Variation in the Hatteras Front density and velocity structure Part 2: Historical setting

On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic Bight and warm salty South Atlantic Bight shelf waters converge alongshelf 90% of the time, causing strong alongshelf gradients in temperature and salinity known as the 'Hatteras Front'. The resulting density gradient supports strong shoreward velocities in the cross-shelf oriented 'nose', of the Front in wintertime. To investigate further, the Frontal Interactions near Cape Hatteras (FINCH) project used shipboard ADCP and a towed undulating CTD to examine Hatteras Front property, density and velocity fields in August 2004, January 2005, and July 2005. Strong property gradients were encountered across the nose of the Hatteras Front in all cases, but the density gradient, dynamic height gradient, and observed along-front cross-shelf velocities evolved in time. FINCH along-Front velocities were strong and shoreward in fall and winter, and weakly mixed shoreward and seaward in July. Several archived data sets were examined, and demonstrate that the density evolution and wind forcing seen in FINCH are characteristic of other years. Evidence suggests the width of the Hatteras Front does not vary dramatically in time, so that consistently large fall and winter density contrast across the Front implies consistently large shoreward velocities along it in winter. Weak density contrasts across the Hatteras Front in spring suggest the magnitude and sign of springtime density gradients and along-Front velocities could vary interannually. Recruitment success of commercially important stocks on the shelf that depend on cross-shelf transport may thus be affected year to year.