Open-ocean deep convection due to thermobaricity 2. Numerical experiments

Abstract

Properties of open-ocean deep convection due to thermobaricity at high latitudes have been investigated by executing three-dimensional, nonhydrostatic numerical experiments in a rotating frame. As obtained with the scaling argument by Akitomo [this issue], the first type of deep convection, which is not so affected by thermobaricity, can occur in a homogeneous ocean with an intermediate water depth (∼2000 m) while the second type in a two-layer ocean can occur through pure thermobaric instability. It is also confirmed that the former could occur in the Greenland Basin and the latter in the Weddell Sea, which suggests the different processes of deep and bottom water mass formation between the two regions. Dependence of convective properties on external parameters is basically the same with the scaling argument. However, some important differences from the theory are found in a homogeneous ocean. The convective size is determined by the product of characteristic velocity and timescale, whether the ocean is shallow or deep, rotating or nonrotating. This is possibly a feature common to nonlinear convective phenomena. The characteristic timescale is given by the shortest one of the diffusion time, the inertial period, and the period of any oscillatory feature accompanying convective motion. Reduced gravity and velocity scales depend on the ocean depth in a similar manner to the nonrotating scalings, even when the rotation is effective. As a result, the same relation is realized in the three-dimensional regime as in the two-dimensional regime, namely, that the convective size is proportional to the "Rossby radius of deformation." Copyright 1999 by the American Geophysical Union.