The time-variable position of the Antarctic Circumpolar Current salinity fronts from Aquarius data

Abstract:

The flow of the Antarctic Circumpolar Current (ACC) is known to be concentrated in a number of jets associated with fronts that are characterized by large horizontal gradients of temperature, salinity, sea surface height, and other variables. The processes of water mass transformation as well as the meridional energy and tracer transports in the Southern Ocean are most intense near the fronts and, therefore, it is important to establish their position and spatial extent. Based on historic hydrographic data, three major ACC fronts from north to south have been identified: the Subantarctic Front (SAF), the Polar Front (PF), and the Southern ACC Front (SACCF). Later, space-borne observations of sea surface temperature (SST) and sea surface height (SSH) as well as ocean model simulations have revealed a more complex frontal structure. As part of the validation of the Aquarius data and a demonstration of its capabilities and utility for oceanographic studies, we use along-track Aquarius data for the determination of the ACC sea surface salinity (SSS) fronts between Africa and Antarctica. The position of the fronts is determined from along-track salinity gradients and validated with concurrent SST- and SSH- derived frontal positions, as well as with the positions of temperature and salinity fronts obtained from several realizations of AX25 XBT and Good Hope CTD transects in 2011-2013. We find that some of the frontal positions that we have determined, although being representative only for one particular day, are close to the time mean positions obtained from hydrography. Thus, Aquarius data demonstrate its utility for studies of the variability of the ACC frontal positions. The SSS fronts are not necessarily collocated with SST and/or SSH fronts. The integral effects of temperature and salinity may compensate each other in the Southern Ocean, so that temperature and salinity fronts are not always associated with SSH fronts. Aquarius measurements thus complement SST and SSH records and can potentially advance our knowledge about the complex structure of the ACC fronts.