NUMERICAL STUDY OF COLD AIR IMPACT ON RAINFALL REINFORCEMENT ASSOCIATED WITH TROPICAL CYCLONE TALIM(2005):I.IMPACT OF DIFFERENT COLD AIR INTENSITY

In 2005,significant rainfall reinforcement and severe disaster was induced by tropical cyclone(TC) Talim after it made landfall on the east of China.Observational analyses show that it has relationship with cold air intrusion.For investigating the impact of cold air intensity,we make use of Weather Research and Forecasting(WRF) model,the synthesizer of NCEP/NCAR reanalysis data and Japan regional spectral model data,to carry out numerical experiments.Results show that rainfall reinforcement occurs in all experiments.Different intensity of cold air can modify the rainfall distribution and intensity significantly.In the rainfall center,the increment maximum of rainfall is twice as large as that of the minimum.Moderate cold air intrusion may result in the strongest rainfall reinforcement.Different cold air intensity can lead to different motion of low-level convergence lines and fronts.There is a good relationship between the rainfall region and the eastern part of the front.On one hand,strong cold air weakens the TC intensity by its intrusion into the TC center and results in weak convergence and a convergent zone and a rain band shifted southward.On the other hand,weak cold air reduces the convergence and moves the convergent zone and rain band northward.Moderate cold air intrusion maintains strong low-level convergence and high-level divergence,keeping strong upward motion over certain regions.Consequently,the rain band begins to stagnate and rainfall reinforces abruptly therein.

Impact of Mid-and Upper-Level Dry Air on Tropical Cyclone Genesis and Intensification:A Modeling Study of Durian(2001)

The impact of mid-and upper-level dry air,represented by low relative humidity(RH)values,on the genesis of tropical cyclone(TC)Durian(2001)in the South China Sea was investigated by a series of numerical experiments using the Weather Research and Forecasting model.The mid-level RH was lowered in different regions relative to TC Durian(2001)’s genesis location.Results suggest that the location of dry air was important to Durian(2001)’s genesis and intensification.The rapid development of the TC was accompanied by sustained near-saturated mid-and upper-level air,whereas low humidity decelerated its development.Water vapor budget analysis showed that moisture at mid and upper levels was mainly supplied by the vertical convergence of moisture flux and the divergence terms,and consumed by the condensation process.The horizontal convergence of moisture flux term supplied moisture in the air moistening process but consumed moisture in the air drying process.With a dryer mid-and upper-level environment,convective and stratiform precipitation were both inhibited.The upward mass fluxes and the diabatic heating rates associated with these two precipitation types were also suppressed.Generally,convection played the dominant role,since the impact of the stratiform process on vertical mass transportation and diabatic heating was much weaker.The vorticity budget showed that the negative vorticity convergence term,which was closely related to the inhibited convection,caused the vorticity to decrease above the lower troposphere in a dryer environment.The negative vorticity tendency is suggested to slow down the vertical coherence and the development rate of TCs.

A primary study of the correlation between the net air-sea heat flux and the interannual variation of western North Pacific tropical cyclone track and intensity

A singular value decomposition （SVD） analysis is carried out to reveal the relationship between the interannual variation of track and intensity of the western North Pacific tropical cyclones （WNPTCs） in the tropical cyclone （TC） active season （July–November） and the global net air-sea heat flux （Q net ） in the preceding season （April–June）. For this purpose, a tropical cyclone track and intensity function （TIF） is defined by a combination of accumulated cyclone energy （ACE） index and a cyclone track density function. The SVD analysis reveals that the first mode is responsible for the positive correlation between the upward heat flux in the tropical central Pacific and the increased activity of western North Pacific （WNP） TIF, the second mode for the positive correlation between the upward heat flux in the North Indian Ocean and the northeastward track shift of WNPTCs and the third mode for the negative correlation between the upward heat flux in mid-latitude central Pacific and the northwest displacement of the WNP TC-active center. This suggests that Q net anomalies in some key regions have a substantial remote impact on the WNP TC activity.

Influence of the Saharan Air Layer on Atlantic Tropical Cyclone Formation during the Period 1-12 September 2003

Atmospheric Infrared Sounder （AIRS） data show that the Saharan air layer （SAL） is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 （TD14） were accompanied with outbreaks of SAL air during the period 1-12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1-12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic ？ows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.

A REGIONAL COUPLED AIR-SEA-WAVE MODEL: SIMULATION OF UPPER-OCEAN RESPONSES TO AN IDEALIZED TROPICAL CYCLONE

In this study a coupled air-sea-wave model system, containing the model components of GRAPES-TCM, ECOM-si and WAVEWATCH III, is established based on an air-sea coupled model. The changes of wave state and the effects of sea spray are both considered. Using the complex air-sea-wave model, a set of idealized simulations was applied to investigate the effects of air-sea-wave interaction in the upper ocean. Results show that air-wave coupling can strengthen tropical cyclones while air-sea coupling can weaken them; and air-sea-wave coupling is comparable to that of air-sea coupling, as the intensity is almost unchanged with the wave model coupled to the air-sea coupled model.The mixing by vertical advection is strengthened if the wave effect is considered, and causes much more obvious sea surface temperature(SST) decreases in the upper ocean in the air-sea coupled model. Air-wave coupling strengthens the air-sea heat exchange, while the thermodynamic coupling between the atmosphere and ocean weakens the air-sea heat exchange: the air-sea-wave coupling is the result of their balance. The wave field distribution characteristic is determined by the wind field. Experiments are also conducted to simulate ocean responses to different mixed layer depths.With increasing depth of the initial mixed layer, the decrease of SST weakens, but the temperature decrease of deeper layers is enhanced and the loss of heat in the upper ocean is increased. The significant wave height is larger when the initial mixed layer depth increases.

THE ROLE OF COLD AIR AND CHARACTERISTICS OF WATER VAPOR IN BOTH TCS NANMADOL (0428) AND IRMA (7427) MAKING LANDFALL ON CHINA IN WINTERTIME

The NCEP/NCAR reanalysis data are used to investigate the role of cold air and moisture characteristics during the evolution of two cases of tropical cyclones (Nanmadol and Irma) which made landfall on China in wintertime. The results are shown as follows. (1) The East Asia trough steered the cold air into the tropical ocean in early winter. The tropical cyclones moved in opposite directions with a high moving out to sea and the enhancement of the pressure gradient at the periphery played a role in maintaining and strengthening the intensity of the storms. The intrusion of weak cold air into the low levels of the tropical cyclones strengthened them by improving the cyclonic disturbance when they were still over the warm sea surface. When the cold air was strong enough and intruded into the eyes, the warm cores were damaged and stuffed before dissipation. (2) The tropical cyclones were formed in a convergence zone of moisture flux and their development could enhance the disturbance of water vapor convergence, thus strengthening the moisture convergence zone. However, when they were outside the moisture zone, the storms could not gain sufficient water vapor and became weak. There were no belts of strong moisture transportation during the wintertime tropical cyclone processes.

A Possible Mechanism of the Impact of Atmosphere-Ocean Interaction on the Activity of Tropical Cyclones Affecting China

In this study, tropical cyclone data from China Meteorological Administration （CMA） and the ECMWF reanalysis data for the period 1958-2001 was used to propose a possible mechanism for the impacts of air- sea interaction on the activity of tropical cyclones （TCs） affecting China. The frequency of TCs affecting China over past 40 years has trended downward, while during the same period, the air sea interaction in the two key areas of the Pacific region has significantly weakened. Our diagnoses and simulations suggest that air sea interactions in the central North Pacific tropics and subtropics （Area 1） have an important role in adjusting typhoon activities in the Northwest Pacific in general, and especially in TC activity affecting China. On the contrary, impacts of the air-sea interaction in the eastern part of the South Pacific tropics （Area 2） were found to be rather limited. As both observational analysis and modeling studies show that, in the past four decades and beyond, the weakening trend of the latent heat released from Area 1 matched well with the decreasing Northwest Pacific TC frequency derived from CMA datasets. Results also showed that the weakening trend of latent heat flux in the area was most likely due to the decreasing TC frequency over the Northwest Pacific, including those affecting China. Although our preliminary analysis revealed a possible mechanism through which the air sea interaction may adjust the genesis conditions for TCs, which eventually affect China, other relevant questions, such as how TC tracks and impacts are affected by these trends, remain unanswered. Further in-depth investigations are required.

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 China's Mainland 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 China's Mainland, which is unfavorable for steering TCs to make landfall in China's Mainland during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in China's Mainland.

Numerical Simulation of Changes in Tropical Cyclone Intensity Using a Coupled Air-Sea Model

A coupled air-sea model for tropical cyclones （TCs） is constructed by coupling the Pennsylvania State University/National Center for Atmospheric Research mesoscale model （MM5） with the Princeton Ocean Model.Four numerical simulations of tropical cyclone development have been conducted using different configurations of the coupled model on the f-plane.When coupled processes are excluded,a weak initial vortex spins up into a mature symmetric TC that strongly resembles those observed and simulated in prior research.The coupled model reproduces the reduction in sea temperature induced by the TC reasonably well,as well as changes in the minimum central pressure of the TC that result from negative atmosphere-ocean feedbacks.Asymmetric structures are successfully simulated under conditions of uniform environmental flow.The coupled ocean-atmosphere model is suitable for simulating air-sea interactions under TC conditions.The effects of the ocean on the track of the TC and changes in its intensity under uniform environmental flow are also investigated.TC intensity responds nonlinearly to sea surface temperature （SST）.The TC intensification rate becomes smaller once the SST exceeds a certain threshold.Oceanic stratification also influences TC intensity,with stronger stratification responsible for a larger decrease in intensity.The value of oceanic enthalpy is small when the ocean is weakly stratified and large when the ocean is strongly stratified,demonstrating that the oceanic influence on TC intensity results not only from SST distributions but also from stratification.Air-sea interaction has only a slight influence on TC movement in this model.

Foreshowing of the Western Pacific tropical cyclone track to PM10 air pollution episode in the Beijing area

By utilizing the air quality monitoring data and the NCEP reanalysis data, the relationship between thePM10 air pollution episode and synoptic situation is analyzed in the Beijing area. It is interesting to findthat PM10 air pollution episode in and around the Beijing area is correlated with the Western Pacifictropical cyclone track to some extent, namely when a tropical cyclone lands southward to the Chang-jiang River, PM10 air pollution episode is not easy to take place generally in the Beijing area; but when atropical cyclone moves northward and finally lands at the Korea Peninsula or the Japanese mainland,and under this condition the Beijing area is generally controlled by weak high or weak low for severaldays, PM10 air pollution episode often takes place in one day or several days. Above findings indicatethat predicting the Western Pacific tropical cyclone track can foretell whether or not PM10 air pollutionepisode takes place in the Beijing area, which can offer a technique for urban air quality prediction andair pollution source management in the Beijing area.

Tropical Cyclone Size Change under Ocean Warming and Associated Responses of Tropical Cyclone Destructiveness: Idealized Experiments

The power dissipation index(PDI),which is defined as the sum of the cube of tropical cyclone(TC)maximum wind speed during TC lifetime,is widely used to estimate the TC destructive potential.However,due to the lack of high-resolution observations,little attention has been paid to the contribution of TC size change to TC destructive potential in response to ocean warming.In this study,sensitivity experiments are performed by using the high-resolution Weather Research and Forecasting(WRF)model to investigate the responses of TC size and TC destructive potential to prescribed sea surface temperature(SST)increase under the present climate condition.The results show that TC size increases with the ocean warming.Possible reasons for TC size change are investigated with a focus on the outer air–sea moisture difference(ASMD).As SST increases,ASMD in the outer zone of the TC is larger than that in the inner zone,which increases the surface entropy flux(SEF)of the outer zone.This change in the radial distribution of SEF causes the increase of tangential wind in the outer zone,which further increases SEF,resulting in a positive feedback between outer-zone SEF and outer-zone tangential wind.This feedback leads to the increase of the radius of gale-force wind,leading to the expansion of TC size.Moreover,to estimate the contribution of TC size change to TC destructiveness,we calculate TC size-dependent destructive potential(PDS)as the storm size information is available in the model outputs,as well as PDI that does not consider the effect of TC size change.We find that PDS increases exponentially as SST increases from 1 to 4°C,while PDI increases linearly;hence the former is soon much greater than the latter.This suggests that the growth effect of TC size cannot be ignored in estimating destructiveness under ocean warming.

Effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit

Air–sea exchange plays a vital role in the development and maintenance of tropical cyclones（TCs）. Although studies have suggested the dependence of air–sea fluxes on surface waves and sea spray, how these processes modify those fluxes under TC conditions have not been sufficiently investigated based on in-situ observations.Using continuous meteorological and surface wave data from a moored buoy in the northern South China Sea,this study examines the effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit. The mooring was within about 40 km of the center of Hagupit. Surface waves could increase momentum flux to the ocean by about 15%, and sea spray enhanced both sensible and latent heat fluxes to the atmosphere,causing Hagupit to absorb 500 W/m^2 more heat flux from the ocean. These results have powerful implications for understanding TC–ocean interaction and improving TC intensity forecasting.

The Dynamic and Thermodynamic Effects of Relative and Absolute Sea Surface Temperature on Tropical Cyclone Intensity

Several numerical experiments were performed to investigate the dynamic and thermodynamic effects of sea surface temperature （SST） on tropical cyclone （TC） intensity. The results reveal that the relative SST within a radius of 2-3 times the radius of maximum wind contributes positively and greatly to TC intensity, while the remote SST far away from the TC center could reduce storm intensity. The change of air sea temperature and moisture differences may be the reason why TC intensity is more sensitive to the relative rather than the absolute SST. As the inflow air moves toward the eyewall, warmer （colder） remote SST can gradually increase （decrease） the underlying surface air temperature and moisture, and thus decrease （increase） the air sea temperature and moisture differences, which lead to less （more） energy fluxes entering the eyewall and then decrease （increase） the TC intensity and make it less sensitive to the absolute SST change. Finally, with all the related dynamic and thermodynamic processes being taken into account, a schematic diagram for the effects of relative SST and absolute SST on TC intensity is proposed.

Opposing Trends of Winter Cold Extremes over Eastern Eurasia and North America under Recent Arctic Warming

Under recent Arctic warming,boreal winters have witnessed severe cold surges over both Eurasia and North America,bringing about serious social and economic impacts.Here,we investigated the changes in daily surface air temperature(SAT)variability during the rapid Arctic warming period of 1988/89–2015/16,and found the daily SAT variance,mainly contributed by the sub-seasonal component,shows an increasing and decreasing trend over eastern Eurasia and North America,respectively.Increasing cold extremes(defined as days with daily SAT anomalies below 1.5 standard deviations)dominated the increase of the daily SAT variability over eastern Eurasia,while decreasing cold extremes dominated the decrease of the daily SAT variability over North America.The circulation regime of cold extremes over eastern Eurasia(North America)is characterized by an enhanced high-pressure ridge over the Urals(Alaska)and surface Siberian(Canadian)high.The data analyses and model simulations show the recent strengthening of the high-pressure ridge over the Urals was associated with warming of the Barents–Kara seas in the Arctic region,while the high-pressure ridge over Alaska was influenced by the offset effect of Arctic warming over the East Siberian–Chukchi seas and the Pacific decadal oscillation(PDO)–like sea surface temperature(SST)anomalies over the North Pacific.The transition of the PDO-like SST anomalies from a positive to negative phase cancelled the impact of Arctic warming,reduced the occurrence of extreme cold days,and possibly resulted in the decreasing trend of daily SAT variability in North America.The multi-ensemble simulations of climate models confirmed the regional Arctic warming as the driver of the increasing SAT variance over eastern Eurasia and North America and the overwhelming effect of SST forcing on the decreasing SAT variance over North America.Therefore,the regional response of winter cold extremes at midlatitudes to the Arctic warming could be different due to the distinct impact of decadal SST anomalies.

Contribution of Winter SSTA in the Tropical Eastern Pacific to Changes of Tropical Cyclone Precipitation over Southeast China

Tropical cyclone precipitation(TCP)accounts for 10%-40%of the boreal summer precipitation that occurs over Southeast China(SEC),causing flood disasters and serious damage.On the decadal scale,TCP increases significantly in SEC while TC frequency decreases in the western North Pacific(WNP)during 1980-2019.Therefore,variations in TCP and the corresponding physical mechanism are investigated in this study.First,an empirical statistical method is introduced to quantify the TCP amount based on accumulated cyclone energy(ACE)and TC frequency with the TCP anomaly decomposed into three items(rainfall frequency,rainfall intensity,and nonlinear item).ACE,as the integration of TC intensity and frequency,is a more effective index than TC frequency for depicting the characteristics of TCP because the contribution of rainfall frequency represented by ACE is higher than that of TC frequency.Then,the physical mechanism affecting the WNP TC activities and TCP in SEC are inspected.Positive sea surface temperature anomaly(SSTA)over the tropical eastern Pacific(TEP)in winter can trigger variations of air-sea interaction over the tropical Pacific,including low-level divergent winds,mid-tropospheric descent flows,high-level convergent winds coupled with negative anomalies of vorticity and humidity over the tropical western Pacific(TWP)in the next summer.These dynamic conditions provide unfavorable environments for TC activities in the WNP and constrain TCP in SEC.Furthermore,more significantly negative SSTA events in the TEP facilitate enhanced ACE along with positive relative vorticity,relative humidity,and upwelling vertical winds anomalies over the coast of SEC after 1998,which is a reasonable explanation for the increasing TCP in SEC.

Evaluation of the Ocean Feedback on Height Characteristics of the Tropical Cyclone Boundary Layer

In this study, the interaction between the tropical cyclone（TC） and the underlying ocean is reproduced by using a coupled atmosphere-ocean model. Based on the simulation results, characteristics of the TC boundary layer depth are investigated in terms of three commonly used definitions, i.e., the height of the mixed layer depth（HVTH）, the height of the maximum tangential winds（HTAN）, and the inflow layer depth（HRAD）. The symmetric height of the boundary layer is shown to be cut down by the ocean response, with the decrease of HVTH slightly smaller than that of HTAN and HRAD. The ocean feedback also leads to evident changes in asymmetric features of the boundary layer depth. The HVTH in the right rear of the TC is significantly diminished due to presence of the cold wake, while the changes of HVTH in other regions are rather small. The decreased surface virtual potential temperature by the cold wake is identified to be dominant in the asymmetric changes in HVTH. The impacts of ocean response on the asymmetric distributions of HTAN are nonetheless not distinct, which is attributed to the highly axisymmetric property of tangential winds. The HRAD possesses remarkable asymmetric features and the inflow layer does not exist in all regions, an indication of the inadequacy of the definition based on symmetric inflow layer depth. Under influences of the cold wake, the peak inflow area rotates counterclockwise distinctly. As a consequence, the HRAD becomes deeper in the east while shallower in the west of the TC.

Results of Numerical Modeling of the Origin of Cyclones and Anticyclones in the Vicinity of the Intertropical Convergence Zone

A review of simulation results, devoted to time-dependent modeling of the initial stage of the formation of large-scale vortices in the troposphere in the vicinity of the intertropical convergence zone, is presented. The simulation results were obtained not long ago with the help of the mathematical model of the neutral wind system of the lower atmosphere, developed earlier in the Polar Geophysical Institute. The utilized mathematical model produces three-dimensional distributions of the atmospheric parameters in the height range from 0 to 15 km over a limited region of the Earth’s surface. Simulation results were obtained for the case when the limited three-dimensional simulation domain, situated at low latitudes, is intersected by an intertropical convergence zone in the west-east direction. The reviewed simulation results were obtained for various initial configurations of the intertropical convergence zone. Results of numerical modeling have indicated that the origin of convexities in the form of the intertropical convergence zone can lead to the formation of different large-scale vortices in the lower atmosphere, in particular, a tropical cyclone, pair of cyclonic vortices, pair of cyclonic-anticyclonic vortexes, and triplet of cyclonic vortices. The simulation results, obtained earlier and presented individually in various editions, are reviewed and summarized in the present paper. A physical mechanism, responsible for the formation of the simulated large-scale vortices in the vicinity of the intertropical convergence zone, is discussed.

干冷空气活动对热带气旋强度变化影响的研究进展与展望

热带气旋的强度变化一直是热带气旋研究中的重要内容之一,干冷空气活动与热带气旋强度变化之间有密切的关系,如快速增强和快速减弱过程。文章对国内外干冷空气影响热带气旋强度变化的研究成果进行了回顾和总结,主要包括干冷空气的强度、侵入位置,以及高空冷性天气系统对热带气旋强度变化的影响三个方面,并对未来的研究进行展望,以期为干冷空气影响热带气旋强度的研究提供参考。

Numerical Modeling of the Initial Formation of Cyclonic Vortices at Tropical Latitudes

To investigate the initial formation of large-scale vortices at tropical latitudes a regional non-hydrostatic mathematical model of the wind system of the lower atmosphere, developed earlier in the Polar Geophysical Institute, is utilized. Three-dimensional distributions of the atmospheric parameters in the height range from 0 to 15 km over a limited region of the Earth’s surface are produced by the utilized model. Simulations are performed for the case when the limited three-dimensional simulation domain is intersected by an intertropical convergence zone in the west-east direction. Simulation results indicated that the origin of two convexities in the north direction in the configuration of the intertropical convergence zone can lead to the formation of three distinct tropical cyclones during the period of about four days.

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short)of the tropical Pacific from January 1970 to December 1989,main spatial patterns of tropical Pacific SSTA and SSWA coupling features in the transform course from the warm phase to the cold phase of El Nino-southern Oscillation(ENSO)cycles are discussed. The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of El Nino(La Nina)are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at the mature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcing of distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with the California cold current and SSTA is mainly responsible for weakening of El Nino;(3)the second sea temperature increase along the South American coast in the decaying course of El Nino results from the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacific forced by anomalous west wind stress:(4)La Nina results from the joint effect of Walker circulation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.