On the dynamics of the slope current system along the West European margin. Part II: Analytical calculations and numerical simulations with seasonal forcing

The seasonality of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula is investigated in a joint analysis of an analytical model and numerical simulations with various forcings. A distinction is made between local winds and basin-scale winds, in which the effect of the latter is indirectly apparent through the basin-scale density gradients. The slope currents are mainly forced by the large-scale structure of the density field. The analysis indicates significant differences in the behavior of the uppermost slope current and of the deeper currents. At all depths, seasonal variations in the large-scale density structure of the ocean alter the strength of the slope currents but are not able to cause robust, long-lasting reversals. Reversals of the uppermost slope current appear to be caused by changes in the alongshore component of the local wind stress, provided that the opposing forcing from the density structure is weak enough. However, the deeper slope currents are not very much affected by the wind stress, so that flow reversals can be explained neither by the wind nor by seasonal changes in the density structure. A numerical simulation suggests that the reversals of the deeper slope currents are at least partly forced by seasonal changes in the flow upstream of the slope current system. The authors demonstrate that the larger part of these seasonal changes is associated with annual baroclinic Rossby waves caused by the seasonal cycle of the large-scale wind stress over the whole basin. © 2008 American Meteorological Society.