The relationship between synoptic factors and intensity changes of typhoons over the western North Pacific Ocean during ENSO and non-ENSO events

The purpose of this study is to find the relationship between synoptic factors and intensity changes of typhoons over the western North Pacific Ocean during ENSO (El Nino/Southern Oscillation) and non-ENSO events. An empirical equation between climatological sea surface temperature (SST) and maximum potential intensity (MPI) of typhoon is derived by using 27 years (1965--1991) of data from the Joint Typhoon Warning Center and Comprehensive Ocean-Atmosphere Data Set. This equation is provided as a capping function for a given SST and its related typhoon intensity. Six years (1992--1997) of data were used to test the derived equation and there was no typhoon intensity exceeding the MPI.; This empirical equation is used to as a measure of intensification potential (POT), one of the six synoptic variables used to relate the intensity changes of typhoons during ENSO and non-ENSO events. The other variables are vertical wind shear (SHEAR), time tendency of the shear (DSHEAR), relative and planetary eddy angular momentum flux convergence (REFC and PEFC), and relative angular momentum (RAM). These six synoptic variables are the independent variables and the intensity change of typhoons is the dependent variable. The standard multiple linear regression is used to relate the synoptic variables and intensity changes at 12, 24, 36, 48, 60, and 72 hours during ENSO and non-ENSO events.; The results of this study reveal that each variable has a different pattern of significance at various time periods. POT is not significant on the intensity changes of typhoon at any time period during ENSO and non-ENSO events. This may result from the rather uniform SSTs in the region where typhoons develop. SHEAR and DSHEAR are significant at most time periods with negative correlation with intensity changes which indicate that they are good predictors. RAM is significant at most time period of non-ENSO events (1996 and 1998) and not significant at all during ENSO (1997). REFC is not significant on 1996 but is significant on 1997 and 1998. The change of pattern of large-scale environment circulation induced by ENSO may affect the performance of these variables.