Continental Shelf Surface Thermal Fronts in Winter off the Northeast U.S. Coast

David S. Ullman and Peter C. Cornillon
University of Rhode Island
Narragansett

Abstract:

Analysis of 12 years (1985-1996) of sea surface temperature (SST) imagery covering the shelf and slope off the northeast U.S. coast has revealed the presence of persistent fronts in winter over the middle shelf. Ongoing work shows that similar fronts occur in other coastal regions, suggesting that these fronts are of more than regional interest. The satellite data from the U.S. east coast make clear that these fronts, which are found with highest frequency in the vicinity of the 50 meter isobath, separate cool water inshore from warmer outer shelf water. The temperature step across the fronts, a measure of the frontal strength, is negatively correlated with estimates of heat flux (latent plus sensible) indicating that winter surface cooling plays an important role in their formation. Although generally the fronts are oriented parallel to the bottom topography, the region around Nantucket Shoals is a location where fronts oriented in the cross-isobath direction occur more often than elsewhere, suggesting that this area is one of enhanced cross-isobath flow. The cross-isobath flow manifests itself in the form of cold tongues extending south and west from the shallowest part of the shoals. Historical hydrographic data from the shelf in winter indicate that the fronts typically have a salinity signal, with water on the inshore sides of the fronts having lower salinity and resulting lower density due to the controlling influence of salinity on density. Weak vertical stratification is often present inshore of the fronts suggesting that the fronts may represent the offshore edge of a freshened coastal zone.