Multi-frequency HF radar observations of the thermal front in the Great Lakes

Abstract

In large fresh water lakes in temperate regions, the spring transition from weak to strong stratification is characterized by the formation of a coastal thermal front. This transition is dominated by high gradients in temperature, nutrient and plankton fields. A combination of solar warming, boundary heat flux, coastal bathymetry and surface wind stress causes the frontal system to develop such that a surface convergence forms at the nearly vertical 4/spl deg/C isotherm (the temperature of maximum density). This isotherm propagates offshore as warming of the nearshore water increases and as storms provide a mechanism by which the two water bodies (warm stratified nearshore waters and cold isothermal offshore waters) mix. As part of the NSF Episodic Events Great Lakes Experiment (EEGLE), HF Radar observations were obtained during the development and progression of the vernal thermal bar in Southern Lake Michigan in April 1999. Two Multi-Frequency Coastal Radars (MCRs) were utilized to provide observations of near-surface current vectors and vertical current shear adjacent to the Lake Michigan shoreline near St. Joseph, Michigan. MCR measurements of nearsurface currents show evidence of theoretical vernal thermal front circulation supported by in-situ measurements of thermal and dynamic structure. A two-week study of surface dynamics in the vicinity of the thermal front is presented and compared with in-situ measurements.