Long-Term Linear Meridional Shift of the Jet Structure of the Antarctic Circumpolar Current South of Africa

Abstract—The meridional shift of the absolute dynamic topography (ADT) gradient field structure in the zone of the Antarctic Circumpolar Current (ACC) south of Africa (10° E–25° W) was analyzed over the observation period 1993–2018 based on ADT satellite altimetry data at https://marine.copernicus.eu. The study was carried out by an original method based on linear regression calculations. Two 26-year time series of dependences of the mean annual absolute values of the ADT gradient on latitude and on the value of the ADT itself have been analyzed. Analysis of the dependence on latitude showed that in the ACC zone (42°–57° S), there are three zones of increased gradients, conventionally corresponding to the cores of the Subantarctic (SAC), South Polar (SPC), and South Antarctic (SthAC) currents. Analysis of the dependence on the ADT showed that there are four such zones in the ACC zone (–130 to 20 cm in ADT units); an additional zone is observed in the SPC. In general, over the 26 years of observations, a northward latitudinal shift in the structure of the gradient field of the ADT by 0.05° ± 0.10° has been noted in the ACC zone. At the same time, in the SAC, SPC, and SthAC zones, the mean shifts are 0.16° ± 0.15° to the south, 0.30° ± 0.14° to the north, and 0.03° ± 0.26° to the north, respectively. The extreme values of the shift in the SAC and SPC zones reach 0.4° to the south and 1.4° to the north, respectively. A positive shift in the structure of the gradient field of the ADT is observed in the ACC zone relative to the ADT, amounting to 8.3 ± 0.9 cm. This shift is generally caused by the corresponding increase in ocean level at geographic points. However, in individual zones within the ACC, the shift can differ significantly from the cited value due to the meridional shift in the structure of the ADT gradient over geographic coordinates. In particular, it reaches 17 cm in the boundary zone between the SAC and SPC. In the ACC zone, an increase in the absolute value of the ADT gradient is also observed (1.9 ± 2.7 × 10^–3 cm/km), which is equivalent to an increase in the difference of the ADT across the ACC by about 3 cm. This corresponds to the difference in the increase in the ADT at geographical points on the southern and northern periphery of the current. At the same time, in the SAC zone, a decrease in the absolute value of the ADT gradient by 8.0 ± 4.2 × 10^–3 cm/km is observed, and in the SPC and SthAC, conversely, there is an increase by 10.6 ± 4.3 × 10^–3 and 8.2 ± 5.4 × 10^–3 cm/km, respectively.