Seasonal modulations of different impacts of two types of ENSO events on tropical cyclone activity in the western North Pacific

The paper examines different impacts of eastern Pacific warm/cold (EPW/EPC) and central Pacific warm/cold (CPW/CPC) events on tropical cyclones (TCs) in the western North Pacific (WNP) by considering the early season of April–June (AMJ), the peak season of July–September (JAS) and the late season of October–December (OND). During AMJ, EPW (EPC) is associated with a significant increase of the TC genesis number in the southeastern (southwestern) sub-region of the WNP, but no class of El Niño-Southern Oscillation (ENSO) events shows a significant change in the TC lifetime and intensity. During JAS, EPW corresponds to an increase (decrease) of the TC genesis number in the southeastern (northwestern) sub-region, but CPW shows no significant change. EPC increases the TC genesis in the northwestern and northeastern sub-regions and decreases the genesis in the southwestern sub-region, whereas CPC suppresses the genesis in the southeastern sub-region. Both the lifetime and intensity of TCs are increased in EPW, but only a shortened lifetime is seen for CPC. During OND, EPW reduces the TC genesis in the southwestern and northwestern sub-regions, whereas CPW enhances the genesis in the southeastern sub-region. Over the South China Sea, CPW and CPC show a significant decrease and increase of the TC genesis, respectively. The TC lifetime is significantly longer in both EPW and CPW and shorter in EPC, and TCs tend to be more (less) intense in EPW (CPC). All of these variations are consistent with the development of ENSO-related SST anomalies during different seasons and are supported by distributions of the genesis potential index—a combination of large-scale oceanic and atmospheric factors that affect TC activity. TCs in the WNP mainly take the straight westward, northwestward and recurving tracks. During AMJ of EPW years, the TC steering flow patterns favor the recurving track and suppress the straight westward and northwestward tracks. During JAS, EPW is associated with the steering flows that are unfavorable for TCs to move northwestward or westward, whereas CPW favors the northwestward track and suppresses the straight westward track. The steering flow patterns during OND are similar to those during JAS, except that EPC may increase the possibility of the northwestward track.     

两类El Niño型对西北太平洋季风槽及热带气旋生成的可能影响

通过对1948~2015年不同El Ni?o事件下西北太平洋季风槽变化和热带气旋(tropical cyclone,TC)生成进行分析,初步探讨了不同El Ni?o型事件对季风槽及其对TC的可能影响。分析结果表明,较东太平洋增暖(eastern Pacific warming,EPW)年,中太平洋增暖(central Pacific warming,CPW)年季风槽偏弱,位置相对偏西、偏北。在CPW年,中(西和东)太平洋海温增暖(降低)引起了从中到西太平洋热带地区的西风异常和中太平洋地区上升运动及对流活动加强,使得季风槽加强东伸,同时西太平洋副高偏弱、偏北,季风槽向北推进;而在EPW年,赤道东(西)太平洋海温增暖(降低)使得赤道地区西风异常显著加强东扩,异常Walker环流的上升支东移至东太平洋,季风活动加强,副高偏强、偏南,这使得季风槽较CPW年相比更强、更偏东。利于TC生成的大尺度环境因子随季风槽强度和位置的变化而发生改变,在CPW年,低层气旋性涡度、高层辐散、高的中层相对湿度以及低垂直风切变区随着季风槽向北移动;而在EPW年,这些因子随季风槽向南、向东偏移。这些大尺度环境因子的变化使得西北太平洋TC生成的位置在CPW年比EPW年更加偏北、偏西。     

1998和1999年西北太平洋热带气旋的异常特征及其大尺度条件

应用联合台风警报中心 (JTWC)的资料 ,研究了 1998和 1999年西北太平洋 (WNP)热带气旋 (TC)的活动 ,发现这两年具有较为相似的异常特征 :台风个数明显偏少 ,台风的生成源地显著偏西。对导致这种异常的大尺度条件场的分析表明 ,1998和 1999年的台风季季风槽、垂直风切变、海平面气压等因子的分布都不利于台风在WNP东部海域的生成 ,因此是导致这两年台风偏少和生成源地异常偏西的重要原因。其中 ,季风槽的异常偏西及在盛夏的异常偏北对TC的异常活动产生了主要影响。对低纬大气环流异常的进一步分析表明 ,WNP地区的环流异常是导致季风槽和垂直风切变场异常的主要原因 ,而 1999年异常环流的出现可能是由于低纬大气对印尼附近异常热源强迫的响应所致。由于该热源的存在 ,在其东侧 (WNP地区 )激发出东传的Kelvin波 ,而相应的异常风场则表现为低层盛行异常东风 ,而高层盛行异常西风 ,因此造成了WNP东部和西部海域垂直切变场的近乎相反的变化。同时 ,由于低层异常强的东风不利于季风西风的向东延伸 ,从而使季风槽明显偏西 ,未到达菲律宾以东的热带气旋频发区的位置。     

2010年西北太平洋与南海热带气旋活动异常的成因分析

利用中国气象局热带气旋（TC）资料、NCEP/NCAR 再分析资料和美国 NOAA 向外长波辐射（OLR）等资料,分析了2010年西北太平洋（WNP）及南海（SCS）热带气旋活动异常的可能成因,讨论了同期大气环流配置和海温外强迫对TC生成和登陆的动力和热力条件的影响。结果表明,2010年生成TC频数明显偏少,生成源地显著偏西,而登陆TC频数与常年持平。导致7～10月TC频数明显偏少的大尺度环境场特征为：副热带高压较常年异常偏强、西伸脊点偏西,季风槽位置异常偏西,弱垂直风切变带位置也较常年偏西且范围偏小,南亚高压异常偏强,贝加尔湖附近对流层低高层均为反气旋距平环流,这些关键环流因子的特征和配置都不利于 TC 在WNP的东部生成。影响TC活动的外强迫场特征为：2010年热带太平洋经历了El Ni?o事件于春末夏初消亡、La Ni?a事件于7月形成的转换;7～10月,WNP海表温度维持正距平,140°E以东为负距平且对流活动受到抑制;暖池次表层海温异常偏暖,对应上空850 hPa为东风距平,有利于季风槽偏西和TC在WNP的西北侧海域生成。WNP海表温度和暖池次表层海温的特征是2010年TC生成频数偏少、生成源地异常偏西的重要外强迫信号。有利于7～10月热带气旋西行和登陆的500 hPa风场特征为：北太平洋为反气旋环流距平,其南侧为东风异常,该东风异常南缘可到25°N,并向西扩展至中国大陆地区;南海和西北太平洋地区15°N以南的低纬也为东风异常;在这样的风场分布型下,TC容易受偏东气流引导西行并登陆我国沿海地区。这是2010年生成TC偏少但登陆TC并不少的重要环流条件。     

Possible Linkage Between Tropical Indian Ocean SST Anomalies and the Date ofFirst and Last Tropical Cyclones Landfalling in the Chinese Mainland

Bases on the NCEP / NCAR reanalysis products, HadISST dataset, and data of tropical cyclone (TC)landfalling in the Chinese mainland during 1960-2019, the possible impacts of Indian Ocean Dipole (IOD) mode andIndian Ocean basin (IOB) mode on the last-TC-landfall date (LLD) and first-TC-landfall date (FLD), respectively, areinvestigated in this study. The LLD is in significantly negative correlation with autumn IOD on the interannual time-scale and their association is independent of El Ni?o-Southern Oscillation (ENSO). The LLD tends to be earlier when theIOD is positive while becomes later when the IOD is negative. An anomalous lower-level anticyclone is located aroundthe Philippines during October-November, resulting from the change of Walker circulation over the tropical Indo-westPacific Ocean forced by sea surface temperature (SST) anomalies related to a positive IOD event. The Philippinesanticyclone anomaly suppresses TCs formation there and prevents TCs from landfalling in the Chinese mainland due tothe anomalous westerly steering flows over southeast China during October-November, agreeing well with the earlierLLD. However, the robust connection between spring IOB and FLD depends on ENSO episodes in preceding winter.There is an anticyclonic anomaly around the Philippines caused by the tropical SST anomalies through modulating theWalker circulation during May-June when the IOB is warming in the El Ni?o decaying phase. Correspondingly, the TCsgenesis is less frequent near the Philippines and the mid-level steering flows associated with the expanded westernPacific subtropical high are disadvantageous for TCs moving towards southeast China and making landfall during May-June, in accordance with the later FLD. By contrast, cooling IOB condition in spring of a La Ni?a decaying year andnegative IOD cases during autumn could produce a completely reversed atmospheric circulation response, leading to anearlier FLD and a later LLD over the Chinese mainland, respectively.     

MODULATION OF TC GENESIS OVER THE NORTHWESTERN PACIFIC BY ATMOSPHERIC INTRASEASONAL OSCILLATION

The influence of intraseasonal oscillation (ISO) on TC genesis over the northwestern Pacific is studied through comparing analyses of the more and less TC years from 1979 to 2006. It is indicated that the ISO strongly affects the TC genesis. In the years for more TC genesis, the ISO is weak and propagates insignificantly in the area to the west of the Philippines, but the ISO is strong in the area to the east of the Philippines and propagates significantly northwestward. In this situation, the Walker cell shifts gradually westward from the tropical western Pacific to the tropical eastern Indian Ocean. Convergent winds appear in the lower atmosphere while divergent winds in the upper atmosphere, suggesting the presence of enhanced ascending flow over the 140-160°E region and a favorable condition for TC genesis. Moreover, in the years for less TC genesis, the ISO gradually becomes stronger in the area to the west of the Philippines and significant eastward propagation prevails from the eastern Indian Ocean to the area around 120°E; the ISO is weak in the area to the east of the Philippines. During these years, the Walker circulation gradually moved eastward, with convergent winds in the upper troposphere and divergent winds in the lower troposphere. Sinking motion was significant, unfavorable for the TC genesis over the Northwestern Pacific.     

Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China

The present study identifies a significant influence of the sea surface temperature gradient(SSTG) between the tropical Indian Ocean(TIO; 15°S-15°N, 40°-90°E) and the western Pacific warm pool(WWP; 0°-15°N, 125°-155°E) in boreal spring on tropical cyclone(TC) landfall frequency in mainland China in boreal summer. During the period 1979-2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific(WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region,meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.     

Dependency of typhoon intensity and genesis locations on El Ni?o phase and SST shift over the western North Pacific

The effects of the El Ni?o-Southern Oscillation (ENSO) phase and the shifting of the ENSO sea surface temperature (SST) on the intensity of tropical cyclones (TC) have been extensively investigated in terms of TC genesis locations in the western North Pacific (WNP). To advance the hypothesis for a relation of genesis location–intensity that the TC formation location hints its intensity, two cases have been compared, which include the phase of the decaying El Ni?o turning over to La Ni?a (type I) and the phase that recovers to a neutral condition (type II). In addition, the shift of ENSO SST to the central Pacific warming (CPW) from the East Pacific warming (EPW) has been examined. The genesis potential index (GPI) and the accumulated cyclone energy have been applied to compare the differences between the ENSO phase and the TC formation location. It was apparent that ENSO influences the WNP typhoon formation location depending on the cycle of the ENSO phase. In addition, the typhoon activity was affected by the zonal shift of the El Ni?o SST. The CPW, which has maximum SST over the central Pacific, tends to have a persistently high GPI over the WNP in September–November and June–August, demonstrating that the formation locations of strong TCs significantly shift southeastward compared with the EPW having SST maximum over the eastern Pacific. CPW years revealed a distinguishable relationship between the TC formation location and the TC between the tropical depression (TD) + tropical storm (TS) and the intense typhoon of category 4?+?5.     

南海夏季风爆发与热带太平洋两类海温型关系的年代际差异

南海夏季风爆发日期在1993/1994年出现年代际偏早的转变,利用海温和再分析资料的研究证实西北太平洋增暖和两类海温型的年代际差异可能是导致此种变化的重要成因。进一步的研究揭示出在南海夏季风爆发出现年代际变化的背景下,南海夏季风爆发日期与太平洋海温的关系也出现明显的变化:1993/1994年之前的第一年代东太平洋(EP)型海温异常起主导作用,而1993/1994年之后的第二年代两类海温型均影响了季风爆发,但以中太平洋(CP)型海温异常为主。第一年代,东太平洋型增温(EPW)通过抑制西北太平洋-孟加拉湾的对流活动,在菲律宾海、孟加拉湾西部激发出两个距平反气旋,使越赤道气流建立偏晚、孟加拉湾低槽填塞、西北太平洋副热带高压增强,进而导致南海夏季风爆发偏晚,且其影响可从4月维持到5月;而中太平洋型增温(CPW)对季风爆发前期的流场无显著影响。第二年代,CPW通过抑制菲律宾-孟加拉湾东部的对流活动,在菲律宾-孟加拉湾激发出一个距平反气旋,使孟加拉湾低槽填塞、南海地区副高增强,进而阻碍季风爆发,且显著影响仅出现在4月;EPW对4月大气环流场的影响与第一年代较为接近,在菲律宾-孟加拉湾一带产生的风场、对流场异常稍弱于CPW,但其影响无法持续到5月。     

夏季热带北大西洋海温异常对西北太平洋热带气旋生成的影响

利用1979—2012年西北太平洋热带气旋最佳路径资料,Hadley中心的海温资料和NCEP/NCAR再分析资料等,研究了夏季(6—10月)热带北大西洋海温异常与西北太平洋热带气旋(Tropical Cyclone,TC)生成的关系及其可能机制。结果表明,夏季热带北大西洋海温异常与同期西北太平洋TC生成频次之间存在显著的负相关关系。热带北大西洋海温的异常增暖可产生一对东—西向分布的偶极型低层异常环流,其中气旋性异常环流位于北大西洋/东太平洋地区,反气旋异常环流位于西北太平洋地区。该反气旋环流异常使得TC主要生成区的对流活动受到抑制、低层涡度正异常、中低层相对湿度负异常、中层下沉气流异常,这些动力/热力条件均不利于TC生成。此外,西北太平洋地区低层涡旋动能负异常,同时来自大尺度环流的涡旋动能的正压转换也受到抑制,不能为TC的生成和发展提供额外能量源。反之亦然。     

Interactions between the 30–60 day oscillation,the walker circulation and the convective activities in the tropical western pacific and their relations to the interannual oscillation

In this paper, interactions between the 30-60 day oscillation, the Walker circulation and the convective activities in the tropical western Pacific during the Northern Hemisphere summer are analyzed by using the observed data of wind fields and high-cloud amounts for the period from 1980 to 1989.The analyzed results show that the 30-60 day oscillation (hereafter called LFO) may be largely affected by the convective activities in the tropical western Pacific. The LFO in the tropical western Pacific during the strong convective activities around the Philippines stronger than those during the weak convective activities around the Philippines. Moreover, in the case of strong convective activities around the Philippines, the LFO in the tropical west-ern Pacific and tropical eastern Indian Ocean generally propagates westward, and it is intensified by the LFO with a westward propagating center of maximum oscillation from the east to 140oE. However, in the case of weak convective activities around the Philippines, the LFO gradually becomes stronger with a eastward propagating center of maximum oscillation from the eastern Indian Ocean to the tropical western Pacific.Corresponding to the 30-60 day oscillation, the Walker circulation is also in oscillation over the tropical Pacific and its circulation cell seems to shift gradually westward from the tropical western Pacific to the tropical eastern In-dian Ocean with strong convective activities around the Philippines. This may maintain the intensification of convective activities there. However, during the weak convective activities around the Philippines, the Walker circula-tion gradually moves eastward and an ascending flow may appear in the equatorial central Pacific. This may cause convective activities to be intensified over the equatorial central Pacific.The analyzed results also show that the LFO in the tropical western Pacific and East Asia may be associated with the interannual oscillation of the SST anomaly in the tropical western Pacific.     

全球变暖背景下西北太平洋冬季热带气旋的活动

Using tropical cyclone （TC） observations over a 58-yr period （1949-2006） from the China Meteorological Administration, the 40-year ECMWF Reanalysis （ERA-40）, NCEP-NCAR reanalysis, and the Hadley Centre sea ice and sea surface temperature （HadISST） datasets, the authors have examined the behaviors of tropical cyclones （TCs） in the western north Pacific （WNP） in boreal winter （November-December-January-February）. The results demonstrate that the occurrences of wintertime TCs, including super typhoons, have decreased over the 58 years. More TCs are found to move westward than northeastward, and the annual total number of parabolic-track-type TCs is found to be decreasing. It is shown that negative sea surface temperature anomalies （SSTAs） related to La Nifia events in the equatorial central Pacific facilitate more TC genesis in the WNP region. Large-scale anomalous cyclonic circulations in the tropical WNP in the lower troposphere are observed to be favorable for cyclogenesis in this area. On the contrary, the positive SSTAs and anomalous anticyclonic circulations that related to E1 Nifio events responsible for fewer TC genesis. Under the background of global warming, the western Pacific subtropical high tends to intensify and to expand more westward in the WNP, and the SSTAs display an increasing trend in the equatorial eastern-central Pacific. These climate trends of both atmospheric circulation and SSTAs affect wintertime TCs, inducing fewer TC occurrences and causing more TCs to move westward.     

Tropical Cyclone Genesis Potential Index over the Western North Pacific Simulated by LASG/IAP AGCM

Tropical cyclone genesis potential index(GPI) is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone(TC) genesis.The performance of LASG/IAP AGCM GAMIL2.0 in the simulation of GPI over the western North Pacific(WNP) is assessed in this paper.Since GPI depends on large scale environmental factors including low-level vorticity at 850 hPa,relative humidity at 700 hPa,vertical wind shear between 850 and 200 hPa,maximum potential intensity(MPI),and vertical velocity,the bias of GPI simulation is discussed from the perspective of thermal and dynamical factors.The results are compared with the ECMWF reanalysis data(ERA40).The analyses show that both the climatological spatial pattern and seasonal cycle of GPI over the WNP are reasonably simulated by GAMIL2.0,but due to the overestimation of relative humidity,the simulated GPI extends to 170°E,about 10°east to that in the reanalysis data.It is demonstrated that the bias in the simulation of monsoon trough,which is about 5°north to the reanalysis,leads to an overestimation of GPI during May-June and September-October,but an underestimation during July-August.Over the WNP,the response of GPI to ENSO is well captured by GAMIL2.0,including the eastward(westward) shift of TC genesis location during El Nin o(La Nin a) years.However,the anomalous convective center associated with El Nin o shifts westward about 20°in comparison to ERA40,which leads to the biases in both vertical velocity and relative humidity.These eventually result in the westward deflection of the boundary between the positive and negative GPI centers along 20°-30°N.The results from this study provide useful clues for the future improvement of GAMIL2.0.     

Intense drought and flooding events in the Rio Negro and relation with the tropical Pacific and Atlantic variability modes

The relationship of the hydrological variability of the Rio Negro in Manaus and the dominant large-scale climate variability patterns for the 1902–2007 period is investigated using the quantile method and composite analyses. Variations of the Rio Negro Level (RNL) during its 3-month high (May to July—MJJ) and low (October to December—OND) phases are examined separately. The El Niño (La Niña) related maximum warming (cooling) in the central tropical Pacific during its mature and decaying stages modulates the atmospheric circulation in the tropics and displaces the Walker circulation cell eastward (westward), so that its sinking (rising) branch occurs over western Amazon and causes negative (positive) precipitation anomalies in this region. These anomalous climate conditions occur before the Rio Negro high phase (MJJ) and contribute to reduce (increase) the RNL and lead to a very low (very high) event in the river. On the other hand, the SST variability modes in the tropical Atlantic mainly during the transition from wet to dry season modulate the precipitation variations over western Amazon in OND. The very high events are more frequent after the 1960’s decade and the very low events, before the 1930’s decade. Therefore, the occurrence of these events contains a multidecadal scale variability. The results also indicate that the variations in the rainfall in western Amazon occur up to 9 months in advance and modulate the RNL in Manaus. The results presented here might be useful for monitoring purposes of the RNL.

     

CHARACTERISTICS OF TROPICAL CYCLONE GENESIS IN THE WESTERN NORTH PACIFIC DURING THE DEVELOPING AND DECAYING PHASES OF TWO TYPES OF EL NI?O

During the developing phase of central Pacific El Nio(CPEN), more frequent TC genesis over the northwest quadrant of the western North Pacific(WNP) is attributed to the horizontal shift of environmental vorticity field.Such a northwestward shift resembles the La Nia composite, even though factors that cause the shift differ(in the La Nia case the relative humidity effect is crucial). Greater reduction of TC frequency over WNP happened during the decaying phase of eastern Pacific El Nio(EPEN) than CPEN, due to the difference of the anomalous Philippine Sea anticyclone strength. The TC genesis exhibits an upward(downward) trend over the northern(southern) part of the WNP,which is linked to SST and associated circulation changes through local and remote effects.     

IMPACT OF CONVECTION OVER THE SOUTH CHINA SEA ON TROPICAL CYCLONE MOTION OVER THE WESTERN NORTH PACIFIC DURING SUMMER MONSOON

The intraseasonal oscillation(ISO) of the South China Sea(SCS, 105-120°E, 5-20°N) convection and its influences on the genesis and track of the western North Pacific(WNP) tropical cyclones(TCs) were explored, based on the daily average of NCEP/NCAR reanalysis data, the OLR data and the western North Pacific tropical cyclone best-track data from 1979 to 2008. The mechanism of the influences of ISO on TC movement and the corresponding large-scale circulation were discussed by a trajectory model. It was found as follows.(1) During the SCS summer monsoon, the SCS convection exhibits the ISO features with active phases alternating with inactive phases. The monsoon circulation patterns are significantly different during these two phases. When the SCS convection is active(inactive), the SCS-WNP monsoon trough stretches eastward(retreats westward) due to the activity(inactivity) of SCS monsoon, and the WNP subtropical high retreats eastward(stretches westward), which enhances(suppresses) the monsoon circulation.(2) The amount of TC genesis in the active phase is much more than that in the inactive phase. A majority of TCs form west of 135 °E during the active phases but east of 135 °E in the inactive phases.(3) The TCs entering the area west of 135 °E and south of 25 °N would move straight into the SCS in the active phase, or recurve northward in the inactive phase.(4) Simulation results show that the steering flow associated with the active(inactive)phases is in favor of straight-moving(recurving) TCs. Meanwhile, the impacts of the locations of TC genesis on the characteristics of TC track cannot be ignored. TCs that occurred father westward are more likely to move straight into the SCS region.     

ENSO发展年与衰减年夏季环境要素对热带气旋生成频数变化的贡献

利用中国气象局上海台风研究所整编的热带气旋(Tropical Cyclone,TC)最佳路径数据集和欧洲中期天气预报中心的ERA再分析资料,分析了El Ni?o-South Oscillation (ENSO)发展年与衰减年西北太平洋(Western North Pacific,WNP)夏季(6—8月)总TC生成频数(Tropical Cyclone Genesis Frequency,TCGF)及其区域性特征,通过潜在生成指数(Genesis Potential Index,GPI)定量诊断各环境要素对TCGF变化的贡献。结果表明,西北太平洋TCGF总数异常在ENSO各位相并不显著,但其东南象限和西部的TCGF异常存在明显差异。在ENSO各位相,GPI异常的空间分布与TCGF异常的空间型相似。同一区域,各环境要素对TCGF异常的贡献不同,反映了ENSO不同位相影响TC生成变化的机理存在差异。WNP东南部(SEWNP)是对ENSO较敏感的区域,El Ni?o发展年,中东太平洋异常增暖激发的Rossby波西传导致SEWNP受异常正涡度环流控制,涡度对TCGF增加的贡献最大;El Ni?o衰减年,西北太平洋出现低层异常反气旋,其东侧异常东北气流将湿度相对较低的水汽输送至SEWNP,相对湿度降低导致TCGF显著减少。La Ni?a发展年,绝对涡度减小和垂直风切变增加对TCGF减少都有影响。WNP西部仅在La Ni?a衰减年出现TCGF显著负异常,低层绝对涡度减小的贡献最大,因为季风槽减弱,抑制了南海附近的TC生成。      

TOGA-COARE IOP时期大尺度流场的分析

根据ＴＯＧＡ－ＣＯＡＲＥＩＯＰ时期（１９９２年１１月—１９９３年２月）包含加密观测的四维同化的高空格点资料,分析了这一时期热带太平洋风场的大尺度特征,发现５００ｈＰａ以下,在中东太平洋频繁出现大范围稳定的强西风,高层出现明显的东风;低层强西风高频出现区或爆发区逐月向东传播,在观测结束时,西风到达了中东太平洋地区;相应的正常冬季东亚－西太平洋局地哈得莱环流被破坏,热带太平洋的沃克（Ｗａｌｋｅｒ）环流明显地向东太平洋地区移动。所有这些环流的异常特征表明,ＴＯＧＡ－ＣＯＡＲＥＩＯＰ时期正处于一次ＥｌＮｉｎｏ事件的发展时期。     

Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation.It has been argued in the literature that during the 20th century the Walker circulation weakened,and that this weakening was attributable to anthropogenic climate change.By using updated observations,we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s.Associated with this enhancement is enhanced precipitation in the tropical western Pacific,anomalous westerlies in the upper troposphere,descent in the central and eastern tropical Pacific,and anomalous surface easterlies in the western and central tropical Pacific.The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific.Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign.We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic.An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa,implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation.Whether these recent changes will be sustained is not yet clear,but our research highlights the importance of understanding the interdecadal variability,as well as the long-term trends,that influence tropical circulation.