An analysis of the wind-forced response of Conception Bay using a reduced-gravity numerical model

Otterson, Timm (1992) An analysis of the wind-forced response of Conception Bay using a reduced-gravity numerical model. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (4MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A reduced-gravity numerical model of Conception Bay, Newfoundland, with a realistic coastline and driven by local wind is developed to analyze the wind-forced response of the pycnocline. Model solutions are compared with temperature and current measurements collected from sub-surface moorings deployed at the month in the spring of 1989 and near the head in the spring of 1990. The observed temperature near the head shows similar behavior at all six moorings indicating upwelling of the pycnocline due to northeastward winds. Upwelling around the head and evidence of phase lag from one mooring to the next are consistent with Kelvin wave propagation. The model reproduces most of this variability at all six moorings including the two moorings on the eastern side of the Bay near Bell Island Tickle where Kelvin waves might be expected to be scattered by bottom topography. At the mouth, only the 2 day temperature signal at the mooring closest to the western shore is successfully reproduced by the model. The model's ability to better reproduce temperature variability at the head is explained by the increased wind-forced response at the head and the likely importance of external forcing at the mouth, either from Trinity Bay or due to eddies entering the Bay from outside. The current field at both the head and mouth is poorly reproduced by the model, probably due to the complicated vertical structure of the current. Coherence analysis of the current data shows that the horizontal scale of coherent motions is small (less than 10 km). -- In developing the model used for comparison with observations, boundary conditions that allowed an accurate solution were developed. Placing a radiation condition perpendicular to the coast "upstream" of the Bay resulted in the generation of spurious fluxes and near-inertial Kelvin waves at the boundary. These problems were eliminated by extending the coastline out to sea on the "upstream" side of the mouth and applying a condition of zero normal gradient in interface displacement on the artificial stretch of boundary. Experiments using sponge layers rather than radiation conditions on the other boundaries showed that sponging only the interface displacement and leaving the velocities undamped gives results that compare well with those obtained using radiation conditions. Sponging the velocities as well as the interface displacement led to significantly different results suggesting that applying highly viscous layers around open ocean parts of model boundaries can be counter-productive.

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