On the association between the Indian summer monsoon and the tropical cyclone activity over northwest Pacific

An analysis of observed typhoon tracks and daily global wind data for 56 years (1948-2003) reveals that large-scale circulation anomalies associated with the inter-annual variability of the Indian monsoon play an important role in influencing the tropical Pacific cyclone activity. The cyclogenesis over northwest and tropical west-central Pacific is found to be about 1.33 times higher during weak monsoon years compared to strong monsoon years. Also, there is greater tendency for the Pacific cyclones to move northward and recurve (to the north of 20degreesN) during weak monsoon years. The enhanced cyclogenesis during weak monsoon years is found to be associated with enrichment of low-level cyclonic vorticity anomalies over a wide region of the subtropical Pacific extending from the China Sea, Taiwan and the Philippines region to the central Pacific; while the movement of the tropical cyclones is associated with anomalies of upper-tropospheric steering currents. Given that the interannual variability of the large-scale circulation over the Indo-Pacific sector is crucially determined by the El Nino/Southern Oscillation (ENSO) conditions, the present findings raise several questions pertaining to interactions among the large-scale circulation anomalies, tropical convection and the Pacific cyclonic disturbances, which are likely to provide better understanding of the dynamical linkages between monsoon variability and ENSO.