Seasonal Environmental Conditions Related to Hurricane Activity In the North East Pacific Basin

A study on interannual hurricane activity in the Northeast Pacific basin is presented, using statistical methods to investigate tropical cyclone frequency and its relationship to seasonal environmental conditions in the years 1972-1997. This follows the work of Collins and Mason (2000) who noted that the NE Pacific has more than one population of tropical cyclones with regard to causal factors, and that tropical cyclone frequencies in the two identified development regions show large differences over time and in their relationships with seasonally averaged environmental variables. Here, we focus on the main factors responsible for variations in hurricane frequency. In the western development region (west of 116°W), mid-tropospheric relative humidity, and sea surface temperature are found to be the dominant factors. These are two of Gray\u27s (1979) necessary controls on tropical cyclone formation. Analyses of the National Center for Environmental Prediction/ National Center for Atmospheric Research reanalysis data show that both variables are probably acting locally on tropical cyclone genesis, as threshold effects provide conducive/non-conducive conditions in different parts of the region. Interannual variations in hurricane frequency therefore occur when considering a spatial averaging over the whole development region. The interannual variability of mid-tropospheric relative humidity in this area, as the dominant control on hurricane frequency, is then studied in detail. It is found that interannual variations in seasonal relative humidity are significantly influenced, via the wind field, by El Niño Southern Oscillation (ENSO) and by the intensity of the thermal low in North America. It is shown that ENSO has different effects on hurricane frequency in the eastern development region (east of 116°W) compared with the western development region