An Idealized Study of the Seasonality of Frontal Instabilities with Implications for the Polar Front over the Iceland Faroe Ridge.

The relaxation of the zonally symmetric polar front under a parameter range representing the effects of convective homogenization within the mixed layer is presented here. The important parameter is the deformation radius, independently defined for the mixed layer front and the polar front. We characterize wintertime conditions by a deep mixed layer and strong density jump across it, resulting in a mixed layer deformation radius comparable to the one associated with the polar front. A shallow mixed layer and small density jump across it, resulting in a small deformation radius, characterizes summertime conditions. During summertime conditions, a large baroclinic wave (~50km) emerges. Relative vorticity and horizontal divergence are small, and the baroclinic wave cycle is similar to that of a two-layer baroclinic instability problem. The nonlinear evolution of the front is associated with the separation of eddies from the front. These eddies have length scales on the order of the internal deformation radius O(10km), transporting cold anomalies south of the location of the surface outcrop. During winter conditions the eddy field is dominated early on by mixed layer instabilities (~8 km) with a much faster growth rate. Large values of relative vorticity introduce significant ageostrophic effects, and geostrophic turbulence dominates the later evolution of the eddy field. It is argued that the polar front modulates the instability within the mixed layer by introducing both a sloping base of the mixed layer, and damping of the larger ageostrophic modes by the presence of the polar outcrop, which functions as a meridional boundary for the perturbation. The frontal jet further introduces a zonal variation over the entire mixed layer. The difference of the eddy field between summer and winter has implications for the formation of intermediate waters, and for the heat transport across strong fronts.