RESEARCHES ON ANOMALOUS CHARACTERISTICS OF THE SUBTROPICAL HIGH OVER THE WESTERN PACIFIC DURING 1998 SUMMERTIME

Using NCEP reanalysis data, the anomalous characteristics and mechanisms of the subtropical high over the western Pacific during 1998 summer was analyzed. Contrast to the case of climate mean, the subtropical of 1998 summer was stronger with its ridge extending westward far more. In addition, the abnormal latitudinal fluctuation of the subtropical high occurred, especially on withdrawing abruptly from north to south at the mid of July and then maintaining at lower latitudes until the beginning of August. This caused the second Meiyu precipitation over the Yangtze regions. The latitudinal fluctuation started at east of the subtropical high belt over the mid-Pacific, propagated westward gradually. It shows that the anomalous variation of the 1998 subtropical high is due to the adjustment of the East Asian circulation, in which anomalous variation of the subtropical westly jet is important of the retreat of the subtropical high. The variation on location and intensity of the jet and the induced secondary circulation at entrance/exit of the jet, as well as the instability impacts on the anomaly of the subtropical high on July. The sensitivity numerical experiment by T106L19 global spectral model indicates that the response of geopotential height to weakening of the jet is consistent with the result by observational analysis. Heat source/vapour sink analysis, OLR analysis and SVD analysis with heat source and geopotential height presents that the diabatic heating over the tropical monsoon region is notably weakened before the retreat of subtropical high, which makes for the subtropical high maintaining at lower latitude. The analysis on the meridional circulation also proves above view.