Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Abstract

Abstract. Changes to the properties of Antarctic BottomWater in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γ n = 28.30 kg m −3 . Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year −1 . Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171-158 • E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23 % reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7 % weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7 % is assumed between the year 1995 and 2005) can explain 51 ± 6 % of the warming and 84 ± 10 % of the freshening observed in AA-AABW.

Dense shelf water production in the Adélie Depression, East Antarctica, 2004–2012: Impact of the Mertz Glacier calving

“…The likely influence of freshened dense shelf water from the Ad elie Land sector is also consistent with the freshening of 0.03 identified over approximately the same period (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) in the AAB bottom layer at 140 E on the continental slope just downstream of the Ad elie Land shelf [Aoki et al, 2005]. Although Shimada et al [2012] have recently suggested that a large (84%) part of the overall AAB freshening could be explained by observed changes in the transport and properties of the RSBW, they acknowledge that localized cooling (as opposed to basin wide warming) and enhanced freshening signals identified over the slope near the ALBW source region do indicate the influence of a fresher, colder, bottom water source there.…”

“…Several studies have reported significant changes in the properties of the AABW in the Australian Antarctic Basin during the past decades [Whitworth, 2002;Aoki et al, 2005;Rintoul, 2007;Johnson et al, 2008;Shimada et al, 2012]. As often stated, such changes may just reflect changes in the properties or proportion of ambient water entrained or mixed with the source waters along their path across the basin.…”

“…As often stated, such changes may just reflect changes in the properties or proportion of ambient water entrained or mixed with the source waters along their path across the basin. Recently, Shimada et al [2012] proposed that increased vertical diffusion of heat into the RSBW layer during its transit to the AAB could explain the bottom warming in the AAB over the last two decades. Yet, when considering the bottom freshening of the AAB, there is also evidence that it may relate, at least partly, to freshening of the source dense waters from the Ross Sea and the Ad elie/George V Land sector.…”

“…Off the shelf in the AAB, as well as on the Ad elie Land shelf itself, most observational studies dealing with bottom freshening have concentrated on trends [Whitworth, 2002;Aoki et al, 2005;Rintoul, 2007;Shimada et al, 2012]. The repeat hydrographic stations were, however, too unevenly spaced in time to estimate year-to-year changes and consequent aliasing on the estimated trends.…”

Southern Ocean Warming

“…Rhein et al, (2013); data from Purkey and Johnson (2010) Linear trends of deep ocean change, constructed from repeat sections between 1992 and 2005, reveal that most of the abyssal ocean is warming, with strongest warming close to Antarctica (Figures 1 and 4c; Purkey and Johnson, 2010;Rhein et al, 2013;Talley et al, 2016). The abyssal waters are also contracting in volume and freshening Johnson, 2012, 2013;Shimada et al, 2012;Jullion et al, 2013;van Wijk and Rintoul, 2014). The observed bottom water changes reflect the responses of bottom water source regions to changes in surface climate, to ocean-ice shelf interaction, and to downstream propagation of the signals by wave and advective processes (Jacobs and Giulivi, 2010;Masuda et al, 2010;Johnson et al, 2014;van Wijk and Rintoul, 2014).…”