Warming, Freshening, and Contraction of Antarctic Bottom Water between the 1980s and 2000s

Over the past three decades, Antarctic Bottom Water (AABW) has warmed, freshened, and declined in volume owing to both water-mass and circulation changes. Using highly accurate, full-depth, ship-based, conductivity-temperature-depth measurements taken along repeated oceanographic sections between 1981 and 2012, we quantify water property changes in the deep Southern Ocean within the bottom limb of the meridional overturning circulation (MOC). We find a contraction of AABW within the Southern Ocean and along three of the four northward outflow routes of AABW, suggesting a global scale slowdown of the bottom limb of the MOC. The contraction can be seen in a downward displacement of potential isotherms with time, associated with warming on isobars. Within the Southern Ocean, potential isotherms near the top of AABW have fallen by approximately 100 m per decade, equivalent to an 8.2 (± 2.6) Sv (1 Sv = 1 × 106 m3 s-1) contraction rate. The deep water-mass contraction is compensated near the surface by an expansion of Circumpolar Deep Water, presumably entering from the north. The observed warming from isotherm heave integrated below 2000 m in the Southern Ocean (south of 30ºS) accounts for a net heat uptake of 34 (±3) TW, a warming signal entering the deep ocean. In addition, AABW has freshened within the Indian and Pacific sectors of the Southern Ocean. Freshening of 0.02 PSS-78 per decade is observed in the newest AABW directly downstream from formation sites along the Antarctic continent, with freshening rates roughly a tenth of this in the deep interior of basins adjacent to Antarctica. The fresh water flux required to account for the observed freshening of AABW in these two basins is 73 (±26) GT yr-1, or roughly half of the estimated recent mass loss from the West Antarctic Ice Sheet. The heave and water mass changes together below the 0 °C isotherm in the Southern Ocean contribute 0.5 (±0.2) mm per year to steric sea level rise.