Mean circulation of the Mid-Atlantic Bight from a climatological data assimilative model

The along-shelf momentum balance of the Mid-Atlantic Bight (MAB) coastal ocean includes a significant contribution from the along-shelf gradient in sea level. This sea level tilt, of order 10⁻⁷, and other features of the mean sea level are not captured well in global mean dynamic topography (MDT) derived from hydrographic observations or satellite altimetry and gravity data, and is poorly represented in global and basin scale dynamical models. This is problematic for applications that would use coastal satellite altimeter data to estimate total water level above datum. We have produced a MDT for the MAB using the Regional Ocean Modeling System (ROMS) with 4-dimensional variational (4D-Var) data assimilation configured to obtain climatological annual and monthly mean results. The observations assimilated were a regional hydrographic climatology of temperature and salinity, and long-term mean velocity from HF-radar, shipboard ADCP, and current-meters. Assimilation adjusts the 3-dimensional ocean state, boundary conditions, and air-sea fluxes to minimize the model-data misfit. The assimilation of mean velocity data is vital to obtaining a realistic circulation result. The MDT exhibits a strong across-shelf sea level slope in geostrophic balance with the southwestward mean flow. The subtle along-shelf tilt is recovered and is relatively uniform throughout the MAB inside the 50 m isobath, but on the southern outer shelf it reverses sign and drives significant across-isobath flow, partially draining the southward mean transport. In the north, across-shelf flow is offshore in the surface and bottom Ekman layers, but largely balanced locally by inflow in the interior depth range.