Variation in the Hatteras Front density and velocity structure Part 1: High resolution transects from three seasons in 2004-2005

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, salinity, and density known as the 'Hatteras Front'. Mechanisms of shoreward transport in this region have long been a topic of interest, since many commercially important species spawn on the outer shelf, but utilize the adjacent Albemarle and Pamlico Sounds for nurseries, requiring some physical transport mechanism to move the eggs and larvae from the outer shelf to these nursery areas. One mechanism providing such shoreward transport is strong shoreward velocity along the cross-shelf oriented 'nose' of the Hatteras Front. 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 evolved in time, and along with it the dynamic height gradient driving the observed along-front cross-shelf velocities in the nose of the Front. In August and January FINCH data, MAB shelf waters on the north side of the Hatteras Front are less dense than SAB shelf waters, driving shoreward velocities along the Hatteras Front. By July, MAB shelf waters are slightly more dense than SAB shelf waters, with areas of weak seaward and shoreward velocities within the Hatteras Front. As Part 1 of a pair of contributions, this article focuses on FINCH data to illustrate the range of density gradients encountered and resulting cross-shelf velocities. Whether these observations are typical of variability in the Hatteras Front is explored in a second article, Part 2.