Mesoscale variability on the New Jersey Shelf: Effects of topography, seasons, winds, and offshore forcing on circulation, hydrography, and transport.

摘要

Continental shelf transport and shelf-slope exchange processes are driven by a combination of meteorological, oceanographic and topographic forcing mechanisms. The varied response of the water column to particular forcing combinations can lead to either cross-shelf dominated or along-shelf dominated shelf transport on different time scales. Using data from a coastal ocean observatory, this study investigates how different physical forcing mechanisms such as changing seasons, wind stress, storms, slopewater eddies/rings, river plumes and large scale alongshelf forcing affect the hydrography and circulation at the mid- to outer New Jersey Shelf. The NJ Shelf undergoes large changes in stratification from well mixed during the winter to highly stratified during the summer. The stratification controls the response of the water column to wind forcing. The wind-driven surface flow oscillates between being alongshelf dominated during spring and autumn, and cross-shelf dominated during winter and summer. Cross-shelf transport takes place on the time scale of 1 to 5 weeks.;When multiple watermasses converge at the shelf-slope front (SSF), complex hydrographic patterns and flow behaviors can emerge, especially during the stratified summer season. The SSF has a characteristic along-shelf scale of 10-30 km and a characteristic cross-shelf scale of 5-20 km. The different types of slopewater salinity intrusions at the outershelf drive the variability of the SSF and stratification on tidal to intra-seasonal time scales. Four types of salinity intrusions outlining the SSF were identified based their hydrographic properties. Along-shelf wind stress affects the location of the foot of the front, river discharge and offshore eddies affect the strength of surface and pycnocline intrusions. Eddies can also drive frontal movement and influence secondary circulation below the pycnocline. Upwelling and downwelling associated with the SSF connect the bottom boundary layer with the pycnocline, bringing heat and salt into the water column interior. Tropical and extra-tropical storms at the beginning of the autumn entrain the heat and salt advected onto the shelf through vigorous transport and mixing, marking the beginning of transition from a summer stratified to a winter well-mixed regime.