INTRASEASONAL VARIABILITY IN THE YANGTZEHUAIHE RIVER RAINFALL AND SUBTROPICAL HIGH DURING THE 1991 MEIYU PERIOD

The NCEP/NCAR reanalysis data and observed daily rainfall from gauge stations were used to investigate the intraseasonal variability on heavy rainfall and its r elated atmospheric circulation anomalies during the 1991 meiyu period. It is emp hasized that the intraseasonal variartions of the subtropical high at different levels have close relationships with the 1991 extreme flood over the YangtzeHuaihe river basin. Wavelet analyses show that the intraseasonal variation in the 1991 meiyu rainfall over the YangtzeHuaihe river basin was mainly controlled by the 15-35 days (25 day mode) oscillation. An examination of circulation featur es suggests that the 15-35 day oscillation exhibited as an anomalous anticyclon e (cyclone) in lower troposphere existing alternatively over the coast of southe astern China, which modulates the western North Pacific subtropical anticyclone shifting southwestward into (northeastward out of) South China Sea. Such a lower tropospheric flow pattern was coupled with a dipole anomaly in the upper tropos phere, and the latter regulated the South Asian anticyclone extending northward and eastward (contracting southward and westward), forming a favorable (an unfav orable) circulation environment for heavy rainfall over the YangtzeHuaihe rive r basin. The intraseasonal oscillation in the Meiyu rainfall was also closely re lated with the 15-35 day mode of the 500 hPa North Pacific subtropical high and its westward propagation. During the active phase with excessive rainfall over the YangtzeHuaihe river basin, the 500 hPa geopotential height exhibited posit ive anomaly over the western North Pacific and negative anomaly over the central North Pacific. Opposite circulation anomaly was for break period. The 15-35day oscillation of the 500 hPa North Pacific subtropical high might originate from around Hawaiian Islands, and then propagated westward along the vicinity of sub tropical high ridgeline into the coast of southeastern China.