URI oceanographer studies seasonal changes in coastal ’jet’ south of Block Island

University of Rhode Island Graduate School of Oceanography physical oceanographer David Ullman and University of Connecticut physical oceanographer Dan Codiga have studied the processes giving rise to a coastal current jet that forms in the Atlantic Ocean south of Block Island. Although the commonly accepted scientific view has been that the flow along the southern New England continental shelf is steady on seasonal timescales, recent collection and analysis of long-term current records as part of a National Oceanographic Partnership Program project carried out by URI and UCONN researchers suggests a contrary view.

In their study, published in the current issue of the Journal of Geophysical Research, Ullman and Codiga use two years of current measurements from shore-based radar and in-water current profilers to describe the properties of the jet. The jet flows southwestward along the frontal boundary that separates the low salinity water emanating from Long Island Sound from open ocean water. By averaging the currents over monthly periods to filter out tidal and storm-driven effects, they found a striking seasonal variability, whereby the jet was most intense during summer and extremely weak in winter.

Analysis of water properties and meteorological data in the region showed that the variability of the jet arises from the interplay of freshwater outflow from estuaries and wind stress. Due to the earth’s rotation, outflows along the southern New England continental shelf, which are strongest in spring, produce westward flow. The predominantly eastward winds in this region, on the other hand, tend to drive eastward currents and this wind-driven flow is strongest in winter. The combined effect of these two forcing mechanisms produces strong westward flow when the outflow effect dominates during summer and weak flow when the two processes balance during winter. The winter weakening of the alongshore current jet is hypothesized to be associated with increased offshore transport of nearshore waters. The current mapping radars deployed for this study continue to operate from shoreline sites in Rhode Island and New York, providing a new capability to monitor coastal circulation in real-time over long time periods.

“In addition to the scientific returns from this observational program, the routine monitoring of surface currents is expected to have considerable societal impact, especially in the realm of coastal search and rescue operations,” said Ullman. “The use of real-time currents by U. S. Coast Guard search and rescue planning teams will significantly improve their ability to predict the trajectories of drifting boats or persons at sea.”