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UBC Theses and Dissertations

Upper ocean response to storms: a resonant system Crawford, Gregory B.

Abstract

At mid-latitudes during the fall and winter, the mixed layer temperature (MLT) of the ocean decreases, mostly in short bursts associated with storms. These events have been dubbed episodic cooling events, although it has been shown (Large et al. 1986) that the associated decrease in MLT is primarily due to vertical heat re-distribution within the water column rather than heat loss to the atmosphere. This thesis describes an investigation of the phenomenon of episodic cooling. The approach taken was to identify a vertical mixing model which reproduced available observations of the ocean response to storms and use that model in a series of numerical experiments to test the importance of initial conditions and atmospheric forcing. Observations from the Ocean Storms Experiment were used to define pre-storm ocean conditions, surface forcing during storms, and several measures of the ocean response. Three one-dimensional mixing models (Mellor-Yamada Level 2 1/2 [MY; Mellor and Yamada1974]; Price-Weller-Pinkel [PWP; Price et al. 1986]; Large-McWilliams-Doney [LMD; Large et al. 1993]) were tested using the Ocean Storms data. All three models reproduced observed dynamical measures of the ocean response. The LMD model reproduced changes in potential energy and mixed layer temperature better than PWP and MY, and was therefore selected for the numerical experiments. Studies with the LMD model showed that the rotation rate of the wind is a key component to large episodic cooling events. The upper ocean operates as a resonant system with a resonant frequency equal to the local inertial frequency, f. For winds rotating at f during a storm, the input of inertial kinetic energy is maximized, as are the net changes in inertial kinetic energy, potential energy, and MLT. Under such conditions, significant mixing is observed within the thermocline, even though the mixed layer depth may notchange drastically. Winds rotating off the inertial frequency lead to significantly reduced ocean responses and vertical mixing below the mixed layer. Large background currents can modulate the ocean response to a storm significantly; large surface heat fluxes add or remove heat from the upper ocean, but have little impact on the vertical mixing.

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