ON THE UNSTEADY SEPARATION INTRUSION OF THE GASPE CURRENT AND VARIABILITY IN BAIE DES CHALEURS - MODELING STUDIES/

A numerical model has been used to study separation/intrusion of an un steady, baroclinic coastal jet, the Gaspe Current (GC) and its impact on dynamics and thermodynamics of the Bale des Chaleurs (BdC, Gulf St; Lawrence, Canada). The model has 2 1/2 layers with primitive equation dynamics and an embedded bulk mixed layer (ML) model. It is forced wi th observed atmospheric fluxes, as well as the GC. The simulations sho w that the variability in the Bale des Chaleurs is controlled by the c haracteristics of the unsteady GC separation/intrusion. On the basis o f the dynamic and vorticity balance analyses, it is found that the sep aration is related to the adverse pressure gradient force which is ind uced 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 pr esence of separation in an accelerating current, as well as, in a line ar current, with strong deceleration when momentum advection is neglec ted, gives new insight into the mechanisms of unsteady boundary, curre nt 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 cir culation in the BdC. Prevailing westerly winds reduce the cyclonic cir culation inside the bay but have little effect on separation/intrusion near the entrance. The GC either intrudes along the coastline (attach ment) or after separation (reattachment). Increasing vertical shear of the GC and offshore movement of the GC axis reduces the tendency to s eparate. The finding of an asymmetric response of GC separation/intrus ion to symmetric GC forcing explains the monthly mean features seen in observations taken in the bay.