Seasonal environmental conditions related to tropical cyclone activity in the Northeast Pacific Basin
This thesis presents the first in-depth study on interannual tropical cyclone activity in the Northeast (NE) Pacific, using statistical methods to investigate tropical cyclone frequency and its relationship with seasonal environmental conditions from 1972 to 1997. An improved method of calculating wind shear is first presented. It is demonstrated that the NE Pacific has more than one population of tropical cyclones with regard to causal factors, and tropical cyclones in the two regions show large differences in trends with time and in their relationships with environmental variables. Large increasing trends are found in the western development region (10˚N to 20˚N , 116˚W to 180˚W), with no significant trends in the eastern development region (10˚N to 20˚N, < 116˚W). No significant relationships were found in the eastern development region between tropical cyclone frequency and any of the environmental variables tested, except outgoing long-wave radiation, implying that the main causal factor here is triggering disturbances and their variations. However, in the western development region, some highly significant relationships exist. Important local variables there include relative humidity (RH) and SST. El Niño Southern Oscillation (ENSO) is also a significant factor. The local relationships are probably largely due to the intensity-frequency effect and the spatial averaging of the variables, with threshold effects acting locally to provide conducive/non-conducive conditions in different parts of the region. Physical influences on the most important of these variables (RH) are investigated. (The reverse influence, of hurricanes on RH, is shown to be negligible. ) RH is shown to be significantly influenced, via the wind field, by ENSO and the intensity of the thermal low in North America. ENSO influences provide significant inverse relationships between tropical cyclone frequencies in the western development region and the North Atlantic.