Influences of Tropical SST Anomalies in Different Regions on the East Asian Summer Monsoon Circulation and Precipitation
|School||Ocean University of China|
|Keywords||East Asian Summer Monsoon West Northern Pacific anticyclone GEFA LIM Sensitivity|
Based on the Generalized Equilibrium Feedback Analysis (GEFA), the LinearInverse Modeling (LIM) method, and Atmospheric General Circulation Model(AGCM) simulations, the influences of sea surface temperature anomalies (SSTA)in different region on the East Asian Summer Monsoon (EASM) circulation andprecipitation are investigated. Some practical problem of GEFA and LIM are firstaddressed. An approach to reduce the sampling error of GEFA is proposed. Thetwo methods are applied in assessing the seasonal cycle of the atmosphericresponses to surface forcing, it is found that GEFA and LIM are able to reproducethe major features of the seasonal response consistently. Next, the relativeimportance of SSTAs in different sea areas in forcing the dominant pattern ofEASM low level atmospheric circulation, the West Northern Pacific Anticyclone(WPAC) anomaly, are investigated using the two statistic combining modelsimulations; the key areas to WPAC in different seasons are identified; thesensitivity of summer Yangtze-Huai River Valley (YHRV) to tropical SSTAs arediscussed. This study has deepened our understandings of SSTA influences on theEASM circulation and precipitation, and provided new basis for the prediction ofEASM. The results are achieved as follows.(1). The performance of GEFA in the SST EOF space is validated using anensemble of ideal model. It is found that the GEFA estimation of the atmosphericresponse to the leading SST EOF modes is much more accurate, robust and lesssensitive to the EOF truncation than the GEFA estimation in physical space.(2). The assessment of the seasonal cycle of the atmospheric response tosurface forcing in GEFA and LIM are compared in both a conceptual climatemodel and the observation. It is found that LIM and GEFA are able to reproducethe major features of the seasonal response consistently. The success of GEFA andLIM for the assessment of the seasonal response is due to the slowly varyingnature of the annual cycle relative to the atmospheric response time. Therefore, the two independent methods for the assessment of the seasonal atmospheric responsein the observation.(3). There are different key sea areas to the WPAC in different seasons. Forboreal winter and spring, the most important region to WPAC are, respectively, acold West Northern Pacific (WNP) and a warm Tropical Indian Ocean (TIO). Insummer, the local ocean-atmosphere interaction in WNP is dominant by theinfluence of atmosphere on the ocean, therefore, the WPAC is forced by non-localSSTs instead of local SSTs. Among all the non-local SSTs, the equatorial centralPacific (Nino4region) cooling is most important to the summer WPAC and theTIO warming comes next; the two SSTAs can both generate anticycloniccirculation though the pattern is a little different. The Nino4influences on theWPAC conform the tropical atmospheric responses to diabetic heating.Previous studies attributed the summer WPAC anomaly to the proceeding ElNino. This study demonstrates that El Nino is not necessarily followed by theWPAC; the WPAC depends on the SST pattern in the following summer,especially the SSTAs in the central Pacific. The author pointed out that thesummer WPAC manifested in the first EOF of EASM circulation is actually amixture of two components induced by the central Pacific cooling and TIOwarming respectively. The former is related to El Nino decay while the latter isrelated to La Nina developing.(4). The sensitivity of YHRV rainfall to the tropical SSTs is investigatedusing two AGCM, CCM3and ECHAM5. It is found that the Northern IndianOcean and the South China Sea-Western Pacific SSTAs is most favorable to theYHRV rainfall due to the enhanced moisture transport from the southwesterliesassociated with intensified Indian monsoon. This SSTA pattern also induces theWPAC which intensifies the monsoonal winds as well as the moisture transportfrom the western Pacific to the subtropical monsoonal region.