Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Weiqing Han

Second Advisor

Jeffrey Weiss

Third Advisor

Gerald A. Meehl

Fourth Advisor

Rajagopalan Balaji

Fifth Advisor

Robert R. Leben

Abstract

The impacts of two consecutive, strong tropical cyclones (TCs) – 04B (10/15-10/19) and 05B (10/25-11/3) in 1999 (hereafter, TC1 and TC2) – on the Bay of Bengal (BoB) heat budget and upper ocean dynamics are examined using the Hybrid Coordinate Ocean Model. A series of diagnostic experiments are conducted to isolate the model’s response to the individual TC-associated forcings by comparing their model results.

During the TCs, the BoB ocean heat content (OHC) is reduced, primarily due to TC-wind induced southward ocean heat transport (OHT) and a reduction in surface downward radiation due to increased cloudiness. BoB OHC is largely restored in the following months via enhanced surface heat fluxes, associated with cold wake restoration, and positive northward OHT. The TCs’ downward heat pumping effect is estimated to be ~1.74×1018J near the end of February 2000, which is less than estimates using previously published methods based on surface observations. The relatively weak heat pumping results from freshwater input by intense monsoon rainfall and river discharge in the BoB, which stabilizes stratification, forms a barrier layer, and generates temperature inversions during seasonal surface cooling. As a result, early stage TC winds entrain the warm barrier layer water and enhance enthalpy loss in the southeastern Bay, while mature stage TC winds erode the barrier layer, decrease SST through upwelling and entrainment of deeper cold water and reduce enthalpy loss in the northwestern Bay. Our findings suggest TC winds may significantly alter the interseasonal BoB heat budget through OHT and surface heat fluxes.

Background southwesterly and TC winds produce positive sea surface height anomalies (SSHAs) along the northern and eastern BoB boundary via Ekman transport and direct seawater pile-up, and produce negative SSHAs along the TC tracks with rightward bias via Ekman divergence and surface cooling. The wind-induced turbulent mixing and upwelling are the primary causes for surface cooling, and the latter does not induce downward heat pumping. The oscillations of top layer currents and temperatures on the right of the TC tracks have period near inertial oscillation (~1.5 days), and the near-inertial currents have strong influence on horizontal temperature advection.

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