Use of a genesis potential index to diagnose ENSO effects on tropical cyclone genesis

ENSO (El Nino-Southern Oscillation) has a large influence on tropical cyclone activity in various basins. Here, we examine how different environmental factors contribute to this influence, using a genesis index developed by Emanuel and Nolan (2004). Four factors contribute to the genesis index: low-level vorticity (850hPa), relative humidity at 600hPa, the magnitude of vertical wind shear from 850 to 200hPa and PI (potential intensity) (Emanuel, 1988). Using monthly NCEP Reanalysis data in the period of 1950-2004, we calculate the genesis potential index on a latitude strip from 60$S to 60N. Composites of the anomalies of the genesis index are produced for El Nino and La Nina years separately. These composites of the genesis index, which was originally trained on spatial variability and the annual cycle, skillfully replicate observed interannual variations of the observed frequency and location of genesis in several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which among the factors are most important in causing interannual variations in genesis frequency. Specific factors can be identified that have more influence in different regions. For example, in El Nino years, relative humidity and vertical shear are important for the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are important for the eastward shift in the mean genesis location in the western North Pacific.