Effects Of The Preceding SST And Water Vapor Transport Path On The Summer Precipitation Over The Yangtze River Valley

The prediction of summer rainfall over Yangtze River valley is an important aspect of summer precipitation forecast in domestic, there are multiple impact factors which have important influence on summer rainfall over Yangtze River valley, including the sea surface temperature(SST), the summer monsoon, atmospheric circulation field (West Pacific Subtropical High), snow and so on. Now researchers also find summer rainfall over Yangtze River valley relates to the early Southern Hemisphere circulation including the Southern Hemisphere Annular mode, Mascarene High and Australia High. This paper explores the relation between the early SST field in southern hemisphere and summer rainfall over Yangtze River valley using some statistical method including EOF analysis, correlation analysis and composite analysis, to provide theoretical support for the summer precipitation forecast over Yangtze River valley.Using HadISST datasets and the NCEP/NCAR (National Centers for Environment Prediction/National Center for Atmospheric Research) monthly reanalysis data, OLR (Outgoing Long-wave Radiation) data and monthly precipitation data of 160 stations in mainland China from 1980 to 2012, the relationship between the summer rainfall over Yangtze River valley in China and the prior-winter sea surface temperature anomalies (SSTA) around Austria is studied. The results show that the leading mode of the monthly SSTA around Austria by using empirical orthogonal function (EOF) method exhibits the characteristics of SST variation trending to consistent mode, and the SSTA around Austria has a seasonal continuity, which can persist from the prior-winter through the following summer. Using Nino3.4 index excluding ENSO linear signal in the SST field, show that consistent model may be a local phenomenon which is independent of ENSO events. According to consistent mode of prior-winter SSTA around Austria, the consistent mode index (CMI) is defined to characterize the influence of SSTA around Austria on the summer rainfall in the Yangtze River valley, which has certain significance for short-term forecasting summer precipitation in the Yangtze River valley. The influence of prior-winter SSTA around Austria on East Asian summer atmospheric circulations may occur in two ways. One way by an anomalous prior-winter SSTA signal around Australia persisting through the following summer which can excite the teleconnection wave train between north and south Hemisphere, causing an abnormal West Pacific Subtropical High (WPSH), and thus affecting the summer precipitation in Yangtze River valley. The other way is the SSTA around Australia inducing an abnormal convection in the tropics in both hemispheres, especially for the abnormal convection activities over the Philippine Sea, and thus leading to an anomalous change of summer rainfall in the Yangtze River valley.In addition, we find that there may be a negative correlation between the consistent mode index (CMI) and the East Asian summer monsoon, when CMI index is high (low) value, from the northern Australia to South China Sea and the Indian Ocean is warming (cooling) which would weaken (enhance) temperature difference between land and sea in East Asia resulting East Asian summer monsoon weakened (enhanced).The prediction of summer rainfall over Yangtze River valley in 1993 is poor basing on the CMI index, analyzes the sea surface temperature anomaly (SSTA) showing that the sea temperature surrounding Australia is cold phase from pre-winter to summer in 1993, the equatorial central Pacific temperature warming anomaly continues from the beginning of spring until the following winter then develops into El Nino event. The sea surface temperature in middle-east equatorial Pacific warm anomaly in spring and in summer with strong convection, and the Western Pacific convection is suppressed exciting the "+" "-" "+" Pacific-Japan teleconnection (P-J) from south to north in East Asia. On the other hand, the westerly jet is more southerly than normal in East Asia, together which result in West Pacific Subtropical High (WPSH) is more southerly than normal inducing more summer rainfall over Yangtze River valley.Different paths of water vapor transport have different effects on precipitation of different areas. South China Sea water vapor transport has a significant negative correlation with summer rainfall over Yangtze River valley, but after in-depth analysis find that it has a direct relationship with the zonal wind vapor transport. That is, when there is easterly anomalies in low latitude region of East Asia (5-15°N) with an anticyclonic circulation in the western Pacific Ocean, which is conducive to water vapor transport to the Yangtze River valley inducing more summer rainfall over Changjiang-Huaihe River Valley; On the contrary when there is westerly anomalies with an abnormal cyclonic circulation in the western Pacific Ocean, the water vapor from the Indian Ocean and Bengal Bay transports to the eastern Pacific, which is not conducive to water vapor transport to the Yangtze River valley inducing less summer rainfall over Yangtze River valley.