AN OCCLUDED COASTAL OCEANIC FRONT

Field observations, including hydrographic, microwave imaging radar, a nd HF radar measurements, reveal the evolution of a complicated fronta l interaction between three water masses on the continental shelf near Cape Hatteras, North Carolina, during a period of incursion of water from the Gulf Stream. The water masses were found to be separated by i ntersecting frontal lines configured in a manner analogous to an occlu ded atmospheric front. The densest water lay between inshore and offsh ore fronts that gradually merged or occluded in the generally downstre am direction, leaving a single surface front. The overall frontal stru cture appeared as a distinct Y-shaped feature in the radar imagery, si milar to historical imagery of the study area. The interpretation of t he observations is aided by the use of a two-dimensional numerical mod el. The model is initialized with two fronts idealized from the ocean measurements. The model fronts quickly sharpen and begin to move toget her, eventually occluding into a single surface front. As a result of the occlusion, the water mass having intermediate density subducts and intrudes under the most buoyant water, carrying with it strong horizo ntal and vertical shears, and a frontal band of diverging currents is created in the densest water mass. The model thus suggests that in the ocean there will be an increase in hydrographic and velocity fine str ucture downstream of the frontal occlusion point.