The circulation and water masses of the Antarctic shelf and continental slope between 30 and 80∘E

The circulation and water masses from the Antarctic continental shelf to 62∘S between 30 and 80∘E are described using hydrographic data collected on seven hydrographic sections during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) experiment. The eastern limb of the Weddell Gyre dominates circulation between 30 and 40∘E, and is significantly cooler and fresher than the region to the east. The Antarctic Circumpolar Current (ACC) extends from the north into the survey region east of 40∘E, reaching as far south as 65.5∘S at 60∘E. This results in increasing observed maximum temperature and salinities progressively towards the east, peaking at 80∘E due to the intrusion of the southern ACC Front (sACCF) to 63∘S. This southward extension is steered by the southern end of Kerguelen Plateau, causing a horizontal shear of over 0.15ms-1 between the eastward ACC and westward-flowing Antarctic Slope Current (ASC). The ASC is observed at all six meridional sections immediately north of the shelf break. It is strongly barotropic and transports a total of 15.8±7.4Sv westwards, while the bottom referenced baroclinic component only contributes 1.3±0.3Sv. At each section this current intensifies to a narrow westward 'jet' with absolute velocities up to 0.3ms-1 over the steepest shelf slope gradients. At 70∘E a 'V' shape is observed in the ASF. This, and the nearby presence of denser shelf water and ice-shelf water, is characteristic of Antarctic Bottom Water (AABW) formation, but no new AABW is found on this section. Instead, significantly warmer, saltier and less oxygenated AABW to the east and newly formed AABW high on the continental slope immediately to the west suggest a formation region just west of 70∘E. This newly formed AABW progressively becomes warmer and saltier west of 60∘E and is observed extending offshore and moving westward below eastward flowing water masses. ACC frontal positions are found to be 1-2° farther north in the survey region than suggested by historical climatology.