A New Mechanism for Mode Water Formation involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts. Part II: Numerical simulations

Abstract

Submesoscale-resolving numerical simulations are used to investigate a mechanism for sustained mode water formation via cabbeling at thermohaline fronts subject to a confluent strain flow. The simulations serve to further elucidate the mechanism and refine the predictions of the analytical model of Thomas and Shakespeare (2015). Unlike other proposed mechanisms involving air-sea fluxes, the cabbeling mechanism, in addition to driving significant mode water formation, uniquely determines the thermohaline properties of the mode water given knowledge of the source water masses on either side of the front. The process of mode water formation in the simulations is as follows. Confluent flow associated with idealised mesoscale eddies forces water horizontally towards the front. The frontogenetic circulation draws this water near-adiabatically from the full-depth of the thermohaline front up to the surface 25 meters, where resolved submesoscale instabilities drive intense mixing across the thermohaline front, creating the mode water. The mode water is denser than the surrounding stratified fluid and sinks to fill its neutral buoyancy layer at depth. This layer gradually expands up to the surface and eddies composed entirely of this mode water detach from the front and accumulate in the difluent regions of the domain. The process continues until the source water masses are exhausted. The T-S relation of the resulting mode water is biased to the properties of the source water that has the larger isopycnal T-S anomaly. This mechanism has the potential to drive O(1 Sv) mode water formation, and may be important in determining the properties of mode water in the global oceans.