Graduate Thesis Or Dissertation
 

The leading edge of a tropical instability wave : interpretation as a gravity current

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  • A sharp temperature front, oriented along the south-west corner of the leading edge of a Tropical Instability Wave (TIW) warm trough, was encountered at 0°N, 140°W on November 2, 2008 and detected by a 0.45°C increase in SST that occurred over 7 s. The distinct SST signal was observed at three different locations within a 5 km radius. The abrupt change in SST was accompanied by a 0.26 psu drop in sea surface salinity and a 0.5 m s⁻¹ increase in the near surface zonal velocity. The front was oriented west-northwest and propagated at a speed of 0.56 ± 0.01 m s⁻¹. Sub-surface adjustments that coincided with the passage of the front reached nearly 70 m deep and increased both stratification and vertical shear above the Equatorial Undercurrent. The warm side of the front was composed of a turbulent, 40 m thick, buoyant fluid that overtook southsouthwestward flowing equatorial cold-tongue water, forcing the denser water to be subducted below and accelerated along its previous trajectory. The lower 40% of the buoyant flow was composed of a mixture of fluids from both sides of the front. These observations indicate that the buoyant fluid behind the front was driven by the hydrostatic pressure gradient across the front and flowed as a gravity current. Comparisons with two-dimensional, analytical, gravity current models suggest that the buoyant current was in a state of decay at the time it was observed, and the propagation speed of the temperature front was not fully accounted for by the driving forces of the gravity current at its leading edge. It is hypothesized that large scale dynamics associated with the TIW sharpened the hydro-static pressure gradient across the front to an extent that allowed for the generation of a buoyant gravity current to propagate ahead of the mesoscale wave, after which vertical mixing between the buoyant current and the subducted ambient fluid became responsible for the decay of the current's internal driving forces.
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