BAROCLINIC INSTABILITY DUE TO THERMOBARIC CONVECTION AND ITS ROLE IN FORMATION OF DEEP AND BOTTOM WATERS

2003 Fiscal Year Final Research Report Summary

• Project/Area Number: 14540408
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Meteorology/Physical oceanography/Hydrology
• Research Institution: KYOTO UNIVERSITY
• Principal Investigator: AKITOMO Kazunori KYOTO UNIVERSITY, GRADUATE SCHOOL OF SCIENCE, ASSOCIATE PROFESSOR, 大学院・理学研究科, 助教授 (10222530)
• Project Period (FY): 2002 – 2003
• Keywords: THERMOBARIC EFFECT / WEDDELL SEA / CONVECTION / BAROCLINIC INSTABILITY / FORMATION OF DEEP AND BOTTOM WATERS

Research 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, such as in the Weddell Sea Antarctica. In contrast with a scenario based on the linear equation of state, convective overturning of the water column enhances the horizontal density gradient (baroclinicity) through the thermobaric and quadratic (cabbeling) effects. If temperature (salinity) controls water density, baroclinicity is intensified at the bottom (surface) of the overturned layer. Such intensification causes further development of baroclinic instability or baroclinic destabilization and more effective vertical heat transport. In the post-overturning stage, surface cooling (convective motion) has two oppositely-operating effects on development of baroclinic instability and associated heat transport. One is that horizontal convergent flow due to convective motion in the surface layer enhances baroclinic instability, as previous studies have reported focusing on strongly stratified oceans at midlatitudes. This is observed when baroclinicity is surface-intensified, but not when it is bottom-intensified. The other is that strong convective motion suppresses baroclinic instability by continuously homogenizing the overturned layer vertically. This effect becomes significant in strong-cooling cases, and has not been observed in strongly stratified oceans studied previously. As a result, the vertical heat transport is most effective at zero or low cooling rates (<125 Wm^<-2>). When baroclinicity is initially weak as in the Weddell Sea, the most effective transport occurs with the cooling rate of 25 Wim^<-2> which is a possible value under sea-ice cover in the actual situation. Inhomogeneous (partial) overturning of the water column resulting from spatial difference in static stability of the water column is another factor for development of baroclinic instability in the actual situation.