Seasonal differences in wind-driven across-shelf forcing and response relationships in the shelf surface layer of the central Mid-Atlantic Bight

Observations of surface currents, wind stress, and adjusted sea level from August 2002 to January 2004 were used to study across-shelf forcing and response relationships in the central Mid-Atlantic Bight (MAB). A commonly observed shelf wide offshore flow pattern was associated with distinctly different wind stress magnitudes and directions during mixed and stratified seasons. During the stratified period, the offshore current flow pattern was associated with relatively weak winds out of the Southwest (upwelling favorable), while the mixed period events were associated with relatively strong across-shelf winds from the Northwest. To quantify these observations, time series of the spatial mean surface current, wind stress, and coastal sea level were analyzed using several types of correlation analyses. Seasonal vector correlations between the surface current and wind stress revealed very high correlations but distinctly different phase angles and transfer coefficients. The stratified (mixed) period current veered to the right of the wind by 30-40 degrees (6-8 degrees) and had a higher (lower) transfer coefficient. Scalar correlations between across-shelf wind stress and across-shelf current showed higher r values than with the along-shelf wind stress during the mixed period. While this pattern did not hold between wind stress and sea level, the correlations did show a stronger (weaker) relationship with across-shelf (along-shelf) wind stress than what was observed in the stratified season. However, conditional sampling of shelf wide events during the weaker stratified periods did show stronger relationships between both across-shelf wind/across-shelf current and across-shelf wind/sea level than with the along-shelf wind stress.