Geostrophic and Wind-Driven Components of the Antarctic Circumpolar Current

Abstract

The contributions of wind and thermohaline factors to the formation of the mean climatic structure of the Antarctic Circumpolar Current (ACC) were studied using numerical simulations with the INMOM ocean general circulation model. The goal of our research was to separate the wind and thermohaline ACC components. The simulations were carried out for summer (February) and winter (August) conditions. The ACC structure of three jets was revealed by numerical simulations using the diagnosis-adjustment approach to the EN4 climatic data of observations. Wind circulation in the surface layer turns to the left with respect to the direction of wind stress (southern hemisphere) according to Ekman’s theory. Due to the stronger winds in winter, the response of drift velocities is stronger than in summer. It is shown that, in spite of strong winds over the Southern Ocean, the thermohaline factor of circulation is generally much stronger than the drift factor. Nevertheless, the contribution of the wind component to the increase in the zonal velocity in the Drake Passage near Antarctica can be as high as 15–20% of the thermohaline velocity in the upper layer. Wind contributes to a decrease in the mean dynamic topography (MDT) by more than 6 cm from the open ocean to the Antarctic coast. The MDT decrease caused by thermohaline factors over this distance exceeds 2 m, which agrees well with the finding that the contribution of thermohaline factors to the ACC dynamics significantly exceeds the contribution of wind. However, winds are subjected to seasonal variations. The effect of wind on the formation of the barotropic stream function is more pronounced than on the MDT. Thermohaline transport in the Drake Passage is almost the same (~110 Sv) in winter and summer. Due to wind forcing, the total ACC transport around Antarctica increases on average by 9–11 Sv in February and by 12–17 Sv in August. In the Drake Passage, the wind transport increases less than over the entire ACC: by 8 Sv in summer and by 12 Sv in winter. Thermohaline factors comprise a significantly greater contribution to the ACC dynamics than wind forcing. A three-jet structure of the ACC was revealed based on simulations using the INMOM model and EN4 data. It is shown that this three-jet structure of the ACC is of a thermohaline nature.