Ocean Circulation Near Cape Hatteras: Observations of Mean and Variability

The convergence of different water masses on the shelf and along the shelfbreak, and cross-isobath shelf-open ocean exchanges contribute to the complex circulation near Cape Hatteras. We examine the mean and variability of these circulations using data from nine bottom-mounted acoustic Doppler current profilers, deployed over the mid- to outer-continental shelf north and south of Cape Hatteras as part of the Processes driving Exchange At Cape Hatteras program. The 18-month-mean depth-averaged shelf flows are mostly aligned with isobaths and oriented toward Cape Hatteras. At two sites just north of Cape Hatteras, mean flows have a strong cross-shelf component. Two dominant spatial patterns in the velocity field are obtained from an empirical orthogonal function analysis. The two leading modes contain 61% of the total variance. The spatial variation of Mode 1 exhibits an along-shelf flow pattern, while that of Mode 2 shows a convergent flow pattern. The principal component (PC) series of Mode 1 is significantly correlated with the local wind stress, confirming that the along-shelf flow is wind-driven as expected. The PC of Mode 2 is highly correlated with the Gulf Stream lateral position as inferred from the current- and pressure-sensor-equipped inverted echo sounders over the slope south of Cape Hatteras, which indicates that Gulf Stream movement drives time-varying shelf flow convergence. Conditionally averaged sea-surface temperature and high-frequency radar-measured surface currents based on PC1 and PC2 confirm these relationships and further illustrate how the wind and Gulf Stream forcing work together to influence the flow regime in this region. Plain Language Summary Several oceanographic processes occur within a small geographic area east of Cape Hatteras. Cool and warm ocean currents meet and collide on the continental shelf, resulting in complex exchange between the shelf and the adjacent open ocean. To better characterize the complicated circulation and understand the processes driving exchanges in this region, we analyze 18-month current velocity data from an observational array deployed near Cape Hatteras on the shelf and the neighboring continental slope. The time-mean circulation is characterized as converging flows both on the shelf and along the shelfbreak. We find two dominant spatial patterns in the current velocity: a wind-driven along-shelf flow pattern and a Gulf Stream-driven convergent flow pattern. Additionally, the combined effect of the wind and Gulf Stream forcings mainly drives the shelf and open ocean exchanges.