Large-eddy simulations of deep-ocean convection: analysis of the vorticity dynamics

Abstract

Results of large-eddy simulations (LES) of oceanic deep convection resulting from localized cooling applied at the surface are presented. The present LES exhibit a very good quantitative agreement with the laboratory experiments by Maxworthy and Narimousa (1994 J. Phys. Oceanogr. 24 865–87). As with the experimental observations, a critical Rossby number such that (Ro ^*)^1/2 ≈ 0.28 is observed under which the flow is dominated by the rotation effects. Three very distinct flow regimes are identified: low, moderate and high rotation regimes. A detailed investigation of the vertical vorticity generation mechanisms demonstrates that the vertical stretching constitutes the main production term and it initiates an asymmetry between cyclonic and anticyclonic vertical vorticity. For the moderate rotation regime, average quantities show a dominance of cyclonic vorticity. However, a more detailed investigation based, in particular, on flow animations reveals the presence of an anticyclonically rotating cold bottom current that is found to be baroclinically unstable. This current exhibits several features in common with those encountered in baroclinic jets. In particular, intense quasi two-dimensional cyclonic vortices form. They extend over the whole vertical length of the domain and they are linked with intense braids of cyclonic vorticity. In the high rotation regime, the cyclonic/anticyclonic asymmetry is much less pronounced. A global cyclonic rotation of the cold water column is observed in this case and no evidence of baroclinic instability is detected.