The seasonal circulation along the northern Iberian Peninsula and in the Bay of Biscay is investigated by means of a regional ocean model. In particular, the modeled velocities and tracers are compared to available observations and used to hypothesize what the circulation may look like in areas where the density of observations is scarcer. Despite a few biases in the thermohaline properties of some water masses, the model is able to represent the various water masses present in the region in an acceptable way. In particular, the density and depth ranges of most water masses are in good agreement with observed ranges. Similarly, the circulation schemes compare generally well with observations, both in annual mean as for the seasonal features. The model simulates a baroclinic slope current system that extends within the upper 2000 m and is subject to a strong seasonal variability. As a result, these slope currents are seen to reverse seasonally at all depths. A numerical Lagrangian analysis indicates that water masses cannot be transported continuously within the slope currents in or out of the Bay of Biscay because of the flow reversals associated with this seasonality. Instead, this analysis highlights the numerous connections with the slope current system and the interior, in agreement with Lagrangian drifter data.