Open‐ocean deep convection due to thermobaricity: 1. Scaling argument

Abstract

Properties of open‐ocean deep convection due to thermobaricity at high latitudes have been investigated by the scaling argument. Two types of open‐ocean deep convection are found. The first type appears in a homogeneous ocean or in a deepening mixed layer. The increased apparent buoyancy flux due to thermobaricity makes scales of convective properties larger than those without thermobaricity. The ratio of thermobaric to nonthermobaric scales is determined only by l0/Hα, where l0 and Hα are the size of nonthermobaric convection and the characteristic depth for thermobaricity to be effective, respectively. In a nonrotating frame, then, thermobaricity becomes effective when the ocean depth H is comparable with Hα, as previous studies show. In a rotating frame, thermobaricity is most effective with slow rotation (the Coriolis parameter ƒ ≪ 10−4 s−1) and low temperatures (approximately the freezing point of seawater), since l0/Hα becomes largest. In the actual situation (ƒ ∼ 10−4 s−1), thus thermobaricity is not so effective because the Earth rotation confines l0 to a smaller level, i.e., l0/Hα < 1. The second type of deep convection is driven by pure thermobaric instability in a two‐layer ocean where a cold, fresh mixed layer overlies a warm, saline deep layer, as often observed in polar oceans. It causes an abrupt overturning of the water column. Scales of convective properties are dependent on the difference of water temperature between the two layers, the initial size of plume, and the Coriolis parameter (or time), but not on the surface cooling rate directly. With actual parameters, convective properties and the associated buoyancy flux are much larger than those of the first type. Observed vertical profiles of water temperature and salinity suggest that the first type of deep convection could occur in the Greenland Basin while the second type could occur in the Weddell Sea.