Wintertime Shoreward Near-Surface Currents South of Cape Hatteras

[1] Cross-isobath flow on continental shelves is of interest for a variety of reasons. Near Cape Hatteras, North Carolina, the transport of larval organisms, pollutants, and oceanic carbon budget constituents to and from the adjacent Albemarle and Pamlico Sounds may depend critically on cross-isobath currents. Shoreward currents in the near-surface waters south of Cape Hatteras are documented herein, on the basis of continuous 2-year time series, encompassing all or part of three consecutive winters. Energetic shoreward currents exist similar to30% of the time from midfall through late spring. These currents are evident over the 20 and 35 m isobaths along a mooring line situated similar to40 km southwestward from Cape Hatteras. Shoreward velocities average similar to12 cm/s, and events persist from 0.5 to 4 days, occurring every 2.5-5 days, except in summer. These events often coincide with southwestward winds but occur under both upwelling and downwelling favorable conditions, such that Ekman veering in the surface layer does not account for the shoreward velocities. In winter the mooring line south of Cape Hatteras is frequently traversed by a strong temperature and salinity front, with light, relatively fresh, cold, stratified water on one side, and denser, more saline, warmer, unstratified water on the other. Hydrography and satellite sea surface temperature imagery help identify this front as the boundary between South Atlantic Bight and Mid-Atlantic Bight coastal shelf waters, the Hatteras Front.\u27\u27 Flow along the Hatteras Front where it crosses the shelf appears to account for the observed shoreward currents. The along-shelf advection of the Hatteras Front may depend on both winds and Gulf Stream distance offshore

Wind and Gulf Stream Influences on Along-Shelf Transport and Off-Shelf Export at Cape Hatteras, North Carolina

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 (similar to 10% of the time just north of Cape Hatteras and similar to 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

Baroclinic Effects and Tides on the Cape Hatteras Continental Shelf

Seasonal variability has been identified on the shelf near Cape Hatteras in the semidiurnal and diurnal frequency bands. Large summertime semidiurnal currents appear to be an M2 internal tide whose propagation shoreward is supported by strong Middle Atlantic Bight (MAB) seasonal stratification. At the southern limit of the MAB, strong MAB stratification gives way to weaker seasonal stratification in the South Atlantic Bight (SAB), and the M2 internal tide propagates shoreward less effectively. Strong diurnal variability appears in K1 and O1 components in summer, achieving magnitudes as large as the M2. The diurnal components are typically much smaller than M2 in winter. However, this summer signal is unlikely to be a diurnal internal tide since at these latitudes (34.5-36.5N) the diurnal frequency is subinertial. Coastally trapped waves (CTWs) are presented as a mechanism to explain the increased variability in the diurnal band under summertime stratification. Alongshore southward propagation of the diurnal variability is evident from moorings on the middle and outer shelf ( phase speed of 2.1-2.6 m/s along the 60-m isobath) as far south as Cape Hatteras, but little energy in this band propagates past Cape Hatteras. While diurnal band CTW propagation will not occur at this latitude under well-mixed conditions, stratification could cause those frequencies to become available for a given wave number, as described in the work of Brink ( 1990). Estimates of the Huthnance ( 1978) stratification and slope parameter A, and the Burger number S, suggest the summertime diurnal signal is consistent with baroclinic CTWs. If so, these are the first observations of diurnal CTWs at Cape Hatteras. Copyright 2007 by the American Geophysical Union

Forcing and Dynamics of Seafloor-Water Column Exchange on a Broad Continental Shelf

Relict sediments of elevated permeability characterize the majority of continental shelves globally (Emery, 1968). In these settings, interactions between benthic boundary layer (BBL) flows and seabed topography generate pressure fluctuations that drive advective and dispersive porewater transport, dramatically increasing the magnitude and variability of porewater solute and particulate exchange across the sediment-water interface (Huettel et al., 1996; Huettel and Rusch, 2000). On broad shallow shelves with a relatively large area-to-volume ratio, the seafloor’s role is magnified. Energetic events may reorganize bedforms across a significant fraction of the shelf, leading to altered exchange dynamics that may persist long after the organizing event. Ecosystem-based management of both resources and environmental status requires improved fundamental understanding of dynamic benthic exchange processes. Scattered, short-time-scale observations are unlikely to capture the full spectrum of events that affect sediment-water exchanges; a persistent observational presence on the seafloor is required

Shelf Edge Tide Correlated Eddies Along the Southeastern United States

High frequency radar observations in the Southeastern United States have revealed sequences of small short‐lived cyclonic eddies along the shoreward edge of the Gulf Stream, that spin up as the local tide turns alongshelf antiparallel to the Stream. Eddies propagate equatorward along the shelf edge, sometimes progressing shoreward before dissipating one to three hours later. They are distinctly different from Gulf Stream meander eddies, which propagate poleward. In this article, radar and mooring data are used to establish three important aspects of these neweddies: they represent an instability process operating at a previously unidentified frequency, scale, and cross‐Stream position; they contribute to shoreward momentum fluxes,defining a link between Gulf Stream and outer shelf subtidal variability and illustrating a mechanism to justify locally large horizontal eddy viscosity estimates; and they transport properties across the shelf edge, importing nutrients onto the shelf and transferring heat between the Gulf Stream and the coastal ocean

Overview of the Processes driving Exchange At Cape Hatteras Program

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Seim, H., Savidge, D., Andres, M., Bane, J., Edwards, C., Gawarkiewicz, G., He, R., Todd, R., Muglia, M., Zambon, J., Han, L., & Mao, S. Overview of the Processes driving Exchange at Cape Hatteras Program. Oceanography, (2022), https://doi.org/10.5670/oceanog.2022.205.The Processes driving Exchange At Cape Hatteras (PEACH) program seeks to better understand seawater exchanges between the continental shelf and the open ocean near Cape Hatteras, North Carolina. This location is where the Gulf Stream transitions from a boundary-trapped current to a free jet, and where robust along-shelf convergence brings cool, relatively fresh Middle Atlantic Bight and warm, salty South Atlantic Bight shelf waters together, forming an important and dynamic biogeographic boundary. The magnitude of this convergence implies large export of shelf water to the open ocean here. Background on the oceanography of the region provides motivation for the study and gives context for the measurements that were made. Science questions focus on the roles that wind forcing, Gulf Stream forcing, and lateral density gradients play in driving exchange. PEACH observational efforts include a variety of fixed and mobile observing platforms, and PEACH modeling included two different resolutions and data assimilation schemes. Findings to date on mean circulation, the nature of export from the southern Middle Atlantic Bight shelf, Gulf Stream variability, and position variability of the Hatteras Front are summarized, together with a look ahead to forthcoming analyses.We gratefully acknowledge NSF funding (OCE-1558920 to UNC-CH, OCE-1559476 to SkIO, OCE-1558521 to WHOI, OCE-1559178 to NCSU); technical support from Sara Haines, Craig Marquette, Trip Patterson, Nick DeSimone, Erran Sousa, Gabe Matthias, Patrick Deane, Brian Hogue, Frank Bahr, and Ben Hefner; cruise participants Jacob Forsyth, Joleen Heiderich, Chuxuan Li, Marco Valero, Lauren Ball, John McCord, and Kyle Maddux-Lawrence; and the crew of R/V Armstrong for their able support during three PEACH cruises

VHF radar measurements of flow in a salt marsh creek

A VHF ocean radar system was deployed for one month in a salt marsh in coastal Georgia, USA. In this environment, radar backscatter from the marsh grass is highly attenuated and scatter from water in the narrow creeks has a broad band of Doppler shift due to the horizontal variation in water speed across the channel. By assuming a monotonic velocity profile from the edge to the middle of the channel, a theoretical relationship is derived to link the broadening of Bragg energy in the echo spectrum to varying water speed. A methodology is developed to derive the cross-creek profile of velocity variation in the main channel of the salt marsh

High Frequency Radar Observing Systems in SEACOOS: 2002-2007 Lessons Learned

From 2002-2007, the Southeast Coastal Ocean Observing System (SEACOOS) deployed high frequency (HF) radars to overlook several venues stretching from the West Florida Shelf to the North Carolina Shelf. Based on extensive deliberations within SEACOOS, we decided to assess the two differing types of coastal ocean current radars within the southeast that were on the commercial market. The long-range SeaSondes (SS) were deployed to sense surface currents at hourly intervals and a 6 km resolution along the West Florida Shelf and the North Carolina Shelf. The medium and long-range Wellen Radars (WERA) were deployed along the Florida Straits and along the South Atlantic Bight with spatial resolutions of 1.2 to 3 km sampling at time scales of minutes. A common theme in these deployments was to sense the Loop Current, Florida Current and the Gulf Stream, which transport heat poleward as part of the gyre circulation.Several lessons were learned as part of these deployments, such as the need to protect against lightening strikes and the challenge of providing robust communication links between the remote sites and a central hub to make the data available in near real-time. Since states in the southeast and surrounding the Gulf of Mexico are prone to the passage of hurricanes, surface current and wave measurements during hurricanes are invaluable for improving storm surge and inundation models that are now being coupled to surface waves. In addition, significant wave heights (and directional surface wave spectra) are critical in the model assessment. Data quality and accuracy of the surface current and wave fields remain a central issue to search and rescue and safe maritime operations and to understanding the limitations of these radar systems. As more phased array systems (i.e., WERAs) are deployed for surface current and wave measurements, more attention needs to be placed on the interoperability between the two types of systems to insure the highest quality data possible is available to meet applied and operational goals. To insure the highest quality data possible, a full-time technician and a half-time IT specialist are needed for each installation as well as access to spares to keep these systems running consistently and to make quality observations available in near real-time

Airborne Remote Sensing of the Upper Ocean Turbulence during CASPER-East

The article of record as published may be found at http://dx.doi.org/10.3390/rs10081224This study takes on the challenge of resolving upper ocean surface currents with a suite of airborne remote sensing methodologies, simultaneously imaging the ocean surface in visible, infrared, and microwave bands. A series of flights were conducted over an air-sea interaction supersite established 63 km offshore by a large multi-platform CASPER-East experiment. The supersite was equipped with a range of in situ instruments resolving air-sea interface and underwater properties, of which a bottom-mounted acoustic Doppler current profiler was used extensively in this paper for the purposes of airborne current retrieval validation and interpretation. A series of water-tracing dye releases took place in coordination with aircraft overpasses, enabling dye plume velocimetry over 100 m to 10 km spatial scales. Similar scales were resolved by a Multichannel Synthetic Aperture Radar, which resolved a swath of instantaneous surface velocities (wave and current) with 10 m resolution and 5 cm/s accuracy. Details of the skin temperature variability imprinted by the upper ocean turbulence were revealed in 1–14,000 m range of spatial scales by a mid-wave infrared camera. Combined, these methodologies provide a unique insight into the complex spatial structure of the upper ocean turbulence on a previously under-resolved range of spatial scales from meters to kilometers. However, much attention in this paper is dedicated to quantifying and understanding uncertainties and ambiguities associated with these remote sensing methodologies, especially regarding the smallest resolvable turbulent scales and to reference depths of retrieved currents.NRLONRNSFNRL program element 61153N WUs BE023-01-41-1C04NRL program element 61153N WUs BE023-01-41-1C02NRL program element 61153N WUs BE023-01-41-6692ONR grant N0001418WX01087NSF grant OCE-154064