Numerical Study on the Effects of Persistent Cold Sea Surface Temperature on the Northwest Pacific Ocean in Atmosphere Circulation During the Early Summer in East Asia

Ocean as a large heat resource, its persistent sea surface temperature anomaly (SSTA) during the winter half year stirs upper air heating anomaly over it, contributing to atmospheric circulation variation in East Asia region in early summer. By employing the improved NCAR/CCM3 climate model, and designing two experiments which use different sea surface temperature anomaly (SSTA) over northwest Pacific ocean (6.66°—51.11°N, from East Asia western coast to about meridian) , using real monthly SSTA in Exp-A and climate mean SSTA in Exp-B, study is undertaken of the effects of cold SSTA on circulation transformation in the early summer in East Asia in 1992. This model considers well the influence of Tibetan Plateau topography, and the surface roughness length is improved in the context of new TIPEX data, thereby improvs the long term prediction and simulation ability over East Asia region. The investigations indicate that persistent cold SSTA from winter to early summer in 1992 over northwest Pacific ocean is significantly unfavorable for south branch westerly jet's northward seasonal shifting and Tibetan Plateau temperature gradient backspin from winter to summer, exhibiting the temperature decrease of 0—1 ℃ in the north side and increase of 1—2 ℃ in the south side of Tibetan Plateau at early summer, maintaining a west wind center of maximum speed of 5 m/s, 2 m/s on 200 hPa and 500 hPa respectively, resulting in the south branch westerly jet's persistence and enhancement. The cold SSTA in the northwest Pacific ocean reduces the heat release into the atmosphere, inducing the low trough's obuious deepening and the average geopotential height in east Asia dropping by 4.66 dagpm, therefore the cold air in the rear of the cold front invades the lower latitude area so the south branch of the front is southing, and is unfavorable for the west Pacific subtropical high's westwards move and strengthening. It also makes surface mean temperature at early summer decrease by 1—2 ℃ in central China or south China, especially decreasing by 2—5 ℃ at northeast China. Thus cold SSTA in northwest Pacific ocean is an important factor for colder summer in northeast China. The experiment results suggest that those phenomena occurring in early summer in 1992, such as the early or late arrival of the Meiyu in the Yangtze-Huaihe River Basins and scarce rainfalls in the rainy season, are in great relation to the cold SSTA in the close vicinity of East Asia land.The result shows that anomaly heating by SSTA in winter half year over northwest Pacific ocean plays an important role in the circulation variation in early summer. Persistent cold SSTA over northwest Pacific ocean can trigger changes of the temperature over Tibetan Plateau, and then of the westerly jet and long wave trough along Asias east coast, also of the sub-tropical high over western Pacific. Those changes are the main background of the planetary scale flow pattern transforming in North Hemisphere from winter to summer. And then, by influencing surface temperature, it affects the circulation transform in East Asia.