Numerical simulation of a response of an oceanic front to an atmospheric frontal passage

Abstract

The influence of middle‐latitude cyclones and their associated cold fronts on open‐ocean fronts is studied with an embedded mixed layer ocean circulation model similar to that of Adamec et al. (1981). Finite differences with horizontal resolution of 5 km in a staggered grid are used to solve the model equations within a domain of total meridional extension of 300 km and 500 m depth. Three numerical experiments were performed to investigate the response of an oceanic front, similar to the South Atlantic subtropical convergence zone, to an atmospheric frontal passage. Under the effect of the cold air outbreak the thermal and dynamic structures of the upper layers are substantially modified with a higher sinking/cooling rate of the mixed layer. The sinking/cooling rate depends strongly on the magnitude of the thermal gradient at the top of the thermocline and on the mixed layer depth. Due to the differences in sinking/cooling rates, a single storm can move the oceanic front up to 20 km from its initial position. For low‐intensity winds, a change in the wind direction does not affect the thermal structure of the upper layer ocean, even in the frontal region. The mixed layer oscillations are entirely a function of the daily scale of the solar heating. For a sudden change in the wind direction, the response of the current velocity field in the mixed layer is almost instantaneous.