We describe the seasonal cycle of the alongshore flows on the Northwestern Iberian Margin and explain what are the important mechanisms forcing the system. We used a 20-year model simulation at 2.3 km horizontal resolution, with realistic atmospheric forcing and covering the whole Western Iberian Margin. The model results are compared with satellite data, with data measured at two moored buoys and with a compilation of current meter data available for the study region. We show that currents over the slope are divided in three different cores: the Iberian Poleward Current (IPC) occupying the top 350 m, a deeper core at Mediterranean Water levels (∼600–1200 m) and in between the two, an equatorward core centered just beneath the IPC core, the Upper Slope Countercurrent (USCC). The IPC is present almost yearlong, including in summer months, when it is close to the shelf-break and topped by the equatorward upwelling jet. After September, the IPC intensifies and its core surfaces. Heat and salinity budgets on the shelf and slope are dominated by advection, confirming the important role of the IPC in driving the temperature and salinity seasonal cycles. Analysis of the seasonal cycle of the barotropic vorticity equation on the slope, shows that the main forcing mechanism of northward flows is the “Joint Effect of Baroclinicity and Relief” (JEBAR), whose contribution is higher in summer than in winter. In December and January, when the IPC is stronger and surface intensified, the main contribution is from southerly winds. The cross-shore analysis of these terms shows that JEBAR decreases substantially at the core of the IPC because, as advection terms become important, the northward density flux diminishes the local meridional density gradients, resulting in the decrease of JEBAR in the months of strong IPC.