Annual and Seasonal Surface Circulation Over the Mid‐Atlantic Bight Continental Shelf Derived From a Decade of High Frequency Radar Observations

Abstract

A decade (2007-2016) of hourly 6-km-resolution maps of the surface currents across the Mid-Atlantic Bight (MAB) generated by a regional-scale High Frequency Radar network are used to reveal new insights into the spatial patterns of the annual and seasonal mean surface flows. Across the 10-year time series, temporal means and interannual and intra-annual variability are used to quantify the variability of spatial surface current patterns. The 10-year annual mean surface flows are weaker and mostly cross-shelf near the coast, increasing in speed and rotating to more alongshore directions near the shelfbreak, and increasing in speed and rotating to flow off-shelf in the southern MAB. The annual mean surface current pattern is relatively stable year to year compared to the hourly variations within a year. The 10-year seasonal means exhibit similar current patterns, with winter and summer more cross-shore while spring and fall transitions are more alongshore. Fall and winter mean speeds are larger and correspond to when mean winds are stronger and cross-shore. Summer mean currents are weakest and correspond to a time when the mean wind opposes the alongshore flow. Again, intra-annual variability is much greater than interannual, with the fall season exhibiting the most interseasonal variability in the surface current patterns. The extreme fall seasons of 2009 and 2011 are related to extremes in the wind and river discharge events caused by different persistent synoptic meteorological conditions, resulting in more or less rapid fall transitions from stratified summer to well-mixed winter conditions. Plain Language Summary A coordinated High Frequency Radar network operated between Cape Cod, MA, and Cape Hatteras, NC, generates hourly maps of ocean surface currents. A decade-long study revealed the detailed structure of the surface flows. These flows were compared to wind and river flow data to explain the patterns observed in the flow. Near the coast, the average currents flow offshore. Away from the coast, the average currents flow along the coast toward the south. Fall is the season with the most variability from year to year. Its higher variability can be traced to different regional weather patterns that change the wind fields and the amount of freshwater delivered by the rivers to the coastal ocean. This is the first study to use a decade of observed surface current maps that uniquely and simultaneously observe the changing patterns of the average flow structure along a segment of eastern United States. The improved understanding of the coastal circulation over a wide area, and what drives its variability, has implications for pollutant transport, plankton transport at the base of the food chain, fish and shellfish reproduction, and multiple ocean-based human activities including fishing, marine transportation, and offshore wind energy development.

New Global Mean Dynamic Topography CNES-CLS-22 Combining Drifters, Hydrological Profiles and High Frequency Radar Data

“…In the same way as for drifter data, the aim is to process these velocities to obtain synthetic velocities with the physical content of geostrophic MDT velocities. We used the cleaned, detited, high-frequency signal-filtered and mean currents over the period 2006-2016, processed by Rutgers University (Roarty et al 2020). Then the mean wind-driven currents over the same period (U Ekman taken from the Copernicus-Globcurrent product MULTIOBS GLO PHY REP 015 004) were removed and finally these mean currents were re-referenced to the 1993-2012 reference period.…”

“…This new CNES-CLS22 MDT also uses velocity data estimated from HF radars located in the Mid Atlantic Bight area, East coast of the US from Cape Hatteras to Cape Cod. We used the cleaned, detited, high-frequency signal-filtered and mean currents over the period 2006-2016 (processed by Rutgers University, Roarty et al 2020).…”

Many Questions Remain Unanswered About the Role of Microbial Transmission in Epizootic Shell Disease in American Lobsters (Homarus americanus)

“…For example, land-sourced microbial communities which are transported in air (Mayol et al, 2017) or water runoff (Adyasari et al, 2020) change the microbial communities found in coastal ocean waters, and this could have a protracted effect in summer when water circulates more slowly. During the summer, the Gulf Stream slows along eastern North America as warming land temperatures alter winds and push them across coastal water currents on an east-west axis, rather than with water currents on a northward path (Roarty et al, 2020). Around Cape Cod, summer winds contribute to a recirculating eddy in Massachusetts Bay which does not mix with deeper water nearly as much as it does in winter (Robinson, 2005).…”

Multi-Timescale Characteristics of Southwestern Australia Nearshore Surface Current and Its Response to ENSO Revealed by High-Frequency Radar

“…In addition, several studies have investigated the small-to medium-scale near-shore processes, such as the cross-shore transport of surface water and the transport of surface biological communities and chemicals [22][23][24], since their transports are of environmental and ecological importance. Meanwhile, the long-term characteristics of the HFR-derived current have also been investigated, such as the characteristics of surface circulation on the west and east coasts of the United States revealed by Kim and Roarty, respectively [25,26]. In Australia, the seasonal and multi-year characteristics of the East Australian Current (EAC) have also been studied by Mao [14] and Archer [11].…”