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

利用旋转框架中的三维非流体静力模型进行了数值实验，研究了弱分层极地海洋中与热压深对流相关的斜压不稳定性及其在输送过程中的作用。模型海洋有两层结构，冷的、新鲜的混合层覆盖在温暖的、含盐的深水细胞上，比如南极洲的威德尔海。与基于线性状态方程的情景相反，水柱的对流倾覆通过温压效应和二次（束带）效应增强了水平密度梯度（斜压性）。如果温度（盐度）控制水的密度，则倒转层底部（表面）的斜压性会增强。这种强化导致斜压不稳定或斜压不稳定的进一步发展和更有效的垂直热输送。在后倾覆阶段，地表冷却（对流运动）对斜压不稳定和相关热输运的发展有两个相反的作用。一种是由于表层对流运动引起的水平辐合流增强了斜压不稳定性，正如先前的研究报道的那样，这些研究集中在中纬度地区的强分层海洋上。这是在斜压性在表面增强时观察到的，而不是在底部增强时。另一种观点认为强对流运动通过使倒转层在垂直方向上不断均匀化来抑制斜压不稳定。这种影响在强冷却的情况下变得显著，而在以前研究过的强分层海洋中没有观察到。因此，垂直热输运在零冷却速率或低冷却速率（<125 Wm^<-2>）时最为有效。当斜压性最初较弱时，如威德尔海，最有效的输送发生在冷却速率为25 Wim^<-2>时，这是海冰覆盖下实际情况下可能的值。水柱静稳定性的空间差异导致的水柱不均匀（部分）倾覆是实际情况下斜压失稳发展的另一个因素。少

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 … More 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. Less