A numerical model has been used to study separation/intrusion of an unsteady, baroclinic coastal jet, the Gasp¿ Current (GC) and its impact on dynamics and thermodynamics of the Baie des Chaleurs (BdC, Gulf St. Lawrence, Canada). The model has 212 layers with primitive equation dynamics and an embedded bulk mixed layer (ML) model. It is forced with observed atmospheric fluxes, as well as the GC. The simulations show that the variability in the Baie des Chaleurs is controlled by the characteristics of the unsteady GC separation/intrusion. On the basis of the dynamic and vorticity balance analyses, it is found that the separation is related to the adverse pressure gradient force which is induced by the ageostrophic terms in the momentum equations and to local vorticity intensification due to the inertial effect in the flow. The simulations demonstrate characteristics of unsteady separation. The presence of separation in an accelerating current, as well as in a linear current, with strong deceleration when momentum advection is neglected, gives new insight into the mechanisms of unsteady boundary current separation. Unsteady separation or intrusion occurrence depends on the magnitude of the GC transport, as well as its rate and duration of deceleration or acceleration. The GC intrusion generates cyclonic circulation in the BdC. Prevailing westerly winds reduce the cyclonic circulation inside the bay but have little effect on separation/intrusion near the entrance. The GC either intrudes along the coastline (attachment) or after separation (reattachment). Increasing vertical shear of the GC and offshore movement of the GC axis reduces the tendency to separate. The finding of an asymmetric response of GC separation/intrusion to symmetric GC forcing explains the monthly mean features seen in observations taken in the bay.¿ 1997 American Geophysical Union