The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 6 8 Sv (1 Sv 5 106 m3 s-1) across the central Weddell Sea and to intensify to 54 6 15 Sv further offshore. This circulation injects 36 6 13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 6 23 mSv of freshwater, including 13 6 1 mSv as sea ice to the midlatitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13 6 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by midgyre upwelling (at a rate of 2 6 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 6 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6 6 2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.

At ime series of the physical and biogeochemical propertieso fS ubantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantialvariability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variationsi nw intertimea ir-sea turbulent heat fluxes and net evaporation modulated by the El Niñ o-Southern Oscillation(ENSO).Despite their spatial proximity, the AAIW in the Drake Passage has avery different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the BellingshausenSea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air-sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula.Aprominent exception to the prevalentmodes of SAMW and AAIW formationo ccurred in 1998, when strong wind forcing associatedw ith constructive interference between ENSO and the southern annular mode (SAM) triggered atransitory shift to an Ekmandominated mode of SAMW ventilation and a1-2-yr shutdown of AAIW production.The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ;0.38C) and salinified (by ;0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed.I n contrast, AAIW underwent an et freshening (by ;0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ;30-yr oscillation in regional air-sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation,w ith aS AMdriven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, af reshening of the Winter Water ventilatingA AIW was brought about by increased precipitation and ar etreat of the winter sea ice edge, which were seemingly Corresponding author address: Dr. Alberto C. Naveira Garabato,School of Ocean and Earth Science, NationalOceanography Centre, Southampton SO14 3ZH, United Kingdom. E-mail: acng@noc.soton.ac.uk forced by an interdecadal trend in the SAM and regional positive feedbacksinthe air-sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemispherec limate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider So...