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.

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The effects of cumulus cloud ensembles on the dynamics of mid-latitude explosive cyclones are studied both from a diagnostic and from a theoretical point of view. / A diagnostic study of an explosive winter storm shows that significant residuals attributable to cumulus convection are present in the horizontal momentum, thermodynamic and vorticity budgets, but not in the divergence budget, and that the finite-amplitude wave is well described by linear dynamics. / A scale analysis appropriate for mid-latitude synoptic-scale waves with intense convection indicates the importance of three mechanisms by which cumulus clouds can influence these storms. These mechanisms are the thermodynamic and dynamical effects of clouds, and certain cloud-induced non-geostrophic effects. / A series of experiments with linear models of moist baroclinic instability using an idealized basic state shows the significant role of the ageostrophic terms and the important damping due to the dynamical effects of clouds. With a realistic wintertime basic state, we conclude that the different cloud forcings are necessary to account for the observed features of explosive storms.

Graduation date:1978

Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1989. / Includes bibliographical references. / Microfiche. / xii, 198 leaves, bound ill. 29 cm

156 p. : ill. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Mawson Institute for Antarctic Research, 1971