A Numerical Study of Oscillatory Shear Dispersion in the Benthic Layer and the Coastal Thermocline

• Main Author: Guan, Xiaorui
• Format: Thesis
• Language: English
• Published: Columbia University 2010
• Subjects: Physical oceanography; Ross Sea
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3447867

Two process oriented modeling studies are performed to examine the effect of tidal shear dispersion in two oceanic phenomena: the dense water discharge in the benthic layer on the continental slope, and in the salty intrusion in the coastal seasonal thermocline. Motivated by the observed dominant tidal current in the benthic layer on the continental shelf of western Ross Sea, numerical studies were carried out to examine the importance of tidal shear dispersion in the benthic layer formation and facilitating the down-slope dense water discharge. In this process study, numerical simulations are employed to assess an existing analytical model, and to aid the interpretation of the observations in the western Ross Sea. Numerical experiments are carried out for both passive and active tracer experiments, which demonstrate the sharp contrast between the tidal and non-tidal cases. On a steep slope, the model result from the non-tidal case shows that the dense water cannot descend much beyond the shelf break due solely to Ekman advection. When tides are included, however, the dense benthic layer would span several times the Ekman depth, which reduces the diabatic mixing across the density interface that would otherwise dilute the density anomaly, hence allowing the dense water to be more efficiently propelled by the Ekman flow and tidal diffusion. The model results are consistent with the analytical model, and are also corroborated by the observations from AnSlope project. Over the mid-Atlantic shelves of North America, there is prominent onshore intrusion of the saltier slope water along the seasonal thermocline. Taking note of the observed prominence of baroclinic shear across the thermocline, we postulate that it may propel the intrusion through the shear dispersion. The same numerical model is utilized to assess the earlier analytical model and explore the important role of baroclinic shear dispersion. The numerical experiment results agree with the analytical model on the horizontal diffusivity in the thermocline. With inclusion of the baroclinic tidal forcing on the shelf, the numerical model has produced a reasonable simulation of the saline intrusion, in support of our hypothesis of shear dispersion as a possible generation mechanism.