Steady Coastal Upwelling in a Continuously Stratified Ocean

Abstract

Steady coastal upwelling driven by wind-stress and surface apparent temperature anomalies in a continuously stratified ocean of infinite depth is examined within the framework of a set of linearized hydrodynamic and thermodynamic equations. Solutions are found for a single Fourier component in the offshore direction. Three distinct internal depth scales are found to exist which correspond to the thermal wind, Ekman and Lineykin structures of the induced coastal circulation. As a result of the counterbalance of the thermal wind and Ekman drift, a subsurface countercurrent develops under an offshore wind. When the wind is along-shore, the situation may arise where the thermal wind balance prevails throughout the depth; in this case, the circulation in a plane normal to the coast vanishes completely. Consequently, coastal upwelling tends to he attenuated by the surface temperature contrast it produces. Under normal circumstances, the wind stress outweighs the surface buoyancy flux or temperature anomaly as a driving mechanism for coastal upwelling. In general, along-shore winds are more effective in producing coastal upwelling than offshore winds. The most favorable angle between the offshore wind vector and the coastline appears to he 13° for a typical stratification along the West Coast of North America. Finally, increased stratification tends to confine the transverse circulation to a shallower near-surface region than for lesser stratification.