Modification of the deep salinity-maximum in the Southern Ocean by circulation in the Antarctic Circumpolar Current and the Weddell Gyre

Abstract

The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is assessed using a set of 37 hydrographic sections collected over a 20-year period in the Southern Ocean as part of the WOCE/ CLIVAR programme. A circumpolar decrease in the value of the salinity-maximum is observed eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, and in the Atlantic sector, this acts to limit the direct poleward propagation of the salinity signal. Limited entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddy-dominated eastern limb. A vertical mixing coefficient, κV of (2.86 ± 1.06) × 10−4 m2 s−1 and an isopycnal mixing coefficient, κI of(8.97 ± 1.67) × 102 m2 s−1 are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar Current (ACC). A κVof (2.39 ± 2.83) × 10−5 m2 s−1, an order of magnitude smaller, and a κI of (2.47 ± 0.63) × 102 m2 s−1, three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector evidenced by the eastward propagation of the deep salinitymaximumsignal, insulating theWeddell Gyre from short-term changes in NADW characteristics.