The Influence of the Circulation Anomalies in the Southern Hemisphere on the Tropical Cyclone Frequency in Summer over the Western Pacific and Its Mechanism

Based on the tropical cyclone data,NCEP/NCAR reanalysis data,and the Outgoing Longwave Radiation(OLR) data from NOAA,the influence of the circulation anomalies in the Southern Hemisphere on the tropical cyclone frequency in summer over the western Pacific(125°E-150°E) is studied.Moreover,the possible mechanism on the influence is researched.The results show that,the circulation anomalies over the east of Australia are closely related to the frequency of tropical cyclones over the western Pacific.The geopotential height anomalies affect the convection over Intertropical Convergence Zone(ITCZ) near the east of the Philippines not only through the cross-equatorial flow,but through the Rossby wave transportation.Accordingly,the convection over the ITCZ results in the difference of tropical cyclone frequency.The proposed mechanism is as follows.If the geopotential height anomalies over the east of Australia are positive,there will be corresponding positive geopotential height anomalies in the east area of the Philippines.The positive anomalies mean the western Pacific subtropical high is stronger,and extends west and south.The convection over ITCZ near the east of the Philippines is weaker under the control of the stronger western Pacific subtropical high.Thereby,the frequency of tropical cyclones is less than normal.On the contrary,if the geopotential height anomalies over the east of Australia are negative,there will be corresponding negative geopotential height anomalies in the east area of the Philippines.The negative anomalies indicate the western Pacific subtropical high is weaker,and retreats east and north.The convection over ITCZ near the east of the Philippines is stronger.Hence, the frequency of tropical cyclones is greater than normal.In addition,the westerlies within the area(5°N-15°N,125°E-150°E) also play an important role in the frequency of tropical cyclones.For the cross-equatorial flow near 90°E is stronger,the westerlies mentioned above are strengthened.The stronger westerlies are beneficial to the convection over the ITCZ near the east area of the Philippines.As a result,the frequency of tropical cyclones over the western Pacific increases,and vice versa.