The Antarctic Bottom Water (AABW) has warmed since the 1980s or 1990s, most noticeably near Antarctica (Aoki et al., 2005; Rintoul, 2007; Johnson et al., 2008a; Purkey and Johnson, 2010; Kouketsu et al., 2011), but with warming detectable into the North Pacific and North Atlantic Oceans (Johnson et al., 2008b; Kawano et al., 2010). The warming of AABW between the 1990s and 2000s contributed to global ocean heat uptake (Section 3.2). The global volume of the AABW layer decreased by 8.2 [5.6 to 10.8] Sv during the last two decades (Johnson et al., 2008b; Mauritzen et al., 2012; Purkey and Johnson, 2012), making it more likely than not that at least the export rate of AABW from the Southern Ocean declined during this period.

The sources of AABW in the Indian and Pacific sectors of the Southern Ocean have freshened in recent decades. The strongest signal (0.03 per decade, between 1970 and 2008) is observed in the Ross Sea and has been linked to inflow of glacial melt water from the Amundsen and Bellingshausen Seas (Shepherd et al., 2004; Rignot et al., 2008; Jacobs and Giulivi, 2010). Freshening has been observed in AABW since the 1970s in the Indian sector (Rintoul, 2007) and between the 1990s and 2000s in the Pacific sector (Swift and Orsi, 2012; Purkey and Johnson, 2013).

In the Weddell Sea (the primary source of AABW in the Atlantic), a contraction of the bottom water mass was observed between 1984 and 2008 at the Prime Meridian, accompanied by warming of about 0.015°C, and by salinity variability on a multi-annual time scale. Transient tracer observations between 1984 and 2011 confirmed that the AABW there has become less well ventilated over that time period. The changes in the AABW, however, seem to be caused by the much stronger trends observed in the Warm Deep Water, as WDW is entrained into the AABW while sinking to the bottom, and not by changes in the AABW formation rate (Huhn et al., 2008; Huhn et al., 2013).

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