Numerical study of baroclinic instability associated with thermobaric deep convection at high latitudes: Idealized cases

Abstract

Numerical experiments with a three-dimensional nonhydrostatic model in a rotating frame have been executed to investigate baroclinic instability associated with thermobaric deep convection in weakly stratified polar oceans and its role in the transport processes. The model ocean has a two-layered structure with the cold, fresh mixed layer overlying the warm, saline deep water cell, as in the Weddell Sea. In contrast with a scenario based on the linear equation of state, thermobaric overturning of the water column enhances the horizontal density gradient (baroclinicity) through nonlinearity of the equation of state. If temperature controls water density (TEM cases), baroclinicity is intensified at the bottom of the overturned layer while at the surface if salinity does (SAL cases). Such intensification causes further development of baroclinic instability or baroclinic destabilization and more effective vertical heat transport. In the post-overturning stage, on the other hand, surface cooling (convective motion) has two oppositely operating effects on baroclinic instability and the associated heat transport. One is that horizontal convergence due to convective motion enhances baroclinic instability in the surface layer, as in previous studies focusing on strongly stratified oceans. This is observed in SAL cases with weak cooling, but not in TEM cases. The other is that strong cooling suppresses baroclinic instability by homogenizing the overturned layer vertically. This effect has not been found in the strongly stratified oceans. As a result, the vertical heat transport is most effective at low cooling rates ( ∼ 125 W m - 2 ) in SAL cases while it monotonically decreases with cooling rate in TEM cases. When baroclinicity is initially weak as in the Weddell Sea, the most effective transport occurs with the cooling rate of 25 W m - 2 which is a possible value under sea-ice cover in the actual situation.