Hydrodynamics between Africa and Antarctica during Austral Summer 2008

Abstract

Under the International Polar Year endorsed project (IPY#70), the southwest Indian Ocean sector of the Southern Ocean was surveyed during the austral summer of 2008 by deploying expendable CTD probes along the ship tracks : Cape Town–India Bay, Antarctica (Track-1) and Prydz Bay, Antarctica–Cape Town (Track-2). The meteorological data revealed that the unstable marine atmosphere boundary layer (MABL) facilitated a turbulent heat loss of 45 Wm − 2 on average, in conditions of variable wind intensity north of 43°S along Track-1; south of 63°S and north of 51°S the ocean was conducive to higher turbulent heat loss amounting to 95 Wm − 2 (on average) along Track-2. Surface imprints of hydrological fronts were determined by using surface gradient and subsurface temperature and salinity indicators. The core of Agulhas Current was identified between 36.5° and 37.5°S along Track-2, while the Agulhas Retroflection (AR) Front was located at 39.7°S south of Cape Town. The Subtropical (STF), Subantarctic (SAF) and Polar Fronts (PF) exhibited double frontal structures, whose meridional meandering is governed by bottom topography and modulated by planetary vorticity. A large southward deviation in the position of southern PF by 3.5° latitude on Track-1 was observed. Northern and southern SAF and southern STF meander by 2°–3.5° northward; their merger just north of Crozet Island facilitate an enhanced baroclinic transport of 12 Sv in the upper 1000 m. Three anticyclones that detach from the AR transport 17 Sv into the southeast Atlantic. The baroclinic transport contributed by AC and its retroflection across Track-2 amounted to 17.6 Sv. More than 50% of the ACC transport was confined to the 100–500 m depth layer. Water masses have been identified and their zonal extent quantified along the tracks. Strong convective cooling is responsible for the production of Subtropical Mode Water in the eastern Crozet Basin, which was detected near 43.5° and 41.5°S along Track-1 and -2, respectively. The Circumpolar Deep Water was encountered between 600 and 900 m at 51°S on Track-1 and spanned 54°–60°S on Track-2. We also offer an example of the modification of the MABL through strong air–sea interaction in the vicinity of South Africa by employing satellite-derived surface winds and heat fluxes.