INFLUENCE OF TIBETAN PLATEAU ON AIR-SEA INTERACTION OVER TROPICAL PACIFIC IN OCEAN-ATMOSPHERIC COUPLED SYSTEM

A hybrid coupled ocean-atmosphere model is designed,which consists of a global atmosphericgeneral circulation model(L9R15 AGCM)and a simple ocean model(ZC ocean model over tropicalPacific).Using the model,experimental predictions are performed for the 1986/87 El Nino eventand the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TPFORC and NTP FORC hereinafter).It is found as follows:(1)The coupled system cansuccessfully predict the El Nino or La Nina event even if the Tibetan Plateau orography is notincluded in the model.The patterns of SSTA and wind anomalies in the model without the TibetanPlateau are similar to those with the Tibetan Plateau,which further verifies the fact that ENSOprocess is mainly caused by the air-sea interaction in tropical Pacific.(2)However.the existenceof the Tibetan Plateau exerts its influences on the intensity and duration of El Nino(or La Nina).It is unfavorable to the development and maintenance of westerly anomalies,so to some extent,restrains the development of El Nino,but favors the development of La Nina.(3)Effects of theTibetan Plateau orography on the wind anomalies in the coupled system are different from those inuncoupled AGCM simulation.     

THE EFFECTS OF TIBETAN PLATEAU ON THE ANOMALOUS VARIATION OF ASIAN MONSOON IN A COUPLED OCEAN-ATMOSPHERE SYSTEM

Experimental predictions with a hybrid coupled ocean-atmosphere model(L9R15 AGCM-ZCocean model)were performed for the 1986/87 El Nino event and the 1988/89 La Nina event withand without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter).Comparison shows that,to some extent,the existence of the Tibetan Plateau orography weakensor restrains(strengthens or facilitates)the formation of the anomalous circulation of Asianmonsoon during El Nino(La Nina)period.Opposite results are found in the uncoupled AGCMsimulation.     

PREDICTION AND VERIFICATION OF THE 1997-1999 EL NINOAND LA NINA BY USING AN INTERMEDIATE OCEAN-ATMOSPHERE COUPLED MODEL*

The numerical simulations,hindcasts and verifications of the tropical Pacific sea surface temperature anomaly(SSTA) have been conducted by using a dynamical tropical Pacific ocean-atmosphere coupled model named NCCo.The results showed that the model had performed reasonable simulations of the major El Nino episodes in the history,and the model forecast skill in 1990s had been significantly improved.NCCo model has been used to predict the tropical Pacific SSTA since January 1997.The comparisons between predictions and observations indicated that the occurrence,evolution and ending of the 1997-1998 El Nino episode have been predicted fairly well by using this model.Also,the La Nina episode that began in the autumn of 1998 and the developing tendency of the tropical Pacific SSTA during the year 1999 have been predictedsuccessfully.The forecast skills of NCCo model during the 1997-1999 El Nino and La Nina events are above 0.5 at 0-14 lead months.     

DIAGNOSTIC PREDICTIONS OF SST IN THE EQUATORIAL EASTERN PACIFIC OCEAN BASED ON FUZZY INFERRING AND WAVELET DECOMPOSITION

Methods and approaches are discussed that identify and filter off affecting factors (noise) above primary signals,based on the Adaptive-Nework-Based Fuzzy Inference System. Influences of the zonal winds in equatorial eastern and middle/western Pacific on the SSTA in the equatorial region and their contribution to the latter are diagnosed and verified with observations of a number of significant El Nino and La Nina episodes. New viewpoints are propsed. The methods of wavelet decomposition and reconstruction are used to build a predictive model based on independent domains of frequency,which shows some advantages in composite prediction and prediction validity.The methods presented above are of non-linearity, error-allowing and auto-adaptive/learning, in addition to rapid and easy access,illustrative and quantitative presentation,and analyzed results that agree generally with facts. They are useful in diagnosing and predicting the El Nino and La Nina problems that are just roughly described in dynamics.     

THE IMPACT OF MID-LATITUDE CIRCULATION OVER THE TIBETAN PLATEAU ON THE TRACK OF TYPHOON OVER THE NORTHWEST PACIFIC*

There is a close relationship between variation of middle latitude synoptic system over the Tibetan Plateau and movement of typhoon over the Northwest Pacific.This paper tries to discuss the mechanism of it through the numerical simulation and dynamic diagnosis.25-year statistical results during the period from 1970 to 1995 indicate that the relationship between middle latitude circulation over the Plateau and the track of typhoon over the Northwest Pacific are as follows.When longwave trough in 500 hPa isobaric surface over the Plateau.it is favourable to typhoon's westward movement:on the contrary,large scale anticyclone over the Plateau is favourable to typhoon's recurvature.We simulated the typhoon under these two flow patterns with MM5 model.Numerical simulation results showed that:(a)Longwave trough on 500 hPa surface over the Plateau makes subtropical high extended westward so as to typhoon south to high moves westward in easterly steering flow.(b)Anticyclone over the Plateau will bring about the longwave trough developing along the east coast of Asia,it'will usually cause typhoon recurring in front of the developed trough.In addition,the results of dynamic diagnosis show the physical mechanism on impact of synoptic system over the Plateau on the downstream synoptic system,that is,the transport of disturbance kinetic energy over the upstream Plateau makes the downstream trough developed.And then it affects the steering flow of typhoon.Because of existence of the high ridge over the Plateau.the energy transport channel moves to north.On the contrary,the trough exists over the Plateau,the energy transport channel moves to south.The southerly in front of trough over the Plateau and the southerly in the east part of typhoon transport low potential vorticity of low latitude into subtropical high.That is beneficial to development of subtropical high and affects typhoon's movement.These results fully reflect the impact of interaction between mid-latitude and low latitude on synoptic system.     

A STUDY OF CLIMATOLOGICAL ASPECTS OF FLOW FIELD AND HEAT EXCHANGE ON TROPICAL PACIFIC SURFACE IN THE EL NINO AND LA NINA EVENTS

In this paper, based on the 2°× 2° grid data COADS from 1950—1987 the flow field and heatexchange anomalies on the tropical (11°S—11°N, 120°E—80°W) Pacific surface (TPS) are studiedin El Nino and La Nina events. During E1 Nino, the zonal pressure gradient and the trade windsdecrease on the TPS, the tropical convergence strengthens on the TPS, especially on the centralTPS, the sensible and latent heat exchanges increase, the net longwave radiation and incident solarradiation decrease and the net gain (loss) of heat reduces (increases) on the central and easternTPS. During La Nina the results turn out the contrary. Finally, two feedback mechanisms whichinclude the dynamic, thermal and hydrological processes during E1 Nino and La Nina are summa-rized and a conceptive model for El Nino-La Nina cycle is given.     

IMPACT OF UPLIFT OF TIBETAN PLATEAU AND CHANGE OF LAND-OCEAN DISTRIBUTION ON CLIMATE OVER ASIA*

Using an improved CCM1/NCAR climate dynamic model and a combination distribution of land-ocean-vegetation during 40-50 MaBP,a series of numerical experiments representing different stages of the Tibetan Plateau uplifting and different land-ocean distributions are designed to discuss the influence of the Plateau uplifting and land-ocean distribution variation on Asian climate change.It is shown that Tibetan Plateau uplifting can firstly increase the precipitation in China during the period from initial uplift to half height of modern Tibetan Plateau and then decrease the rainfall during the time from the half height to the present plateau. At the same time.the uplifting can reduce surface air temperature over China.Besides.the effects of the uplift and land-ocean distribution change on the variation of winter and summer Asian monsoon circulation are also discussed.     

IMPACTS OF THE TROPICAL PACIFIC COUPLED PROCESS ON THE INTERANNUAL VARIABILITY IN THE INDIAN OCEAN

The basic features of climatology and interannual variations of tropical Pacific and Indian Oceans were analyzed using a coupled general circulation model (CGCM), which was constituted with an intermediate 2.5-layer ocean model and atmosphere model ECHAM4. The CGCM well captures the spatial and temporal structure of the Pacific El Ni?o-Southern Oscillation (ENSO) and the variability features in the tropical Indian Ocean. The influence of Pacific air-sea coupled process on the Indian Ocean variability was investigated carefully by conducting numerical experiments. Results show that the occurrence frequency of positive/negative Indian Ocean Dipole (IOD) event will decrease/increase with the presence/absence of the coupled process in the Pacific Ocean. Further analysis demonstrated that the air-sea coupled process in the Pacific Ocean affects the IOD variability mainly by influencing the zonal gradient of thermocline via modulating the background sea surface wind.     

APPLICATION OF THE VARIATIONAL METHOD OF TOVS DATA OVER THE TIBETAN PLATEAU IN IMPROVEMENT OF THE INITIAL FIELD OF NUMERICAL MODELS*

In this paper,TOVS satellite data are used through variational method on the data-sparse plateau area.Diagnoses are carried out to find a way to solve the large error problem of model initial field.It is put forward that TOVS retrieval data can be used to improve the initial field of numerical prediction model on Tibetan Plateau area.Through variational method,TOVS data are processed and the liability of the initial information on the plateau is improved.Diagnostic results confirm further that the application of TOVS retrieval data can improve our capability to describe the dynamic system features on the plateau and the objectivity of related initial information such as the distribution of water vapor channel and stratification stability.     

MAIN HEATING MODES OVER THE TIBETAN PLATEAU IN JULY AND THE CORRELATION PATTERNS OF CIRCULATION AND PRECIPITATION OVER EAST ASIA*

Based on the 1958-1999 monthly averaged NCEP/NCAR reanalysis data,the REOF analysis is applied to obtain the main spatial modes of normalized atmospheric heating source over the Tibetan Plateau(TP) in July.Results show that the four leading modes are located over the northeast TP,southwest TP.Kashmir and southeast TP respectively,and the cumulative variances are no more than one third of the total.It indicates that the heating source distribution is very complicated over the TP in July.In other words.it is difficult to depict the heating spatial distribution with a few modes.By using wavelet analysis,a 2-4-year variation period is identified in these modes.Moreover,correlation coefficients between each RPC and zonal wind U, meridional wind V.zonal moisture flux Q_v,meridional moisture flux Q_v,and precipitation rate over East Asia are calculated to construct correlation fields,Results show that different heating modes over the TP correspond to different circulation,moisture flux as well as precipitation patterns,Precipitation over North China(or Kashmir) is negatively(or positively) correlated with REOF1.Similarly.notable negative(or positive) correlation can be found between the rainfall over south part of Southwest China.South China,and the Philippines(or Japan) and the REOF3. Due to high localization of diabatic heating over the TP.it is not enough to study the influence of TP thermal forcing on the climate with an area averaged heating index.     

PRELIMINARY NUMERICAL STUDY OF TOPOGRAPHIC EFFECTS OF THE TIBETAN PLATEAU ON SURFACE DIRECT RADIATION*

To improve the simulating ability of a model,this paper presents a scheme of calculating direct radiation at land surface with topography in the model.A numerical study is conducted for the topographic effects of the Tibetan Plateau on the direct radiation using NCEP terrain data.Results show that,after taking account into the topographic radiation effect,the regional average of the radiation over the Plateau obviously increases in the local early morning and late afternoon,but changes less around noon.The effect is stronger in winter than that in summer.And heterogeneous topography has also affected the distribution of the radiation in this area.A simple numerical experiment shows that considering the effect will lead ground temperature to increase on the slope having more sunshine,and vice versa.     

THE IMPACT OF MID-LATITUDE CIRCULATION OVER THE TIBETAN PLATEAU ON THE TRACK OF TYPHOON OVER THE NORTHWEST PACIFIC

There is a close relationship between variation of middle latitude synoptic system over theTibetan Plateau and movement of typhoon over the Northwest Pacific.This paper tries to discussthe mechanism of it through the numerical simulation and dynamic diagnosis.25-year statistical results during the period from 1970 to 1995 indicate that the relationshipbetween middle latitude circulation over the Plateau and the track of typhoon over the NorthwestPacific are as follows.When longwave trough in 500 hPa isobaric surface over the Plateau.it isfavourable to typhoon's westward movement:on the contrary,large scale anticyclone over thePlateau is favourable to typhoon's recurvature.We simulated the typhoon under these two flowpatterns with MM5 model.Numerical simulation results showed that:(a)Longwave trough on500 hPa surface over the Plateau makes subtropical high extended westward so as to typhoon southto high moves westward in easterly steering flow.(b)Anticyclone over the Plateau will bringabout the longwave trough developing along the east coast of Asia,it'will usually cause typhoonrecurring in front of the developed trough.In addition,the results of dynamic diagnosis show the physical mechanism on impact ofsynoptic system over the Plateau on the downstream synoptic system,that is,the transport ofdisturbance kinetic energy over the upstream Plateau makes the downstream trough developed.And then it affects the steering flow of typhoon.Because of existence of the high ridge over thePlateau.the energy transport channel moves to north.On the contrary,the trough exists over thePlateau,the energy transport channel moves to south.The southerly in front of trough over thePlateau and the southerly in the east part of typhoon transport low potential vorticity of lowlatitude into subtropical high.That is beneficial to development of subtropical high and affectstyphoon's movement.These results fully reflect the impact of interaction between mid-latitude andlow latitude on synoptic system.     

REGIONAL DIFFERENCES OF TEMPORAL-SPATIAL CHARACTERISTICS OF AIR-SEA INTERACTIONS IN TROPICAL OCEANS*

The singular value decomposition (SVD) of air-sea interaction in the tropical western,central,and eastern Pacific,and the tropical Atlantic and Indian Oceans has been conducted by using the NCEP/NCAR 40-year reanalysis 1000 hPa monthly wind field and COADS monthly sea surface temperature (SST).Comparisons of the results suggest that these areas can be divided into three types from the viewpoint of air-sea interaction:tropical central-eastern Pacific belongs to monistic type,in which ENSO is the sole important process;tropical western Pacific and Indian Oceans belong to dualistic type,in which in addition to ENSO.there should be an another important process;tropical Atlantic Ocean belongs to pluralistic type,in which the process is complicated and the ENSO cycle is not evident.     

CHARACTERISTICS OF ATMOSPHERIC HEATING AND ATMOSPHERIC CIRCULATION DURING ACTIVE PERIOD OF 500 hPa HIGH OVER THE TIBETAN PLATEAU IN SUMMER*

In correspondence with the establishment of the "upper high and lower high" pressure pattern due to the activities of 500 hPa high over the Tibetan Plateau in summer,a series of changes of the East Asia atmospheric circulation will take place.In this paper,the distributions of divergence and vertical velocity of 500 hPa high,the evolutions of atmospheric heat source,the variations of vorticity and zonal wind at 100 hPa level and vertical meridional cell over the Tibetan Plateau etc.are statistically analyzed.Thus,we can see that the ascending motion and the convective heating over the Tibetan Plateau,the South Asia high and the westerly jet on the north of the Plateau at 100 hPa level are weakned.The northern branch and the southern branch of the easterly jet on the south of the Plateau merge into a single whole and situate on the south of the former northern branch.In the meantime,thermodynamic land-sea discrepancy in South Asia and the convective heating over the Bay of Bengal is enhanced.It will play an important role in the maintenance of the easterly jet and the South Asia monsoon.     

A STUDY ON THE VARIATIONS OF ANNUAL FREQUENCY FOR TROPICAL CYCLONE IN NORTHWEST PACIFIC DURING THE LAST HUNDRED YEARS

In this paper, a study on the variation of annual frequency of tropical cyclone (TC) and its relation with SST, Southern Oscillation index, sunspot relative number and number of days for specific circulation patterns was made by using 1884-1988 data of annual frequency for Northwest Pacific TC occurrence, which had been corrected to tendencies.Preliminary results indicated that in the variation of annual TC frequency there exist obvious p.6riods of 21, 31, 15 and 6 years and sustaining periods lasting 12 years in average.Well-defined processes of inflexion were observed in 1931, 1959 and 1977 over the past hundred years.The results also suggested insignificant statistic tendency of annual TC frequency increasing (decreasing) in winter/spring (summer/autumn )in the El Nino years.When the stratosphere was in the zonally westerly phase, the northern zonal circulation would abnormally develop and solar activity would enhance to favour the generation and development of TC.     

青藏高原春夏季对流异常及其对西太平洋副高的影响

高原冬春季净辐射多寡可以显著影响高原地区大气环流季节转换进程和后期高原对流活动。净辐射偏多,初夏高原同低纬度之间温度梯度反转时间提前,高原对活跃,夏季高原主体对流异常,可以通过辐散环流造成西太平洋异常的下沉运动,影响副高的强度、南北位置和西伸程度,最终导致夏季降水的异常分析。     

青藏高原对1998年长江流域天气异常的影响

利用NCEP再分析资料分析了青藏高原对 1998年夏季长江流域洪涝及天气异常的影响 ,并讨论了第 2段梅雨期的暴雨与长江流域洪涝灾害的关系。研究结果表明 :由于青藏高原的热力作用 ,夏季高原东北部斜压性强 ,多短波槽活动。 1998年长江流域两段梅雨期间 ,高原东移的短波槽加强了梅雨锋 ,并引起梅雨锋上强暴雨。 1998年长江流域的 8次洪峰均与高原东侧短波槽东移有关。由于梅雨期前面几场暴雨已使得土壤水份饱和 ,沿江各支流及湖泊水位很高 ,梅雨期的最后一场暴雨的大量雨水往往作为径流流入江河或湖泊 ,与长江洪峰汇合后易造成洪涝灾害     

ON THE RELATIONSHIPS BETWEEN THE SUMMER RAINFALL IN CHINA AND THE ATMOSPHERIC HEAT SOURCES OVER THE EASTERN TIBETAN PLATEAU AND THE WESTERN PACIFIC WARM POOL

The relationships between the summer rainfall in China and the atmospheric heat sources over the eastern Tibetan Plateau and the western Pacific warm pool were analyzed comparatively, using the NCEP/NCAR reanalysis daily data. The strong (weak) heat sou…     

DIAGNOSTIC ANALYSIS OF THE INFLUENCE OF ANOMALOUS SURFACE SENSIBLE HEAT FLUX IN THE TIBETAN PLATEAU AND ITS VICINITY ON THE EAST ASIAN WINTER MONSOON*

Based on NCEP/NCAR reanalysis monthly data,the relation between the surface sensible heat flux,(SHTFL) in the Tibetan Plateau and its vicinity and the East Asian winter monsoon is revealed as follows:on the inter-annual and longer time scales,the difference between SHTFL anomalies in the east and southern slope of the Tibetan Plateau last spring has influence on the East Asian winter monsoon,that is,SHTFL anomaly in the east of the Tibetan Plateau was positive and that in the southern slope was negative last spring,then the East Asian winter monsoon would become more vigorous,and vice versa.Both the most significant period of the difference between SHTFL anomalies in the east and southern slope of the Tibetan Plateau and that of the East Asian winter monsoon index are 2 to 4-year time scales.On the 2 to 4-year time scales,the heterogeneous spatial distribution of SHTFL anomalies in the east and southern slope of the Tibetan Plateau last spring has effect on the East Asian winter monsoon,after SHTFL anomaly in the east of the Tibetan Plateau was positive and that in the southern slope was negative last spring,then the East Asian winter monsoon would be more powerful,and vice versa.The lag influence of the difference of SHTFL anomalies in the east and southern slope of the Tibetan Plateau on the East Asian winter monsoon brings into effect mainly on 2 to 4-year time scales.In the end an reasonable explanation for their relationship has been discussed.     

CHARACTERISTICS OF SEASONAL VARIATION OF RAINFALL OVER THE TIBETAN PLATEAU DURING SUMMER 1998 AND ITS IMPACT ON EAST ASIAN WEATHER*

The seasonal variation of rainy season over the Tibetan Plateau in summer 1998 is analyzed by using daily observational rainfall data for Lhasa from 1955 to 1996,and rainfall data at 70 stations from January to August of 1998 over the Tibetan Plateau (TP) and adjacent regions,as well as TBB data from May to August of 1998.The onset date of rainy season for Lhasa is climatologically 6 June.Among the analyzed years,the earliest onset date is 6 May,while the latest may delay to 2 July.The obvious inter-decadal variation can be found in the series of onset date.The onset date of summer 1998 over middle TP (onset date of Lhasa) is 24 June,which is relatively later than the normal case.The onset for rainy season of 1998 started over southeast and northeast parts of TP and then propagated westward and northward.The convection over east and west parts of TP shows that there is a quasi 12-15 day oscillation.In June,the convection over middle and lower reaches of Yangtze River is formed by the westward propagation of convection over subtropical western Pacific.while in July.it is formed by the eastward propagation of convection over TP.Besides,it is also found that there exists good negative and obvious advance and lag correlation between the convection over the middle and western TP and that over the subtropical western Pacific and southern China.Therefore it can be inferred that a feedback zonal circulation with a quasi two-three week oscillation exists between the ascending region of TP and descending region of subtropical western Pacific,i.e.the convection over TP may affect the subtropical high over western Pacific and vice versa.