Direct injections of coastal water across the continental shelf of the southeastern United States: Atmospheric forcing?

Persistent cross-shelf oriented jets of coastal water are commonly observed during late autumn and winter in SeaWiFS and MODIS satellite imagery of the southeastern U.S. continental shelf in the South Atlantic Bight (SAB) region. These plumes of coastal water can persist for extended periods, through several cycles of meteorological forcing by transient extratropical cyclones down to mesoscale processes. The plumes appear to bypass the conventional diffusive time scales and provide direct mass flux to the open ocean, with concomitant nutrient as well as pollutant impacts from entrained river runoff. The waters of the coastal SAB is typically turbid and have a strong optical signal due to colored dissolved organic matter (CDOM) discharged from rivers draining the piedmont, coastal plains, and swamps of North and South Carolina, Georgia and NE Florida.

Mesoscale convective systems (MCS) have intense interaction with the coastal waters. These are strong, but limited scale, episodic events that are poorly understood because of their limited extent in both spatial and temporal scales and the fact that measurements over the ocean at mesoscales during such extreme events are rare. However, maximum wind speeds observed so far at the SABSOON towers have been associated not with hurricanes or winter extratropical cyclones, but instead with the passages of squall lines and convective thunderstorms. These extreme wind events were associated with rapid decreases in air temperature, downpours, high-frequency barometric pressure fluctuations (~1 cph), and 6-minute-averaged wind speeds of 40-60 m/s. Such observations suggest that these events are intense downbursts, either from single convective cells or more organized MCS outflow boundaries, with their larger spatial scales and durations.

Several mechanisms can partially account for these plumes, from air-sea wind stress at convective to mesocales, extratropical cyclone passage and associated wind stress changes to reactions to Gulf Stream flow forcing interacting with the SAB hydrography. The unique instrument assets of the SABSOON tower array provide some clues, and a coordinated study of this phenomenon is planned for 2006-2007.