Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model

Abstract. A regional-scale ocean model was used to hindcast the coastal circulation over the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) from 2004 to 2013. The model was nested inside a data assimilative global ocean model that provided initial and open boundary conditions. Realistic atmospheric forcing, tides and observed river runoff were also used to drive the model. Hindcast solutions were compared against observations, which included coastal sea levels, satellite altimetry sea surface height, in situ temperature and salinity measurements in the GOM, and observed mean depth-averaged velocities. Good agreements with observations suggest that the hindcast model is capable of capturing the major circulation variability in the MAB and GOM. Time- and space-continuous hindcast fields were used to depict the mean circulation, along- and cross-shelf transport and the associated momentum balances. The hindcast confirms the presence of the equatorward mean shelf circulation, which varies from 2.33 Sv over the Scotian Shelf to 0.22 Sv near Cape Hatteras. Using the 200 m isobath as the shelf/slope boundary, the mean cross-shelf transport calculations indicate that the shelfbreak segments off the Gulf of Maine (including the southern flank of Georges Bank and the Northeast Channel) and Cape Hatteras are the major sites for shelf water export. The momentum analysis reveals that the along-shelf sea level difference from Nova Scotia to Cape Hatteras is about 0.36 m. The nonlinear advection, stress, and horizontal viscosity terms all contribute to the ageostrophic circulation in the along-isobath direction, whereas the nonlinear advection plays a dominant role in determining the ageostrophic current in the cross-isobath direction.