of tropical cyclone (TC) genesis location and atmospheric circulation on interannual variability of TC intensity in the western North Pacific (WNP). In each experiment, bogus TCs are placed at different initial locations, and simulations are conducted with identical initial and boundary conditions. In the first three experiments, the specified atmospheric and SST conditions represent the mean conditions of El Nio, La Nia, and neutral years. The other two experiments are conducted with the specified atmospheric conditions of El Nio and La Nia years but with SSTs exchanged. The model results suggest that TCs generated in the southeastern WNP incurred more favorable environmental conditions for development than TCs generated elsewhere. The different TC intensities between El Nio and La Nia years are caused by difference in TC genesis location and low-level vorticity (VOR). VOR plays a significant role in the intensities of TCs with the same genesis locations between El Nio and La Nia years.  相似文献   

6.
Interannual variability of landfalling tropical cyclones(TCs) in China during 1960-2010 is investigated.By using the method of partial least squares regression(PLS-regression),canonical ENSO and ENSO Modoki are identified to be the factors that contribute to the interannual variability of landfalling TCs.El Ni o Modoki years are associated with a greater-than-average frequency of landfalling TCs in China,but reversed in canonical El Ni o years.Significant difference in genesis locations of landfalling TCs in China for the two kinds of El Ni o phases occurs dominantly in the northern tropical western North Pacific(WNP).The patterns of low-level circulation anomalies and outgoing longwave radiation(OLR) anomalies associated with landfalling TC genesis with different types of El Ni o phases are examined.During canonical El Ni o years,a broad zonal band of positive OLR anomalies dominates the tropical WNP,while the circulation anomalies exhibit a meridionally symmetrical dipole pattern with an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the tropics.In El Ni o Modoki years,a vast region of negative OLR anomalies,roughly to the south of 25°N with a strong large-scale cyclonic anomaly over the tropical WNP,provides a more favorable condition for landfalling TC genesis compared to its counterpart during canonical El Ni o years.For more landfalling TCs formed in the northern tropical WNP in El Ni o Modoki years,there are more TCs making landfall on the northern coast of China in El Ni o Modoki years than in canonical El Ni o years.The number of landfalling TCs is slightly above normal in canonical La Ni a years.Enhanced convection is found in the South China Sea(SCS) and the west of the tropical WNP,which results in landfalling TCs forming more westward in canonical La Ni a years.During La Ni a Modoki years,the landfalling TC frequency are below normal,owing to an unfavorable condition for TC genesis persisting in a broad zonal band from 5°N to 25°N.Since the western North Pacific subtropical high(WNPSH) in La Ni a Modoki years is located in the westernmost region,TCs mainly make landfall on the south coast of China.  相似文献   

7.
The performance of the Climate version of the Regional Eta-coordinate Model (CREM), a regional climate model developed by State Key Laboratory of Nu- merical modeling for Atmospheric Science and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP), in simulating rainfall anomalies during the ENSO decaying summers from 1982 to 2002 was evaluated. The added value of rainfall simulation relative to reanalysis data and the sources of model bias were studied. Results showed that the model simulated rainfall anomalies moderately well. The model did well at capturing the above-normal rainfall along the Yangtze River valley (YRV) during El Nio decaying summers and the below and above-normal rainfall centers along the YRV and the Huaihe River valley (HRV), respectively, during La Nia decaying summers. These features were not evident in rainfall products derived from the reanalysis, indicating that rainfall simulation did add value. The main limitations of the model were that the simulated rainfall anomalies along the YRV were far stronger and weaker in magnitude than the observations during El Nio decaying summers and La Nia decaying summers, respectively. The stronger magnitude above-normal rainfall during El Nio decaying summers was due to a stronger northward transport of water vapor in the lower troposphere, mostly from moisture advection. An artificial, above-normal rainfall center was seen in the region north to 35°N, which was associated with stronger northward water vapor transport. Both lower tropospheric circulation bias and a wetter model atmosphere contributed to the bias caused by water vapor transport. There was a stronger southward water vapor transport from the southern boundary of the model during La Nia decaying summers;less remaining water vapor caused anomalously weaker rainfall in the model as compared to observations.  相似文献   

8.
In summer, water vapor over the eastern China monsoon region(ECMR) comes mainly from low latitudes and is modulated by tropical cyclone(TC) activity in East Asia(EA). This study examines the variability of water vapor transport over the ECMR, especially of the moisture inflow via the southern and eastern boundaries. The results of composite and correlation analyses, using data from 1979 to 2010, reveal significant differences in moisture budgets along the boundaries between TC days and non-TC days. Almost 80% of the water vapor transport via the eastern boundary occurs during TC days, while at the southern boundary most inflow occurs on non-TC days. The ratio of the total water vapor transport between TC and non-TC days is about 4:6. In addition, the El Nio–Southern Oscillation(ENSO) exhibits a remarkable influence on moisture transport over EA and the contributions of moisture inflow on TC days increase(reduce) in El Nio(La Nia) years. Moreover,different types of TCs, based on their tracks, have different effects on the moisture budgets along the southern and eastern boundaries. When TCs enter EA(but not the ECMR), they favor the moisture inflow via the eastern boundary and hinder the moisture inflow via the southern boundary. After TCs enter the ECMR, the inhibition of moisture inflow via the southern boundary will be weakened, and more water vapor can be brought into the ECMR. For some recurving TCs with an increase in TC activity in the midlatitudes, the influence is uncertain in different cases. The results herein suggest that TC activity is an important factor that influences the boundary moisture budgets in the ECMR.  相似文献   

9.
After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible mechanisms between Indian Ocean Dipole(IOD) and two types of El Nio were investigated.IOD events,originating from Indo-Pacific scale air-sea interaction,are composed of two modes,which are associated with EP and CP El Ni o respectively.The IOD mode related to EP El Nio events(named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean.Besides,it shows a quasi-symmetric distribution,stronger in the south of the Equator.The IOD mode associated with CP El Nio(named as IOD2) has strongest signal in tropical southern Indian Ocean surface.In terms of mechanisms,before EP El Nio peaks,anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean,resulting in upwelling in the east,decreasing sea temperature there;a couple of anomalous anticyclones(stronger in the south) form off the Equator where warm water accumulates,and thus the IOD1 occurs.When CP El Nio develops,anomalous Walker circulation is weaker and shifts its center to the west,therefore anomalous easterlies in equatorial Indian Ocean is less strong.Besides,the anticyclone south of Sumatra strengthens,and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone.Meanwhile,there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean,making sea temperature fall and rise separately.Therefore,IOD2 lies farther south.  相似文献   

10.
This study investigates how the El Ni?o–Southern Oscillation(ENSO) modulates the intraseasonal variability(ISV) of Pacific–Japan(PJ) teleconnection pattern. The PJ index during boreal summer is constructed from the empirical orthogonal function(EOF) of the 850-hPa zonal wind(U850) anomalies. Distinct periods of the PJ index are found during El Ni?o and La Ni?a summers. Although ISV of the PJ pattern is significant during 10–25 days for both types of summers, it peaks on Days 30 and 60 in El Ni?o and La Ni?a summers respectively. During El Ni?o summers, the 30-day ISV of PJ pattern is related to the northwestward propagating intraseasonal oscillation(ISO) over the western North Pacific(WNP), which is originated from the tropical Indian Ocean(IO). During La Ni?a summers,the 60-day ISV of PJ pattern is related to the northeastward propagating ISO from the tropical IO. The low-frequency ISV modes in both El Ni?o and La Ni?a summers are closely related to the boreal summer ISO(BSISO), and the high-frequency ISV modes over WNP are related to the quasi-biweekly oscillation. The underlying mechanisms for these different evolutions are also discussed.  相似文献   

11.
利用1979—2010年观测和再分析资料,诊断分析ENSO对中国华南冬半年降水的影响及其机制。结果表明,在El Ni?o冬半年期间,东亚沿岸上空对流层低层南风的增强导致了水汽输送明显偏多,水汽在华南辐合,使得大气可降水量和比湿增加,降水显著偏多。而在La Ni?a冬半年期间,这些大气要素并没有呈现显著的相反变化,负异常的量值很弱并在统计上不显著。通过进一步分析El Ni?o和La Ni?a冬半年期间季节内振荡的特点,给出一种华南冬半年降水对ENSO信号不对称响应的物理解释。El Ni?o期间,热带西太平洋到南海地区的季节内振荡不活跃,与El Ni?o相联系的西北太平洋反气旋性环流异常造成的水汽输送以及水汽辐合在华南能够稳定维持,致使华南降水明显偏多。但在La Ni?a冬半年期间,季节内振荡很活跃,与La Ni?a相联系的西北太平洋气旋性环流异常受到季节内时间尺度的扰动影响,ENSO的年际变化信号被季节内振荡破坏,使得西北太平洋和华南的年际异常信号不能得到稳定维持,导致与ENSO信号相联系的年际变化在统计上不显著。因此,热带西太平洋到南海地区的季节内振荡强度在El Ni?o和La Ni?a冬半年期间的差异,是华南冬半年降水对ENSO信号不对称响应的一个主要原因。   相似文献   

12.
利用江西省83个站1970—2010年春季(3—5月)降水资料和NOAA的Ni觡o 3.4区海表温度资料,分析了ENSO事件对江西省春季降水的影响。结果表明,ENSO在衰亡年对江西省春季降水有显著的影响,ElNi觡o衰亡年春季江西地区降水偏多,易发生暴雨天气过程,江西北部和南部降水偏多最为显著;La Ni觡a衰亡年春季江西地区降水偏少,不易发生暴雨天气过程,江西东南部降水偏少最为显著。结合对应天气形势分析发现,El Ni觡o(La Ni觡a)衰亡年春季低层(850 hPa)南海的水汽输送偏强(偏弱)、上升运动偏强(偏弱)以及高层(200 hPa)辐散抽吸偏强(偏弱),是造成江西地区降水偏多(偏少)的主要原因。  相似文献   

13.
使用NCEP/NCAR再分析资料、中国气象局台站降水资料和GPCC降水资料,系统研究了在冬季平流层准两年振荡(Quasi-Biennial Oscillation, QBO)调制下,厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation, ENSO)不同阶段与中国夏季降水的可能联系。根据两者的位相和强度,可将它们的配置分为QBO西风/El Ni?o、QBO西风/La Ni?a、QBO东风/El Ni?o、QBO东风/La Ni?a。研究结果表明,在年际时间尺度上,ENSO和QBO无显著相关关系。冬季QBO西风位相时,El Ni?o发展年夏季,我国整体偏旱,而华南偏涝;衰减年夏季,华南、华东北部偏旱,东北、长江流域偏涝。La Ni?a发展年夏季,我国东部降水异常呈负-正-负的三极分布;衰减年夏季,东南沿海偏涝。冬季QBO东风位相时,El Ni?o发展年夏季,长江以北偏旱;衰减年夏季,我国东部降水异常呈负-正-负的三极分布。La Ni?a发展年夏季,江淮和华南南部偏旱;衰减年夏季,我国东部沿海偏涝。ENSO是影响我国夏季降水异常的重要因子,而QBO的调制作用在ENSO衰减年夏季更为显著。相比冬季QBO东(西)风位相,QBO西(东)风位相时El Ni?o (La Ni?a)期间赤道西太平洋负(正)海温异常更强,衰减年夏季位于西太平洋的异常下沉(上升)运动和印度洋的异常上升(下沉)运动更强更深厚,西太平洋副热带高压范围更大(小),南亚高压更偏东(西)。   相似文献   

14.
利用1979—2012年Hadley中心海表温度、中国2 474个台站逐日降水和NCEP/NCAR全球再分析资料,分析了不同类型ENSO事件秋冬季和次年春季中国南方地区10~30 d降水低频变率的变化特征。结果表明,中国南方地区10~30 d降水低频变率对不同类型ENSO事件的响应存在显著的季节差异。EP型El Ni1o的冬季和次年春季,低频降水变率显著增强; CP型El Ni1o秋冬季低频降水强度呈现相反的异常,秋季低频降水偏弱,而冬季则偏强; La Ni1a事件期间中国南方低频降水变率的变化较小且不稳定。进一步分析发现,ENSO对南方地区10~30 d低频降水变率的影响与西北太平洋地区季节平均大气环流背景场对ENSO的响应密切相关。相比正常年份,EP型El Ni1o冬春季菲律宾反气旋性异常环流的强度较强且范围较大,其西侧的异常西南风向中国南方地区输送了大量水汽,从而有利于低频降水的增强; CP型El Ni1o年秋季西北太平洋表现为气旋性环流异常,抑制了热带水汽向东亚大陆的输送,而冬季却产生了与EP型El Ni1o年类似的异常反气旋环流,只是强度有所减弱,因此中国南方地区低频降水强度在秋冬季呈相反异常。La Ni1a年菲律宾附近虽然存在气旋性环流异常,但强度较弱,因而我国南方地区低频降水变率的响应也较弱。  相似文献   

15.
The spatio-temporal variations of eastern China spring rainfall are identified via empirical orthogonal function (EOF) analysis of rain-gauge (gridded) precipitation datasets for the period 1958–2013 (1920–2013). The interannual variations of the first two leading EOF modes are linked with the El Niño–Southern Oscillation (ENSO), with this linkage being modulated by the Pacific Decadal Oscillation (PDO). The EOF1 mode, characterized by predominant rainfall anomalies from the Yangtze River to North China (YNC), is more likely associated with out-of-phase PDO–ENSO events [i.e., El Niño during cold PDO (EN_CPDO) and La Niña during warm PDO (LN_WPDO)]. The sea surface temperature anomaly (SSTA) distributions of EN_CPDO (LN_WPDO) events induce a significant anomalous anticyclone (cyclone) over the western North Pacific stretching northward to the Korean Peninsula and southern Japan, resulting in anomalous southwesterlies (northeasterlies) prevailing over eastern China and above-normal (below-normal) rainfall over YNC. In contrast, EOF2 exhibits a dipole pattern with predominantly positive rainfall anomalies over southern China along with negative anomalies over YNC, which is more likely connected to in-phase PDO–ENSO events [i.e., El Niño during warm PDO (EN_WPDO) and La Niña during cold PDO (LN_CPDO)]. EN_WPDO (LN_CPDO) events force a southwest–northeast oriented dipole-like circulation pattern leading to significant anomalous southwesterlies (northeasterlies) and above-normal (below-normal) rainfall over southern China. Numerical experiments with the CAM5 model forced by the SSTA patterns of EN_WPDO and EN_CPDO events reproduce reasonably well the corresponding anomalous atmospheric circulation patterns and spring rainfall modes over eastern China, validating the related mechanisms.  相似文献   

16.
El Niño–Southern Oscillation (ENSO) events significantly affect the year-by-year variations of the East Asian winter monsoon (EAWM). However, the effect of La Niña events on the EAWM is not a mirror image of that of El Niño events. Although the EAWM becomes generally weaker during El Niño events and stronger during La Niña winters, the enhanced precipitation over the southeastern China and warmer surface air temperature along the East Asian coastline during El Niño years are more significant. These asymmetric effects are caused by the asymmetric longitudinal positions of the western North Pacific (WNP) anticyclone during El Niño events and the WNP cyclone during La Niña events; specifically, the center of the WNP cyclone during La Niña events is westward-shifted relative to its El Niño counterpart. This central-position shift results from the longitudinal shift of remote El Niño and La Niña anomalous heating, and asymmetry in the amplitude of local sea surface temperature anomalies over the WNP. However, such asymmetric effects of ENSO on the EAWM are barely reproduced by the atmospheric models of Phase 5 of the Coupled Model Intercomparison Project (CMIP5), although the spatial patterns of anomalous circulations are reasonably reproduced. The major limitation of the CMIP5 models is an overestimation of the anomalous WNP anticyclone/cyclone, which leads to stronger EAWM rainfall responses. The overestimated latent heat flux anomalies near the South China Sea and the northern WNP might be a key factor behind the overestimated anomalous circulations.  相似文献   

17.
Warm and cold phases of El Ni?o–Southern Oscillation(ENSO) exhibit a significant asymmetry in their decay speed.To explore the physical mechanism responsible for this asymmetric decay speed, the asymmetric features of anomalous sea surface temperature(SST) and atmospheric circulation over the tropical Western Pacific(WP) in El Ni?o and La Ni?a mature-to-decay phases are analyzed. It is found that the interannual standard deviations of outgoing longwave radiation and 850 h Pa zonal wind anomalies over the equatorial WP during El Ni?o(La Ni?a) mature-to-decay phases are much stronger(weaker) than the intraseasonal standard deviations. It seems that the weakened(enhanced) intraseasonal oscillation during El Ni?o(La Ni?a) tends to favor a stronger(weaker) interannual variation of the atmospheric wind, resulting in asymmetric equatorial WP zonal wind anomalies in El Ni?o and La Ni?a decay phases. Numerical experiments demonstrate that such asymmetric zonal wind stress anomalies during El Ni?o and La Ni?a decay phases can lead to an asymmetric decay speed of SST anomalies in the central-eastern equatorial Pacific through stimulating different equatorial Kelvin waves. The largest negative anomaly over the Ni?o3 region caused by the zonal wind stress anomalies during El Ni?o can be threefold greater than the positive Ni?o3 SSTA anomalies during La Ni?a, indicating that the stronger zonal wind stress anomalies over the equatorial WP play an important role in the faster decay speed during El Ni?o.  相似文献   

18.
This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Ni?o decaying pattern before 1990 while rapid transformation from El Ni?o to La Ni?a after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Ni?o SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.  相似文献   

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