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ArticleA shelf water cascading event near Cape Hatteras(American Meteorological Society, 2021-06-01) Han, Lu ; Seim, Harvey E. ; Bane, John M. ; Todd, Robert E. ; Muglia, MichaelCarbon-rich Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf waters typically converge on the continental shelf near Cape Hatteras. Both are often exported to the adjacent open ocean in this region. During a survey of the region in mid-January 2018, there was no sign of shelf water export at the surface. Instead, a subsurface layer of shelf water with high chlorophyll and dissolved oxygen was observed at the edge of the Gulf Stream east of Cape Hatteras. Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore surface waters. Driven by the density gradient, the denser shelf waters cascaded beneath the Gulf Stream and were subsequently entrained into the Gulf Stream, as they were advected northeastward. Underwater glider observations 80 km downstream of the export location captured 0.44 Sv (1 Sv ≡ 106 m3 s−1) of shelf waters transported along the edge of the Gulf Stream in January 2018. In total, as much as 7 × 106 kg of carbon was exported from the continental shelf to a greater depth in the open ocean during this 5-day-long cascading event. Earlier observations of near-bottom temperature and salinity at a depth of 230 m captured several multiday episodes of shelf water at a location that was otherwise dominated by Gulf Stream water, indicating that the January 2018 cascading event was not unique. Cascading is an important, yet little-studied pathway of carbon export and sequestration at Cape Hatteras.
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ArticleOverview of the Processes driving Exchange At Cape Hatteras Program(Oceanography Society, 2022-05-12) Seim, Harvey E. ; Savidge, Dana ; Andres, Magdalena ; Bane, John M. ; Edwards, Catherine ; Gawarkiewicz, Glen G. ; He, Ruoying ; Todd, Robert E. ; Muglia, Michael ; Zambon, Joseph B. ; Han, Lu ; Mao, ShunThe 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.
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ArticleCharacteristics of an advective Marine Heatwave in the Middle Atlantic Bight in early 2017(Frontiers Media, 2019-11-22) Gawarkiewicz, Glen G. ; Chen, Ke ; Forsyth, Jacob S. T. ; Bahr, Frank B. ; Mercer, Anna Malek ; Ellertson, Aubrey ; Fratantoni, Paula S. ; Seim, Harvey E. ; Haines, Sara ; Han, LuThere has been wide interest in Marine Heatwaves and their ecological consequences in recent years. Most analyses have focused on remotely sensed sea surface temperature data due to the temporal and spatial coverage it provides in order to establish the presence and duration of Heatwaves. Using hydrographic data from a variety of sources, we show that an advective Marine Heatwave was initiated by an event in late December of 2016 south of New England, with temperature anomalies measuring up to 6°C and salinity anomalies exceeding 1 PSU. Similar features were observed off of New Jersey in February 2017, and are associated with the Shelfbreak Front migrating from its normal position to mid-shelf or further onshore. Shelf water of 34 PSU was observed just north of Cape Hatteras at the 30 m isobath and across the continental shelf in late April 2017. These observations reveal that the 2017 Marine Heatwave was associated with a strong positive salinity anomaly, that its total duration was approximately 4 months, and its advective path extended roughly 850 km along the length of the continental shelf in the Middle Atlantic Bight. The southward advective velocity implied by the arrival north of Cape Hatteras is consistent with previous estimates of alongshelf velocity for the region. The origin of this Marine Heatwave is likely related to cross-shelf advection driven by the presence of a Warm Core Ring adjacent to the shelfbreak south of New England.
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ArticleOcean circulation near Cape Hatteras: observations of mean and variability(American Geophysical Union, 2022-11-19) Han, Lu ; Seim, Harvey ; Bane, John ; Savidge, Dana ; Andres, Magdalena ; Gawarkiewicz, Glen ; Muglia, MikeThe 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.