Abstract:In this research study,historical SST data provided by the Japan Meteorological Agency and Hadley Center,along with NCEP/NCAR reanalysis data,and the ten-day precipitation data in Jiangnan (25°-28°N,110°-120°E;112 stations),were used to examine the relationship between the yearly precipitation in Jiangnan from April to June(Jiangnan rainy season(JRS),also referred to as the first flood period in South China) and the preceding heat content anomalies over the western Pacific Ocean warm pool.Next,the possible mechanism for this relationship was investigated.In this study,consideration was given to the depth of the lower boundary of the thermocline,and the mean temperature in the 0 to 400 m range was defined as the value of the heat content in the upper level of the ocean.Due to the interdecadal increases in the SST,the linear trends of the calculated heat content and precipitation during the Jiangnan rainy season(JRS) were removed,and only the interannual relationship was discussed.Furthermore,the statistical methods which were used in this study included time-delay correlations,regression,and synthetic analyses.The results showed that the precipitation levels during the Jiangnan rainy seasons were negatively related to the heat content anomalies over the warm pool of the western Pacific Ocean.It was found that the heat content over the key areas (3.5°-11.5°N,130.5°-150.5°E) during the months of July and August could be regarded as a good precursor of the winter rainfall.The preceding heat content was found to have important influences on the precipitation and circulation from the month of April until the following June.During the cold years,it was been observed to be negative over northern Pacific Ocean and positive over the western Pacific Ocean warm pool at 500 hPa,which has been referred to as the anti-WP teleconnection waves or the enhancement of the subtropical highs.These are controlled by the anomalous anticyclone activities around the Philippines,and the anomalous cyclone activities at high latitudes over the western Pacific Ocean at the 850 hPa wind field,with distributions of significant anomalous anticyclonic water vapor flux around the Philippines.The anomalous anticyclone activities over the Phillipines tend to intensify the western extensions of the subtropical highs,which subsequently strengthen the transportation of the warm and wet water vapor flow from the west side of the anticyclone regions to Jiangnan.Also,with the divergences in the upper troposphere,the anomalous convective rising motions become enhanced in Jiangnan,and vice versa.These enhancement trends indicate that the precipitation which occurs during the Jiangnan rainy season will be more(less) during the cold(warm) years.In regard to the mechanism,the strong westerly anomalies to the south of the key areas during the previous summer will result in the development of anomalous anticyclone activities in the Philippines during the late autumn months,and cold(warm) SSTs to the west(east) of the key areas.Up until the month of April,the upwelling of the subsurface cold water will sustain the cold SSTA and enhance the anticyclone activities,which continuously transports water vapor to Jiangnan.Meanwhile,the anomalous zonal vertical circulations tend to become enhanced in the tropical Indian Ocean with the development of warm SSTs in the western Indian Ocean,and easterly anomalies in the Equatorial regions.Then,the sinking branch in the western Pacific Ocean sustains the anomalous anticyclone activities,which further induces the water vapor convergence and rising motions in Jiangnan.Moreover,the upper strong divergences which are induced by the southward movements of the subtropical westerly jet stream also favor the increasing convective rising motions in Jiangnan.Therefore,the above-mentioned processes lead to increased (lower) rainfall levels during the Jiangnan rainy season,which are known to be related to the preceding negative (positive) heat content anomalies over the key areas.