Abstract
Abstract. A high-resolution, 3-dimensional coupled biophysical model is used to simulate ocean circulation and ecosystem variations at the shelfbreak front of the Middle Atlantic Bight (MAB). Favorable comparisons between satellite observations and model hindcast solutions from January 2004 to November 2007 indicate the model has intrinsic skills in resolving fundamental physical and biological dynamics at the MAB shelfbreak. Seasonal and interannual variability of ocean physical and biological states and their driving mechanisms are further analyzed. The domain-wide upper water column nutrient content is found to peak in late winter-early spring. Phytoplankton spring bloom starts 1â2 months later, followed by zooplankton bloom in early summer. Our analysis shows the variability of shelfbreak nutrient supply is controlled by local mixing that deepens the mixed layer and injects deep ocean nutrients into the upper water column and alongshore nutrient transport by the shelfbreak jet and associated currents. Nutrient vertical advection associated with the shelfbreak bottom boundary layer convergence is another significant contributor. Spring mean nutrient budget diagnostics along the Nantucket transect are compared between nutrient rich 2004 and nutrient poor 2007. Physical advection and diffusion play the major role in determining strong interannual variations in shelfbreak nutrient content. The biological (source minus sink) term is very similar between these two years.
Highlights
The Middle Atlantic Bight (MAB) shelfbreak region contains a sharp front that separates the cold, fresh water on the shelf from warmer, more saline water in the slope sea
The MAB shelfbreak ROMS is one-way nested inside a regional-scale Middle Atlantic Bight and Gulf of Maine ROMS, which itself is nested in the global HyCOM circulation model
A more robust statistical skill assessment of the temporal evolution of the domain-averaged surface chlorophyll is shown by year in the form of a Taylor diagram (Taylor, 2001), where correlation coefficients, centered root mean square differences (RMSD) between observed and simulated domain-averaged chlorophyll concentration, and normalized standard deviations are all presented in a single plot (Fig. 3)
Summary
The Middle Atlantic Bight (MAB) shelfbreak region contains a sharp front that separates the cold, fresh water on the shelf from warmer, more saline water in the slope sea. In situ and satellite observations often show higher levels of chlorophyll within the MAB frontal region, known as the shelfbreak pigment maximum (Marra et al, 1982; Ryan et al, 1999a, b, 2001). This locally enhanced productivity provides energy to upper trophic level predators. We utilize a coupled biophysical model to study MAB shelfbreak physical and biological dynamics and their associated seasonal and interannual variability. 3. Causes of temporal and spatial variability of physical, biological and nutrient fields are discussed, followed by summary and conclusions in Sect.
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