SUBTIDAL CIRCULATION ON THE SCOTIAN SHELF - ASSESSING THE HINDCAST SKILL OF A LINEAR, BAROTROPIC MODEL

Currents measured during the winter of 1985-1986 by four moorings on t he inner Scotian Shelf are used to assess the hindcast skill of a thre e-dimensional circulation model forced by local wind stress and coasta l sea level. The model is linear and barotropic. The integration schem e is based on the modification of the Galerkin spectral method propose d recently by Sheng and Thompson [1993]. The skill of the model is mea sured by the variance of the hindcast errors divided by the variance o f the observations (henceforth gamma(2)). The model is most effective within 30 km of shore (0.51 less than or equal to gamma(2) less than o r equal to 0.66). At the mooring in the Nova Scotia Current, a surface intensified southwestward jet with its center approximately 50 km fro m shore, the model fails to capture the bulk of the variance at curren t meters within 70 m of the surface (0.81 less than or equal to gamma( 2) less than or equal to 0.87). The skill of the model is lowest at th e mooring 65 km from shore (0.92 less than or equal to gamma(2) less t han or equal to 0.94). To put these measures of skill into perspective , the currents are hindcast using a linear statistical model with the same inputs as the circulation model. The statistical model is optimal in the sense no other linear model with these inputs can achieve a lo wer hindcast error variance. For half of the current meters within 30 km of shore and two in the Nova Scotia Current the skill of the circul ation model is not significantly lower than that of the statistical mo del. The largest discrepancies in the skill of the two models are foun d at the offshore mooring. We show that the suboptimal performance of the circulation model is due in part to the assumption of a spatially uniform wind field. We speculate that another contributing factor is t he assumed form of the sea level profile along the open boundary that is upstream in the sense of coastal trapped wave propagation. Finally, the statistical model is used to estimate the increase in skill of th e circulation model that may result from the assimilation of hydrograp hic data and additional coastal sea levels.