Wind and Gulf Stream influences on along‐shelf transport and off‐shelf export at Cape Hatteras, North Carolina

Abstract

Along‐shelf transports across three cross‐shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992–1994. The along‐shelf convergence has been used to infer off‐shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along‐shelf transport variability is primarily wind‐driven and highly correlated with sea level fluctuations at the coast. Both winds and along‐shelf transport exhibit a near‐annual period variability. Along‐shelf transport is not well correlated with Gulf Stream offshore position. Along‐shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along‐shelf convergence by hours to a few days. Long‐period variability of 14–16 months and 1–3 months is apparent in both Gulf Stream position and transport convergence. Variability in along‐shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along‐shelf transport convergence is that the Gulf Stream is directly influencing cross‐shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along‐shelf divergence and shoreward cross‐shelf transport exist (∼10% of the time just north of Cape Hatteras and ∼34% of the time just south of Cape Hatteras during episodes of up to 3–8 days duration). Implied onshore flows of a few cm s−1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along‐shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf.