资源环境科技发展态势分析平台

The upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time and zonal mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here an observationally constrained ocean model and an observation-based seasonal climatology are analyzed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two-stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near-surface winter water via wintertime mixing. Second, winter water warms to form intermediate water through summertime surface heat fluxes. The mixing-driven pathway from deep water to winter water follows mixing lines in thermohaline coordinates indicative of nonlinear processes.

Plain Language Summary Deep, cold and salty water is drawn to the surface ocean around Antarctica. This is controlled by the combined action of winds blowing over the sea surface and the stirring by large 100-km eddies. Studying the ocean around Antarctica is notoriously challenging. We therefore have sparse ocean measurements in this region. This limits our understanding of some of the processes involved in the circulation of deep water, notably, a stage involving the warming and freshening of deep water to form intermediate water. Intermediate water plays a key role in drawing excess anthropogenic heat and carbon dioxide from the sea surface into the interior ocean. Our results show that the conversion of deep to intermediate water involves a two-stage cold transit that takes place over the course of the austral season. First, during the winter, deep water sits below the very cold and fresh surface water. Ice formation around Antarctica makes this surface water slightly saltier, decreasing the density difference between the deep and surface water. This drives a mixing which cools and freshens the deep water while drawing it to the surface. Second, during summer surface warming and melting sea ice warm and freshen this deep/surface water mixture forming intermediate water.