Theses and Dissertations

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Defense Date

1992

Document Type

Dissertation - NSU Access Only

Degree Name

Ph.D. Oceanography/Marine Biology

Department

Oceanographic Center

First Advisor

Julian P. McCreary

Second Advisor

Jeffrey A. Proehl

Third Advisor

Russell L. Snyder

Fourth Advisor

Robert H. Weisberg

Fifth Advisor

Richard E. Dodge

Abstract

The goals of this research are to understand the dynamical processes that lead to the seasonal variation of the tropical instability waves (TIWs) in the central and eastern Pacific Oceans and to their meridional asymmetry about the equator. Two types of ocean models are used, namely, the fully nonlinear and linearized versions of the 21/2-layer model used by McCreary and Yu (1992). The nonlinear model, which is forced by climatological data, is used to simulate the TIWs and to identify differences in their character at different times of the year. The linearized model is used to investigate the effect of various background states on generating asymmetric unstable waves.

Solutions to the fully nonlinear model simulate the seasonal variation and asymmetry of the TIWs reasonably well. Results from our main run show that the TIWs seen in the upper-layer temperature field share most of the features of the TIWs observed in both the Pacific and Atlantic Oceans: they have periods of 15-20 days, propagate westward, are most active in the summer and fall, and tend to be concentrated north of the equator.

Various budget analyses reveal that the TIWs in the upper layer are a mixed type of barotropic and frontal instabilities, that the Reynolds-stress term associated with these waves significantly weakens the shear within the two branches of the South Equatorial Current (SEC), and that their eddy heating is as large as the surface heat flux during the wave season. The instabilities in the lower layer, which have longer periods of around 30 days, are of the Kelvin-Helmholtz type. Energetics analyses indicate that the seasonal variation of the TIWs is determined by the sharpness of the sea-surface-temperature (SST) front. The same analyses show that their asymmetry is due to the asymmetry about the equator of the two branches of the SEC and of the SST front, but not to the presence of the North Equatorial Countercurrent. These properties are confirmed by the linear instability analysis.

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