A multi-level numerical model of coastal upwelling: A diagnostic study

A two-dimensional baroclinic model is described for coastal upwelling in a vertical plane perpendicular to the coast. The model consists of equations of motion, continuity and turbulence energy along with equations for salinity and thermal energy and an equation of state. The role of density gradient in the baroclinic pressure gradient is investigated to understand the dynamics during the upwelling process. To represent the surface and bottom boundaries corresponding to a fixed computational level in the discretized equations, a set of non-dimensional co-ordinates is used. These co-ordinates are then transformed onto logarithmic co-ordinate axes to resolve effectively the boundary layers. The first experiment is carried out with a flat bottom to understand the dynamics of the upwelling and the structural features of the process by diagnostic analysis of the balance between various terms of the momentum equation. Starting from a state of rest, a spatially uniform alongshore wind stress corresponding to the mean monthly wind stress for the month of May is applied and held constant thereafter. The fluid is assumed to be incompressible and stratified, with the initial temperature and salinity having no horizontal variations but a uniform vertical gradient. As the upwelling phenomenon is transient in nature and keeping in mind the additional computational overheads, the response of the model is studied day-wise up to 4 days. In the second experiment the model is applied to study the upwelling off the east coast of India in a plane normal to the coast of Visakhapatnam. The analysis area extends to 100 km offshore with real topography. The results are presented day-wise for 4 days, comparing the balance between various terms in the upwelling region and in the open sea, and the dynamics of the baroclinic coastal jet is explained.