Review of Recent Climate Conditions and Forecasts
Abstract

Abstract Author: Tom Murphree and David Meyer

Abstract Title: Long Term Changes in Tropical Cyclone Activity in the Western North Pacific

Abstract: We have examined long term changes in tropical cyclone (TC) activity in the western North Pacific during 1970-2006. Our primary goal was to assess the relative roles of interannual-decadal climate variations and global warming related processes in altering TC activity. Our analyses focused on the large scale environmental factors (LSEFs) that determine TC formations and intensities. Our primary data sets were Joint Typhoon Warning Center best track data for TC activity and NCEP reanalysis fields for the LSEFs. For our study region and period, the TC data appear to be sufficient to allow a relatively accurate assessment of long term variations in TC activity. We used linear regression models to describe TC formations and intensities as linear functions of the LSEFs at intraseasonal and sub-basin scale resolutions. Our results indicate that: (1) both TC formations and intensities can be well described by a limited set of LSEFs; (2) the spatial and temporal variations in the LSEFs since 1970 have both favored and disfavored increased TC formations and intensities; (3) TC formations (TC intensities) per year have undergone a relatively small (large) long term increase since 1970; and (4) the LSEF variations that underlie these increases are consistent with both known interannual and decadal variations (e.g., El Nino - La Nina) and with changes associated with global warming (e.g., changes in SST, vertical shear). Finally, we have used the regression models, in combination with hindcasts of the LSEFs, to conduct preliminary hindcasts of TC activity at intraseasonal lead times. Our results indicate that such a multi-model forecast system has significant potential for generating intraseasonal, sub-basin, probabilistic predictions of TC formations and intensities.