Abnormal Characteristics Of Western Pacific Warm Pool Heat Content And Its Impact On Summer Rainfall Over Eastern China

Recently it is still hard to predict the summer rainfall over eastern China. In these years, the heat content of the western Pacific warm pool (WPWP) has been noted a lot as one of the important factors which affects the summer rainfall in advance. The variation features of shapes and heat content of the WPWP have been analyzed, the relationship between the summer rainfall over the middle and lower reaches of Yangtze River and Northeast China and heat content in the WPWP, and the way how heat content anomaly probably affects the summer rainfall by using historical ocean temperature analysis data provided by Japan Meteorological Agency, NCEP/NCAR reanalysis data, Hadley Centre sea surface temperature (SST) dataset and rainfall data. The main conclusions are as follows:1) The area of the WPWP is the smallest in winter and largest in summer, and its subject moves Northward from winter to summer and Southward in the fall. On average, the thickness of the WPWP is100m or shallower and the depth of its thermocline is200-250m or shallower; Compared with SST, heat content is charactered with slowly varying. The water temperature near the sea surface in the eastern Pacific along9.5°S always changes first, and then gradually influences that along equator9.5°N. The sub-surface temperature shows severe inverse variation along9.5°N and0.5°N. The variability of the heat content of the WPWP is the largest in the Pacific all the year round, and its high value distribution range is the largest in autumn and winter, followed by spring. The winter heat content of the WPWP experienced a climate shift from positive to negative anomaly in the early1990s.2) There is a close negative correlationship between the summer rainfall over both of the middle and lower reaches of Yangtze River and Northeast China and heat content for the previous period of the WPWP, and the predictors for the summer rainfall over those two regions are the0-200-m heat content averaged from November to January in the key area I (166.5°E-173.5°W,7.5°S-3.5°N) of the WPWP and the0-200-m heat content averaged from October to November in the key area (125.5°-135.5°E,15.5°-20.5°N) of the WPWP, respectively.3) The previous heat content anomaly averaged from November to January in the WPWP lasts for about two seasons can lead to persistent influences on the general circulation over the East Asia-Pacific area, which may be the main reason that influences the summer rainfall over the middle and lower reaches of Yangtze River. When the previous heat content anomaly is negative, there is a negative-positive-negative longitudinal wave train from low to high latitude over eastern Asia in the vorticity field in winter and spring, i.e. an anticyclone or negative vorticity anomaly exists in the sea area East to the Philippines, a cyclone anomaly exists over the South China and a negative vorticity district exists near Japan. In summer, the Philippines anticyclone strengthens and moves northward slightly, the cyclone anomaly over the South China moves to the sea area East to the eastern China and enhances, and the negative vorticity district moves northward to the Okhotsk and forms anticyclone anomaly there, which lead to the formation of evident Pacific-Japan (PJ) or East Asia-Pacific (EAP) teleconnection. The middle and lower reaches of Yangtze River, East China Sea, the Yellow Sea and the south of Japan are all under control of the cyclone anomaly, which is conducive to moisture convergence. At the same time, it is good for more rain that the middle and lower reaches of Yangtze River is the intersection of cold air from the North and warm air from the South. Otherwise, the summer rainfall over the middle and lower reaches of Yangtze River is suppressed.4) The EAP teleconnection is stimulated by the heat content anomaly for the previous period of the WPWP, at the same time the wave train propagating eastward along the Asian westerly jet stream exists in the upper level of mid-latitude region, which may be the main reason that influences the summer rainfall over Northeast China. When the0-200-m heat content anomaly averaged from previous October to November in the key area is negative, the anomalous Philippine Sea anticyclone persists and EAP teleconnection appears during the summertime leading to westward extending and enhancing of the western Pacific subtropical high (WPSH) as well as the formation of local anomalous cyclone over Northeast China and Okhotsk blocking high. Meanwhile there exists a teleconnection wave train propagating eastward along the Asian westerly jet stream in the upper troposphere so that local anomalous cyclone over Northeast China and WPSH near Japan can be enhanced, which is conducive to more rain in Northeast China. Otherwise, the summer rainfall is suppressed.