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
