Structure and Temporal Variability of the Northern Branch of the Antarctic Circumpolar Current in the Drake Passage

The Antarctic Circumpolar Current plays a key role in the circulation of the Southern Ocean and affects redistribution of heat by the ocean on a global scale. The study of the dynamics and structure of this current becomes especially relevant in a changing climate. The current is well manifested based on the satellite altimetry data, which makes it possible to study the variability of its structure in time and space on different scales. The methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data becomes most important in solving these tasks. In this paper, we compare different approaches to the detection of fronts. To do this, the structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability have been considered based on the satellite altimetry data from 1993 to 2020 and also on the results of a hydrographic section made onboard the R/V Akademik Mstislav Keldysh across the northern part of the Drake Passage in February 2020. To assess the dynamics of the front, a section of the TOPEX/Poseidon, Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south, 350 km long, was selected. Criteria are presented for determining the position of the northern and main branches of the Subantarctic Front based on the satellite altimetry data. A long-term trend of shifting the position of the fronts with respect to the previously accepted levels of absolute dynamic topography has been found. It has been established that the accuracy of determining the position of fronts from fixed values of dynamic topography can decrease in time, in particular, due to the variations in the mean sea level. A statistically significant long-term trend in sea level rise in the region of the Subantarctic Front at a rate of 4 mm/year for the northern branch and 2.5 mm/year for the main branch was found.