Publication Date

2008-04-20

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2008-04-09

First Committee Member

Lynn K. Shay - Committee Chair

Second Committee Member

Bruce A. Albrecht - Committee Member

Third Committee Member

Mark A. Donelan - Committee Member

Fourth Committee Member

Kevin D. Leaman - Committee Member

Fifth Committee Member

Peter G. Black - Outside Committee Member

Abstract

The ocean mixed layer response to a tropical cyclone within, and immediately adjacent to, the Gulf of Mexico Loop Current is examined using a combination of ocean profiles and a numerical model. A comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy and circulation evolution in response to Hurricane Lili's (2002) passage. Mixed-layer temperature analyses show that the Loop Current cooled <1 degree C in response to the storm, in contrast to typically observed larger decreases of 3-5 degrees C. Correspondingly, vertical current shears, which are partly responsible for entrainment mixing, were found to be up to 50% weaker, on average, than observed in previous studies within the directly-forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical water from the cooler, heavier Gulf Common water, was found to decrease in intensity by -0.18 plus/minus 0.25 m/s over an approximately 10-day period within the mixed layer. Contrary to previous tropical cyclone ocean response studies which have assumed approximately horizontally homogeneous ocean strucutre prior to storm passage, a kinetic energy loss of 5.8 plus/minus 6.3 kJ/m^2, or approximately -1 wind stress-scaled energy unit, was observed. Using near-surface currents derived from satellite alimetery data, the Loop Current is found to vary similarly in magnitude, suggesting storm-generated energy is rapidly removed by the pre-exiting Loop Current. Further examination of the energy response using an idealized numerical model reveal that due to: 1) favorable coupling between the wind stress and pre-existing current vectors; and 2) wind-driven currents flowing across the large horizontal pressure gradient; wind energy transfer to mixed-layer kinetic energy can be more efficient in these regimes as compared to the case of an initially horizontally homogeneous ocean. However, nearly all of this energy is removed by advection by 2 local inertial periods after storm passage, and little evidence of the storm's impact remains. Mixed-layer vorticity within the idealized current also shows a strong direct response, but little evidence of an near-inertial wave wake results.

Keywords

Tropical Cyclone; Hurricane; Air-sea Interaction; Mixed Layer; Ocean Response; Mechanical Energy

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