Topographically controlled circulation and mixing in a lake

Abstract

It is well known that the wind-driven circulation in a wide lake may be controlled by its topography. Wind stress acting on the smaller mass of the water column in the shallows results in downwind how there and return flow in the deeper water. In lakes with large areas of shallow water this circulation may cause rapid mixing in weakly stratified conditions. In more strongly stratified conditions, topographically induced horizontal wind driven circulation may influence the vertical stratification of the lake. Lough Mask is about 10 km long and has a maximum width of about 8 km. The lake may be divided into shallow eastern and deeper western portions with mean depths of 10 and 35 m respectively. In late Autumn, water at depth in the deep part of the lake moves upwind at speeds of up to 50 cm s(-1) in response to axial winds of up to 15 m s(-1) At the same location a weaker upwind how is observed in the near surface water. This return flow is topographically driven. Temperature differences between near surface and near bottom meters were small (< 0.5<degrees>C) and inversely correlated with the deep current. In a cool, windy summer, temperature time series in the deep part of the lake indicate a deep (> 20m) surface mixed layer and a cooler layer at depth whose temperature fluctuates in response to wind stress along the axis of the lake. Modelling results, supported by drogue measurements, indicate that the return how associated with axial winds in these summer conditions is confined to the surface mixed layer. If this layer is thin, shear between the surface layer and the underlying water can be sufficient to cause mixing and a thickening of the surface layer until the Richardson number is similar to 1. Thus the vertical temperature structure of the deep part of the lake can be influenced by flows driven by the lake topography. In the shallow part of the lake, there is evidence of thermally driven motion in periods of calm weather in summer.