Freshening drives contraction of Antarctic Bottom Water in the Australian Antarctic Basin

Abstract

Antarctic Bottom Water (AABW) in the Australian Antarctic Basin has become fresher and lighter since the late 1960s, with largest changes observed near the sources of AABW. The volume of AABW with neutral density (γ n ) > 28.30 kg m À3 decreased by ≥50% between 1969-1971 and 2008-2012, thinning at a rate of >100 m per decade. Contraction of dense AABW was balanced primarily by expansion of water lighter than γ n = 28.15 kg m À3 prior to 1995 and by inflation of less dense classes of AABW after 1995. Oxygen concentrations remained unchanged in the densest AABW and increased on neutral surfaces for AABW with γ n > 28.31 kg m À3 , between 1995 and 2011-2012, consistent with continued ventilation over this time period. The observed changes in potential temperature, salinity, density, volume, and oxygen of AABW can be accounted for by freshening of the source waters but cannot be explained by changes in formation rate alone.

Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning

“…Recent observations show that the high-latitude Southern Ocean is becoming increasingly stratified due to freshening of its surface 25,26 . Future implications of this change, in terms of its effect on the AIS, can be assessed by comparison with periods of the recent past when similar oceanic conditions occurred.…”

Southern Ocean Warming

“…Ship-based observations, floats, and bathythermographs have been used to measure the total heat content change of the ocean in the upper 700 m, 1,000 m, or 2,000 m (Trenberth et al, 2009) to better understand the total Earth energy imbalance caused by global warming. The increasing greenhouse gases in the atmosphere create an energy imbalance at the top of Earth's atmosphere: there is more energy coming into the system than energy radiated out, because greenhouse gases trap heat in the atmosphere, which creates warming (IPCC, 2007;Trenberth et al, 2009).…”

“…The increasing greenhouse gases in the atmosphere create an energy imbalance at the top of Earth's atmosphere: there is more energy coming into the system than energy radiated out, because greenhouse gases trap heat in the atmosphere, which creates warming (IPCC, 2007;Trenberth et al, 2009). This imbalance in Earth's energy budget is one of the best metrics for determining the rate of current global warming (von Schuckmann et al, 2016).…”

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“…The global sea level rise is predicted to be between 40 cm and 1.2 m by the year 2100 and up to 3 m by the year 2300 (Horton et al, 2014). Rising ocean levels are largely a result of melting polar ice caps and are associated with a mean decrease of ocean salinity (van Wijk and Rintoul, 2014). In contrast to decreasing salinity of the pelagic ocean, rising sea level leads to increased salinization of coastal areas due to flooding and seawater invasion into freshwater aquifers.…”