The cross-shelf structure of wind- and buoyancy-driven circulation over the North Carolina inner shelf

Abstract

Circulation and transport over the North Carolina shelf were studied as part of an interdisciplinary CoOP study in 1994. Shipboard data and moored instruments reveal two important influences on the density distribution. Winds in the area reverse every few days, driving upwelling and downwelling circulation alternately, and a low-salinity coastal buoyancy current from Chesapeake Bay is present only during downwelling. Empirical orthogonal functions reveal that variability in temperature and cross-shore circulation is dominated by upwelling and downwelling, while patterns in salinity and along-shore circulation are primarily due to the buoyancy current. The observed density distribution goes through a cycle, showing each influence in turn. After a period of equatorward winds, the thermocline bends downward at the nearshore end, and a coastal wedge of low-salinity water is observed. When the winds turn poleward, the thermocline bends upward and the low-salinity water moves offshore. This cycle repeats four times during our study period with modest variations.