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《Chinese Journal of Atmospheric Sciences》 2006-02
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Influence of Abnormal East Asian Winter Monsoon on the Northwestern Pacific Sea Temperature

QIN Zheng-Kun and SUN Zhao-BoKey Laboratory of Meteorological Disaster and Environmental Variation,Nanjing University of Information Science and Technology,Nanjing 210044  
The strength of cold air acts as an intense signal of midlatitude air-sea interaction and represents a primary feature indicating the difference from the counterpart at lower latitudes.Researchers have shown that East Asian winter monsoon anomaly(WMA) leads to an abnormal mode of the tropical Pacific sea surface temperature(SST) on a synchronous and subsequent basis,and the anomaly would persist beyond one season,so that it would obviously impose great influence on atmosphere circulations in the following season.As a result,based on the WMA,a study of the relationship between the monsoon and SST has been the topic of great interest for a large number of meteorologists.East Asian winter monsoon and Pacific SST are innegligible factors for the rainy-season precipitation in the Yangtze River and the Huaihe River valleys of China.And the monsoon exerts remarkable influence on the precipitation in a season-spaced way.It has been shown that the SST effect links the monsoon with the season-spaced rainfall.Further researches point out that the northwestern Pacific SSTA is particularly important to the Yangtze River and the Huaihe River valleys rainfall,because of its proximity to the Asian mainland. The SSTA of the western Pacific near-equatorial warm pool,Kuroshio and Oyashio currents produces different impact on the rainfall in the studied basins,indicating that SSTA both in large and small mainland-close areas shows its pronounced effect on the basin precipitation.Using simulation and statistic analysis,the authors investigate the regional characteristics of East Asian WMA influencing the Northwestern(NW) Pacific SST.The WMA with its effect on the NW Pacific SST has been addressed both statistically and via modeling. Statistic analysis indicates that in years of intense WMA,up to the depth of 300 m,the NW Pacific SST exhibits remarkable variation on a regional basis, with its decrease thought the Kuroshio area in sharp contrast to the rise in the warm pool and Oyashio on both sides,and vice versa in years of weak WMA.As demonstrated by further statistics and simulation,the regional variation depends largely on varying-degree responses of the air and waters there to the WMA for its formation.Amplified winter monsoon brings plenty of dry,cold air from the north,with the wind field exhibiting pronounced singularity.The Kuroshio area is covered by the anomalies of stronger northerly wind and an anomalous intense anticyclonic circulation shows up in the middle of the North Pacific,with its impacts reaching as far as the western equatorial Pacific.No doubt,the anomaly of the wind field is the cause of the abnormality of oceanic currents.As a result,an extraordinary anticyclonic oceanic current occurs in the eastern part of the Kuroshio and a weaker(stronger) counterpart in the southern part of the warm pool(the west of the North Pacific),with the characteristics of the oceanic currents in rough correspondence to those of low-level wind fields.On the basis of WMA-caused anomalous features of the atmospheric and oceanic fields,it is showed that during the strong monsoon the dry,cold air and northerly abnormal winds give rise to the great intensification of seaair heat fluxes,leading to the drop of Kuroshio SST due to excessive loss of heat,and remarkable temperature decrease also can be found in the deep levels in virtue of convergent sinking of cooled surface water.As revealed by researchers,in years of strong winter monsoon there occurs apparent rise in sea water temperature in the equatorial warm pool due dominantly to the winter monsoon-caused abnormal cyclonic oceanic currents that lead to convergent sinking of water for warming.This study has reached the same conclusion.But the sea water warming in the north of the warm pool is due to other reasons,where sea-air heat flux reduces on account of moist,warm flows of the southwestern part of the cyclone wherein sea-air flux is diminished,other being the cause of reduction of latent heat from condensation resulting in weakened heat loss from water in this region,which is supported by observations.Also,under the effect of warmer airflow in the northeastern part of the cyclone,the sea - air heat fluxes are markedly decreased over the Oyashio and under the impacts of divergent extraordinary cyclonic oceanic currents,low-latitude warm water goes poleward,which serves as an noteworthy factor for water warming. Besides,the convergent sinking of warm water in the western boundary layer is the most important cause of the warming in the deep levels of Oyashio area. In summary,due to the difference in the anomaly of air-sea heat fluxes and sea current which result from the WMA,opposite SST variation is found in the Kuroshio vs the NW Pacific and Oyashio.Because SST acts as a link between winter monsoon and season-lagged SST,the present study is of significance to in-depth study of the WMA effect on summer rainfall over the Yangtze River and the Huaihe River valleys in China.
【Fund】: 国家自然科学基金资助项目40175023
【CateGory Index】: P732
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