A Model of Mesoscale Air-Sea Interaction in a Sea Breeze-Coastal Upwelling Regime

Abstract

A time-dependent, two-dimensional numerical model is constructed by coupling a four-layer atmosphere to a two-layer ocean through fluxes of heat and momentum. Idealized experiments are performed to investigate the oceanic response to sea breeze forcing, changes induced in the sea breeze by coastal upwelling, and air-sea feedback during periods of active coastal upwelling. This problem is motivated by the fact that the time scale of the coastal upwelling response is short compared to most oceanic response times and is comparable to the sea breeze time scale. When forced with a longshore wind stress, the model ocean reproduces several features commonly observed in coastal upwelling regimes, including an equatorward surface jet, a poleward undercurrent, and a region of low sea surface temperatures near the coast. For the cases considered here, the sea breeze contributes significantly to the mean longshore wind stress and, consequently, plays a role in driving the coastal upwelling circulation. It also substantially increases the kinetic energy of the nearshore ocean by forcing inertial oscillations and internal gravity waves with a diurnal period. When the sea surface temperature is held constant and the land temperature is varied diurnally, the model atmosphere cyclically reproduces a realistic simulation of the sea breeze-land breeze circulation which includes such features as the sea breeze forerunner and the sea breeze front. However, a rapid decrease in sea surface temperature near the coast characteristic of coastal upwelling produces important alterations of the sea breeze-land breeze circulation. Low-level cooling of the atmosphere over the cold water leads ultimately to the formation of a shallower, sharper, faster and longer lasting sea breeze front that penetrates more than twice as far inland than it would without the upwelling. In general, the cold water causes an increase in the low-level sea breeze intensity landward of ?6 km inland but a decrease seaward of this point. The cold water decreases the land-sea thermal contrast at night and weakens the low-level land breeze everywhere. Since the cold water in the upwelling zone perturbs the atmosphere on a horizontal scale that is small compared to the internal radius of deformation for the atmosphere, the increase in the longshore geostrophic wind it induces near the coast is small. Furthermore, the reduction in low-level sea breeze amplitude over the cold water compensates the effect of slightly increased mean longshore wind such that the change in mean longshore wind stress is negligible. Thus, although the sea breeze affects the upwelling and the upwelling affects the sea breeze, the air-sea feedback loop to the coastal upwelling process is exceedingly weak.