Seasonal environmental conditions related totropical cyclone activity in theNortheast Pacific basin

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