Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Bottom Water in the Eastern Weddell Gyre – Remotely induced Warming

A warming of the Antarctic Bottom Water (AABW) by ~ 0.1^oC and a lightening of ~0.02-0.03 kg per m³ of the Antarctic Bottom Water entering the eastern Weddell gyre from the Indian Ocean sector of the Southern Ocean between the mid-1990s and the late 2000s was revealed by 4 repeat hydrographic sections across the eastern Weddell gyre at 30^oE. It is suggested by historical hydrographic and altimetric measurements in the region that the most likely explanation for the change is an increase in entrainment of warmer mid-depth Circumpolar Deep Water by cascading shelf water plumes close to Cape Darnley, where the Antarctic Bottom Water sourced from the Indian Ocean entering the Weddell gyre from the east is ventilated. This entrainment change is associated with a concurrent southward shift of the southern boundary of the Antarctic Circumpolar Current (ACC) in the region. Wherever the Antarctic Circumpolar Current flows close to the coast of Antarctica may be affected by warming of the Antarctic Bottom Water by this mechanism.

A key mechanism by which the heat, carbon and other tracers that are climatically important is transported around the planet is the meridional overturning circulation (e.g. Ganachaud & Wunsch, 2000). An important part of this circulation is dense bottom water, collectively known as Antarctic Bottom Water (AABW), which is produced at a few specific high-latitude regions of the Southern Ocean. The modification of the density of seawater drives the formation of Antarctic Bottom Water by air-sea-ice interactions near the margins of Antarctica (Orsi et al., 1999) that are sensitive to climate forcing.

In recent decades the Antarctic Bottom Water has undergone pronounced property changes. Widespread warming and contraction of volume of the Antarctic Bottom Water has been reported (Purkey & Johnson, 2010, 2012) across much of the global ocean abyss since the 1980s. Between the late 1960s and the 1990s AABW that had formed in the Ross Sea and off Adélie Land freshened by, respectively, ~0.01 between the late 1960s and the 1990s (Jacobs et al., 2002; Rintoul, 2007; Jacobs & Giulivi, 2010) and ~0.03 between the mid-1990s and the mid-2000s (Aoki et al., 2005; Rintoul, 2007). This has been linked tentatively to freshening of shelf water (Jacobs et al., 2002; Jacobs & Giulivi, 2010) which, according to Couldrey et al., has possibly resulted from the loss of mass of the West Antarctic Ice Sheet due to enhanced basal melting (Pritchard et al., 2012).

In this paper Couldrey et al. expand on the evidence for climatic changes in AABW properties, documenting a warming that is remotely induced of the AABW entering the Weddell gyre from the Indian section of the Southern Ocean. Couldrey et al. found by exploring the likely mechanisms for this change that a southwards migration of the southern boundary of the Antarctic Circumpolar Current off the Cape Darnley/Prydz Bay region may have brought mid-depth waters that are relatively warm close to the edge of the continental shelf, where they are entrained by cascading dense shelf water.

Conclusions

Warming by 0.1^oC and lightening by 0.02 – 0.03 kg per m³ on a decadal scale of the AABW entering the eastern Weddell gyre from the Indian sector between the mid-1990s and the mid-2000s was documented by this study. Couldrey et al. suggest the findings of their study add to recent findings of decadal scale variations in the Antarctic Bottom Water θ-S characteristics, which are used to describe either freshening (Aoki et al., 2005; Rintoul, 2007; Jacobs & Giulivi, 2010) or warming that is linked to variable water export that is wind forced (Meredith et al., 2008, 2011; Jullion et al., 2010). The work of Couldrey et al. attributes the warming of the Antarctic Bottom Water that has been observed to a new mechanism: a change in the entrainment of warm, mid-depth waters into cascading shelf water plumes. The bottom water mixture that results from this entrainment is shifted to a warmer, lighter class, which is consistent with the contraction of the coldest Antarctic Bottom Water that has been observed (Purkey & Johnson, 2010, 2012). The property changes of the AABW that was measured by Couldrey et al. are explained most plausibly by this process and is concomitant with the shift towards the pole of the southern boundary of the Antarctic Circumpolar Current in the Prydz Bay region observed between 1996 and 2008. Couldrey et al. conjecture, insofar as the southward migration of the southern boundary off Cape Darnley is part of a recent adjustment on a large scale of the Antarctic Circumpolar Current to climatic changes in wind forcing (Sallee et al., 2008; Sokolov & Rintoul, 2009), that the induction of AABW warming changes in Cold Deep Water entrainment may affect the area of formation near the coast of Adélie Land anywhere the Antarctic Circumpolar Current flows close to the Antarctic continental shelf. The way in which the wind-forced translations of the Antarctic Circumpolar Current affect the properties of the Cold Deep Water that enters the Weddell gyre, which then influences the AABW that is formed, is explained by Fahrbach et al., 2011). Therefore, such shifts of the Antarctic Circumpolar Current as the one described by Couldrey et al. could also affect indirectly the AABW that is produced in the Weddell gyres.

Sources & Further reading

1. Couldrey, M. P., L. Jullion, A. C. Naveira Garabato, C. Rye, L. Herráiz-Borreguero, P. J. Brown, M. P. Meredith and K. L. Speer (2013). "Remotely induced warming of Antarctic Bottom Water in the eastern Weddell gyre." Geophysical Research Letters 40(11): 2755-2760.