A model of the circulation on the outer Scotian shelf with open boundary conditions inferred by data assimilation

Abstract

The circulation on Western Bank is described using data collected in spring 1991 and 1992 as part of an interdisciplinary study of the early life history of Atlantic cod (Gadus morhua). The mean circulation over the crest of the bank is weak, of the order of a few centimeters per second, and is flanked to the south and west by an anticyclonic flow with a speed of about 10 cm s-1. A thermal wind calculation shows the mean circulation is due primarily to horizontal variations in the density field. The M2 tidal constituent is dominant and reaches speeds of about 20 cm s-1. The tidal residuals have a standard deviation of about 10 cm s-1 and characteristic time and length scales of several days and tens of kilometers, respectively. In order to help collect and interpret biological data from Western Bank, we developed a hydrodynamic model of the near-surface flow that could be used operationally. The model is based on a decomposition of the flow into components driven by (1) local wind stress, (2) horizontal density gradients, and (3) flows through the open boundaries of the model. The wind-driven component is calculated using a simple slab model driven by the observed wind. The other two components are estimated through the assimilation of observed bottom pressures, dynamic heights, and currents into the hydrodynamic model. The hindcast skill of the model is quantified by cross validation and shown to be higher than that of four simple, statistically based schemes