Dynamical response to differential rates of temperature change over sloping topography: Lough Corrib, Ireland

Abstract

An enclosed water body, subjected to a uniform surface heat flux, exhibits differential rates of temperature change between shallow peripheral waters and adjacent deeper waters. The result is the formation of horizontal temperature gradients in regions of sloping topography, referred to as topographic temperature gradients (TTGs). An integrated observational and numerical study has been undertaken, considering the impact of TTGs on the basin-scale dynamics of Lough Corrib, Ireland. Temperature time series recorded during the winter of 2001 revealed sustained surface temperature gradients parallel to sloping boundaries. Cold, dense water formed in peripheral regions of the lake cascaded down slope under the influence of gravity, forming density plumes that extended over horizontal scales of kilometers. Plumes were typically 8-12 in thick and traveled down slope at speeds in the range 4-10 cm s-1. Down-slope cascading of cool water was typically initiated when the daily mean surface heat flux fell below -55 W m-2, meaning cascading occured almost persistently throughout the period of seasonal cooling (from early September until early January). While cascading plumes exhibited a diurnal periodicity, individual cascading events were linked to the passing of weather systems, persisting for between 5 hours and 3.6 days. The cascading of dense water formed an important mechanism for the transport of peripheral waters to depth, greatly increasing rates of lake overturning and preventing stagnation during cool, calm conditions. The offshore transport owing to cascading was balanced by a weaker and more diffuse onshore flow within the upper 5 m of the water column. Quasi-geostrophic alongshore currents, evident in the modeled velocity field, formed a basin-scale anticyclonic gyre during calm conditions. Copyright 2007 by the American Geophysical Union.