2007-2016 Surface Circulation over the Mid Atlantic Bight Continental Shelf derived from a Decade of High Frequency Radar Observations

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 long term annual and seasonal mean surface flows.  Across the 10 year time series, temporal means, inter- and intra-annual variability are used to quantify the variability of spatial surface current patterns from season to season and year to year.  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 shelf break, 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 ten-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 current speeds are larger and correspond to a time when the 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 interannual 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.