Determine the location of a thermal front in the Iroise Sea by using HF radar data and tide model results

Abstract

The Iroise Sea is a shallow sea, located on the North West European Shelf close to Western Brittany (France) and North of the Bay of Biscay. Due to strong gradients in the bathymetry close to the coast and strong local winds, tidal currents can reach up to 3.5 m/s during spring tide and induce a strong vertical mixing along the coast. During summer, solar radiation and thus warming of the sea surface, together with mixing along he coast, and thus colder temperatures in this area, leads to a formation of a thermal front, where the differences between the sea surface temperatures on both sides can reach up to 3degC. During the SURLITOP experiment, that took place from August to November 2005, two HF radar stations have been installed at the French coast. Both stations were working simultaneously at a working frequency of 12.4 MHz and a measurement cycle of 12 min. The surface current fields processed from the radar data have been interpolated to a regular 2times2 km grid. For the same period, tide model simulations (MARS 2D) have been processed with the same temporal and spatial resolutions. Because the model has been run under barotropic conditions, the location of the front, which is induced by density gradients, can not be reproduced. In contrast to the model, the radar measures the total circulation including the baroclinic signal, so that the difference between radar data and model results can be exploited to find the location of the thermal front. To validate this result, satellite remote sensed sea surface temperature data have been used. The result is very promising, as the region of highest differences between radar data and model results coincide with the location of the thermal front. The location of the front itself can be determined by the narrowness of the isotherms. Also the structures of the front isotherms and the strongest gradients of the currents are very similar. Thus it is possible, to determine the location and dynamics of a thermal front by using HF radar data in combination with model results, if the model is working under barotropic conditions.