Observations of East Coast upwelling conditions in synthetic aperture radar imagery

Seasonal coastal upwelling in the U.S. Mid-Atlantic coastal ocean normally occurs during the summer months because of generally alongshore southerly wind episodes. Southerly winds force an offshore surface Ekman flow over the inner continental shelf. Colder and nutrient-rich waters from below upwell toward the surface replacing offshore-flowing surface waters. Synthetic Aperture Radar (SAR) observations from the European Remote Sensing (ERS) satellite ERS-2 before and after upwelling-favorable wind episodes in early summer 1996 along the New Jersey coast are presented. Lower backscatter conditions appearing in the SAR imagery after the onset of upwelling demonstrate the influence of the upwelling regime on the sea surface roughness. Satellite sea surface temperature (SST) observations and in-situ sea temperature vertical profiles confirm upwelling conditions. Three key mechanisms are suggested to explain the lower radar returns observed under upwelling conditions, an increase in the atmospheric marine boundary layer stability, an increase in the viscosity of surface waters, and the presence of biogenic surfactants in the upwelling region.