Transient Coastal Upwelling and Water Circulation in Bantry Bay, a Ria on the South-west Coast of Ireland

Abstract

Water column stratification and circulation were measured in Bantry Bay, a ria on the south-west Irish coast, during the Summer of 1991. Upwelling and downwelling appeared on several time scales. During July, cool dense offshore water penetrated the bay at depth for several weeks, lifting up the thermocline at a mean rate of 1·2 m day −1. In early August, the cool shelf water receded, thermocline depth rapidly deepened at 8·5 m day −1and water in the bay was replaced by warmer coastal surface water. Strong oscillatory residual flows with longer excursion scales than tidal oscillations were also detected throughout the water column. They were caused by the passage of a series of Atlantic depressions, giving a periodicity of a few days. During upwelling, currents were predominantly towards the head of the bay in the bottom 20 m, and seaward in the upper 20 m: at middle depths, the direction of flow varied according to the position of the thermocline. During downwelling, this flow pattern was reversed. A simple two-layer model of frictionally damped wind-driven flows constructed on the basis of hydrostatic equilibrium was used to examine the contribution of local winds to upwelling and downwelling on time scales of about a day or more. Two unknown parameters in the model were fitted statistically but took physically realistic values. The model accounts for about three-quarters of the variance in the record of the residual current and mimics the observed fluctuations in thermocline depth. It successfully reproduces the general late July upwelling and August downwelling and also the shorter period of cyclic upwelling and downwelling associated with the passing Atlantic depressions. Some of the small discrepancy between the model and observed residuals may owe to changing density on the shelf.