Surface temperature fronts in the Great Lakes

The distribution and variability of surface temperature fronts in the Great Lakes is studied using an II year time series (1985 to 1995) of Advanced V ery High Resolution Radiometer (AVHRR) images. After the removal of cloud c ontaminated pixels as well as those potentially ice covered, an edge detect ion algorithm identifies surface temperature fronts in each image. The prob ability of detecting a front in the Great Lakes is highly variable from lak e to lake as well as seasonally. Fronts during the winter months generally have cold water on their shallow side and appear in progressively deeper wa ter as winter progresses. The properties of the winter fronts are consisten t with a formation mechanism involving strong surface cooling over a slopin g bottom. A spring transition occurs, whereby these fronts disappear and ar e replaced by nearshore thermal bar fronts with warm water found on their s hallow side. The time of this transition depends upon the lake, occurring f irst in Lake Erie, next in Lakes Ontario, Huron, and Michigan, and finally in Lake Superior. These fronts are also observed to progress slowly into de eper water as nearshore waters warm. The classical thermal bar front (4 deg rees C) is seen in all of the lakes but most of the fronts during the sprin g peak in frontal activity have temperatures greater than 4 degrees C, sugg esting that the thermal bar marks the offshore edge of a more extensive fro ntal zone. Frontal activity declines during summer in all lakes except Supe rior.