An analytical model describing thermally forced upwelling in a semi-enclosed deep bay is presented. The model is 2-dimensional (integrated in the lateral direction) and models the vertical circulation as response to a surface heat flux. It is assumed that the water outside the bay is kept cold. The cold water enters the bay where it becomes exposed to intense heating and thus becomes less dense. The heated and low-density water forms an outward directed divergent surface current which gives rise to an upwelling spread over the bay and a compensating inflow of cold water at depth. It is assumed that the scales are such that the surface layer outflow is geostrophic. The model is thought to be applicable to a bay at a low-latitude coast where large-scale upwelling along the coast brings cold water to the surface (e.g. Chile). Four equations are used: conservation of heat in the surface layer, geostrophic surface outflow, continuity of volume and a mechanical-energy equation. The model is forced by a constant surface heat flux. The system of equations is solved using dimensional analysis, and thus leaves a universal non-dimensional constant to be determined by observations. The model predicts a divergent outflow in the heated surface layer, which increases linearly (for a rectangular bay) from the head of the bay. The upwelling velocity is thus constant and so is the surface excess temperature. Numerical experiments using a 3D-model in a simple setup show similar response after a few days of spin up. The model predictions are found to be in reasonable good agreement, both in a qualitative and a quantitative sense, with observations from Bahia de Concepción, Chile, during some summer days with low winds. The intense upwelling in the bay those days resulted in a very high primary production as was indicated by the green colour of the water and the small Secchi-disk depth).
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 4203 Analytical modeling and laboratory experiments