A probe into the maintaining mechanism of one type of the double-ridges processes of West Pacific subtropical high
一类西太平洋副热带高压双脊线过程维持机制初探

Based on statistical analysis of the 156 double-ridges processes of West Pacific subtropical high from May to Oct.in 1979~2005,it is found in this paper that 87.82% of the double-ridges processes occur because a new ridge is born on the southern flank and there is distinct "seasonal phase-lock",i.e.there are two peaks in the occurring frequency during mid-Jul.and mid-Sep.The composite analysis of the most typical pattern of "born in south and existing in south" shows that the formation,maintenance and disappearance of this type of double-ridges processes have close relationship with propagation of the easterly waves from southeast to northwest on the periphery of the southern flank of subtropical high.The evolution of the anomalous circulation at 500 hPa shows weakening and disappearing of the ridge on the northern flank accords well with the climate laws.The anomalous cyclone before the trough of easterly waves affects only the position of the northern ridge,while the anomalous anticyclonic circulation behind the trough of easterly waves is favorable to the development and maintenance of the southern ridge,therefore,the subtropical high is anomalously in the south when this type of double-ridges processes come to end.It can be seen after further analysis that the interaction between the easterly waves and West Pacific subtropical high changes the distribution of vertical motion in the subtropical high regions.On one hand,it changes the distribution of meridional wind,thus the geostrophic vortivity develops in the ridge region on the south of the subtropical high at 500 hPa.On the other hand,it causes inhomogeneity of the vertical distribution of diabetic heating,triggering anomalous cyclonic circulation between two ridges and anticyclonic circulation in the ridge region on the south.Therefore,the interaction between the easterly waves and West Pacific subtropical high may result in discontinuous retreat of the subtropical high to the south,which has ominous significance for the prediction of subtropical high.