Observations of Shelf‐Ocean Exchange in the Northern South Atlantic Bight Driven by the Gulf Stream

Abstract

Between Florida and Cape Hatteras, North Carolina, the Gulf Stream carries warm, salty waters poleward along the continental slope. This strong current abuts the edge of the South Atlantic Bight (SAB) continental shelf and is thought to influence exchange of waters between the open ocean and the shelf. Observations from a pair of instruments deployed for 19 months in the northern SAB are used here to examine the processes by which the Gulf Stream can impact this exchange. The instrument deployed on the SAB shelf edge shows that the time‐averaged along‐slope flow is surface‐intensified with only few flow reversals at low frequencies (>40‐day period). Time‐averaged cross‐slope flow is onto the SAB shelf in a lower layer and off‐shelf above. Consistent with Ekman dynamics, the magnitude of lower‐layer on‐shelf flow is correlated with the along‐slope velocity, which is in turn controlled by the position and/or transport of the Gulf Stream that flows poleward along the SAB continental slope. In the frequency band associated with downstream‐propagating wave‐like meanders of the Gulf Stream jet (2‐15 day period), currents at the shelf‐edge are characterized by surface‐intensified flow in the along‐ and cross‐slope directions. Estimates of maximum upwelling velocities associated with cyclonic frontal eddies between meander crests occasionally reach 100 m/day.

Nitrogen uptake rates and phytoplankton composition across contrasting North Atlantic Ocean coastal regimes north and south of Cape Hatteras

“…The positive P * values across the northern MAB suggest an excess PO 4 relative to N and the Redfield Ratio, a trend contrasting with the mostly near-zero or negative P * values observed in the SAB ( Figure 5C ). This was likely due to: (1) the MAB being influenced by the Cold Pool waters intruding from the sub-Arctic whose source waters have already undertaken denitrification, resulting in an excess PO 4 relative to NO 3 − ( Harrison and Li, 2007 ; Fennel, 2010 ; Sherwood et al, 2021 ), and (2) the SAB shelf region has more direct interactions with the oligotrophic Gulf Stream and has limited riverine inputs ( Andres et al, 2023 ). Specific N uptake rates were significantly higher in the MAB than the SAB ( p < 0.05), showing a positive correlation with ambient P concentrations ( r = 0.31, p < 0.05, Figure 12 ).…”

“…The southern coastal MAB is highly influenced by the outflow from the Chesapeake Bay, the southward flow from the northern MAB, and the Slope Sea ( Flagg et al, 2002 ; Todd, 2020 ); the latter two form a shelf-break frontal zone where mixing is often enhanced and exports of shelf water can occur ( Gawarkiewicz et al, 1996 ; Todd, 2020 ). In contrast, south of Cape Hatteras, the SAB continental shelf circulation is impacted by shelf water moving north toward Cape Hatteras, and the northward flowing Gulf Stream ( Andres et al, 2023 ). The strong convergence of the alongshore flows at the Hatteras Front can induce cross-shelf exchanges just north of Cape Hatteras due to density instabilities, transporting significant portions of the shelf water to the open ocean ( Savidge and Savidge, 2014 ; Todd, 2020 ), although there is high temporal and spatial variability in the location of the front due to complex hydrodynamics ( Seim et al, 2022 ).…”

Evidence for Kilometer‐Scale Biophysical Features at the Gulf Stream Front

“…The physical interplay and amount of upwelling driven by divergence at the meander crest versus cyclonic frontal eddies is at a similar scale and thus challenging to differentiate, but should be further investigated. It is possible this balance may shift from the Charleston gyre to further downstream from Cape Hatteras (Andres et al., 2023).…”

Evidence for kilometer-scale biophysical features at the Gulf Stream front

“…While frontal eddy formation may include substantial upwelling as indicated by a cold dome and uplifted isotherms, from a Eulerian perspective off Cape Hatteras, the majority of positive vertical velocities in the top 150 m appear to occur at the crest of the meander (Muglia et al, 2022), which may be cause to reconsider assumptions and applicability of previous work further upstream in the SAB to this region and downstream (Glenn & Ebbesmeyer, 1994b; Lee et al, 1991; Yoder et al, 1981). The physical interplay and amount of upwelling driven by divergence at the crest and cyclonic eddies should be further investigated and this balance may shift from the Charleston gyre to further downstream from Cape Hatteras (Andres et al, 2023).…”