Spatio-temporal distributive character istics of extratropically transitioning tropical cyclones over the Northwest Pacific

The spatiotemporal distributive characteristics of the extratropically transit ioning (ET) tropical cyclones (TC) over the Northwest Pacific Ocean are investig ated based on the “Typhoon Annuals” 1961-2000 compiled by Shanghai Typhoon Ins titute and the NCEP/NCAR reanalysis data in the same period. The results show th at the annual number of ET TCs over the Northwest Pacific exhibited obvious inte rdecadal variations. It was high in the 1960s, and then decreased evidently in t he 1970s and 1980s, increased appreciably in the first half of 1990s, and was ra ther low in the second half of 1990s. We also found that the annual number of ET TCs actually showed an interannual decreasing trend, which was consistent with the trend of annual total TC number over the Northwest Pacific Ocean. Most of ET TCs indeed occurred in summer and autumn, especially in the transition period f rom summer to autumn. The locations of ET distributed over a quite large area in summer and autumn, and the area of ET overall shifted eastwards from summer to autumn. The tracks of ET TCs before and after their extratropical transition wer e mainly concentrated in the sea area south of Korea peninsular and east of Japa n, respectively. The ET TCs often weakened within the first 6-hour after extrat ropical transition, and continuously weakened in the second 6-hour, except in June and July when the ET TCs after extratropical transition obviously strengthen ed. Besides, the EOF analysis of the NCEP/NCAR monthly 500 hPa geopotential height reanalysis data from 1961 to 2000 reveals that in summer and autumn the zonal circulation index was remarkably negatively correlated with the number of ET TCs. The positive anomaly area of 500 hPa anomalous height field in midhigh lati tudes in the summer half year corresponds to high pressure, and under such a cir culation background, the cold air frequently intruded into the ET TCs moving nor thwards, thus accelerating the extratropical transition of the ET TCs.