Large-eddy simulations of deep-ocean convection: analysis of the vorticitydynamics

Results of large-eddy simulations (LES) of oceanic deep convection resultin g from localized cooling applied at the surface are presented. The present LES exhibit a very good quantitative agreement with the laboratory experime nts 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 approximate to) 0.28 is observed under which the flow is dominated by the rotation effects. Three very distinct flow regimes are identified: low, mo derate and high rotation regimes. A detailed investigation of the vertical vorticity generation mechanisms demonstrates that the vertical stretching c onstitutes the main production term and it initiates an asymmetry between c yclonic and anticyclonic vertical vorticity. For the moderate rotation regi me, average quantities show a dominance of cyclonic vorticity. However, a m ore detailed investigation based, in particular, on flow animations reveals the presence of an anticyclonically rotating cold bottom current that is f ound 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 v ertical length of the domain and they are linked with intense braids of cyc lonic vorticity. In the high rotation regime, the cyclonic/anticyclonic asy mmetry is much less pronounced. A global cyclonic rotation of the cold wate r column is observed in this case and no evidence of baroclinic instability is detected.