Upper ocean modelling in Baie des Chaleurs

An eddy-resolving upper ocean model is developed to study the dynamics and thermo-dynamics of Baie des Chaleurs (BdC, 47.5-48.5N, 65.5-66.5W), Gulf of St. Lawrence (GSL), Canada. The model has primitive equation dynamics with two active layers embedded with a Kraus-Niiler type mixed layer model at the top.Forced by observed wind, atmospheric heat fluxes, river runoff and appropriate remote forcing (in particular, the Gaspe Current, GC), the model demonstrates that the mean cyclonic general circulation pattern in the bay is a consequence of the intrusion of the GC. In the mixed layer, atmospheric heat fluxes and horizontal thermal advection play a key role in the thermal balance at the eastern part of the bay. The local mixed layer fluctuations are controlled by wind and GC induced divergence. The entrainment (and its corresponding heat flux) is important at the western part of the bay and changes the mean mixed layer depth on a time scale of more than a week. Varying GC intensified the flow variations induced by the wind in the bay and improved simulation results as compared with observations.Sensitivity runs are conducted to study the effects of external forcing, important physical processes and the internal physical parameterisation on the model results and to compare these with the main model run. Experiments show that nonlinearity is very important in determining the circulation pattern in the bay. Changing external thermal forcing also modifies dynamical processes in the BdC. The fluctuations in the near surface temperature are mainly due to latent and sensible heat changes. The parameter study indicates that, the model is not overly sensitive to changes in most of the parameters, but suggests that sensitivity of the mixed layer physical parameters depends on the dynamical and thermodynamic system applied.Hydrographic and current meter data are used first to study the variability of both the dynamics and thermodynamics in the BdC and its relation to the separation/intrusion of the unsteady GC. A numerical model is then applied to gain insight into the problem. The time scales of interest range from tidal to seasonal.The results show that the kinetic energy in the BdC is dominated by the semi-diurnal tide (M$ sb2$) and periods of 5-10 days for high and low frequency bands, respectively. Most of the energy in the low frequency band is found to be induced by wind-related forcing.Both observations and model results indicate that seasonal variations in the BdC are strongly related to the characteristics of separation/intrusion of the GC, which is mainly controlled by its transport magnitude as well as phase, duration and strength of its acceleration (or deceleration). The separation occurs when (adverse) vorticity having an opposite sign from that existing upstream is generated near the separation area. Although the separation can be generated in a decelerating GC, it can also occur in an accelerating GC when the GC is strong enough to advect upstream vorticity necessary to form a recirculation and the related adverse vorticity downstream. Nonlinearity is critical to the separation. Nevertheless, separation can be generated in a linear current with strong deceleration. The GC intrudes either along the coastline (attachment) into the bay by a non-separated GC or following the separation of the GC (reattachment). Effects of various physical processes on the separation/intrusion and variability of eddies in the BdC are examined