Statistics of low frequency currents over the western Norwegian shelf and slope II: Model

Abstract

We examine velocity statistics from a numerical simulation of the Nordic Seas with 4 km resolution, with a focus on the Norwegian shelf and slope. The model mean flow is dominated by its version of the Norwegian Atlantic Current, with two branches, one near the shelfbreak and the other near the 2000 m isobath. The model variances are surface-intensified and increase with water depth over the shelf; the variance ellipses also indicate topographic steering. Seasonality is more pronounced on the shelf than on the slope and the velocity probability distributions are weakly non-Gaussian, reflecting an excess of extreme velocities. All these aspects are broadly consistent with the observations. There are, however, differences with the observations. The topographic steering of the mean flow and of the variance ellipses is less pronounced in the model, a probable consequence of the model bathymetry, etopo5, being too smooth. The temporal and spatial coherence scales are too large by about a factor of 2, probably due to the model resolution. And correlations between velocity time series from the model and in situ moorings are generally small, despite the model having realistic forcing. The low predictability presumably reflects the degree of chaos in the flow and highlights the need for data assimilation.