Seasonal characteristics of bottom boundary layer detachment at the shelfbreak front in the Middle Atlantic Bight

Abstract

[1] The seasonality of various characteristics of the detached bottom boundary layer of the Middle Atlantic Bight shelfbreak front is examined using a collection of highresolution transects across the front. The analysis follows previous methodology in which accumulated temperature change along isopycnals within the front is used to infer the location of the detached layer. The seasonal mean isopycnal at which detachment occurs (approximately 26.0 kg m À3 ) is fairly constant throughout the year. However, the vertical scale of the detached layer varies significantly with season, extending 60À80 m above the bottom in winter and spring, but only 20À40 m above the bottom in summer. The vertical scale is controlled by the strength and depth of the seasonal pycnocline. The observations suggest that the detached layer is capable of extending into the euphotic zone during winter and spring.

Seasonal and interannual variability of physical and biological dynamics at the shelfbreak front of the Middle Atlantic Bight: nutrient supply mechanisms

“…On one hand, stronger local wind forcing in winter and spring deepens the mixed layer, making more deepocean nutrient available to the upper water column. On the other hand, the shelfbreak jet reaches its highest intensity in spring (Linder et al, 2004;Chen and He, 2010), maximizing the upstream nutrient influx into the MAB shelfbreak area. Taking 0.25 ms −1 as the mean shelfbreak jet speed in spring, it would take it about a week for a fluid parcel in the jet to traverse the shelfbreak domain under study.…”

Linear and nonlinear stratified spindown over sloping topography

“…Linder et al (2004) note that the vertical extent of along-isopycnal upwelling is important for sustaining enhanced levels of phytoplankton at the shelfbreak (Malone et al 1983, Marra et al 1990. During the summer, the development of the thermocline tends to suppress the vertical extent of the upwelling and the supply of nutrients to the euphotic zone (Linder et al 2004). For the spring transition from well-mixed to stratified conditions (mid-April to June), Ryan et al (1999) used the pigment concentrations from the Coastal Zone Color Scanner to show that a band of enhanced levels of chlorophyll occurred along the Middle Atlantic Bight shelfbreak, which could be explained by upwelling of nutrients.…”

“…This region corresponds to a portion of the orange boxed region for the Middle Atlantic Bight shown in the previous figure (modified from Ryan et al (1999)). Linder et al (2004) note that the vertical extent of along-isopycnal upwelling is important for sustaining enhanced levels of phytoplankton at the shelfbreak (Malone et al 1983, Marra et al 1990. During the summer, the development of the thermocline tends to suppress the vertical extent of the upwelling and the supply of nutrients to the euphotic zone (Linder et al 2004).…”

“…Pickart (2000) also estimated a vertical upwelling of 23 m day −1 from ADCP measurements. In a seasonal depiction of the detached bottom boundary layer during the Shelfbreak PRIMER experiment, detachment occured along the 26.0 kg m −3 isopycnal throughout the year (Linder et al 2004). Following Pickart (2000), the detached bottom boundary layer was estimated to reach 80 m above the bottom in the winter, whereas it was only able to reach 25 m above the bottom in the summer owing to strong stratification near the surface.…”

“…This upwelling leads to a detached bottom boundary layer, in which tracer gradients are weakened along the isopycnal where upwelling occurs (Pickart 2000, Linder et al 2004). Upwelling brings nutrients up from depth, supporting biological productivity, and hence its rate is important to quantify.…”