Australia: The Land Where Time Began

A biography of the Australian continent 

Bottom Water Export from Western Ross Sea, 2007-2010

Bottom water exported from the Ross Sea between February 2007 and January 2011 exhibited seasonal and interannual variability. In the late austral summer into the autumn temperature minima coupled with salinity maxima indicate input from High-Salinity Shelf Water (HSSW). In spring secondary temperature minima without the high-salinity trait, characteristic of Low-Salinity Shelf Water (LSSW), appear. In the winter and early summer warmer bottom water that is similar to modified Circumpolar Deep Water (mCDW) is observed. As the HSSW pool retreats polewards from the shelf break in response to increased winter polar easterlies that allow less dense overlying water to spill into the deep ocean within the benthic layer, LSSW and mCDW may be drawn from the Drygalski Basin. From 2007 – 2011 salinity at the bottom decreased by 0.007 per year which is significantly higher than regional decadal trends, which Gordon et al. propose results from the retreat of HSSW that is induced by strengthening easterlies.

Dense water descends at several places along the Antarctic margin over the continental slope within gravity currents and entrains ambient water that is less dense, all of which contribute to Antarctic Bottom Water (AABW) (Legg et al., 2009; Heywood et al., 2009). Primary sites of dense shelf water are the Weddell Sea and the Ross Sea, as well as other sites along the margins of Antarctica to the south of Australia and within the Indian Ocean sector (e.g. Baines & Condie, 1998; Legg et al., 2009; Ohshima et al., 2013). The total volume of Antarctic shelf water exported in this way is estimated as 5.4 Sv, based on the concentrations of chlorofluorocarbon, which forms 17.5 Sv of Antarctic Bottom Water (1 Sv – 10⁶m³s¹ = 10 million cubic metres per second) (Orsi et al., 2002). A particularly salty variety of AABW is formed in the western Ross Sea site (Gordon, 1974; Jacobs et al., 1985; Orsi & Wiederwohl, 2009) which accounts for the formation of about 25 % of the AABW that is formed (Orsi et al., 2002).

There has been speculations as to how the rate of formation and properties of AABW respond to global climate change (Stewart & Thompson, 2013, 2015) that was triggered by changes in the global abyssal waters (Purkey & Johnson, 2013) as well as observations of stratification changes on the continental shelf of Antarctica that are associated with increased glacial melt (Jacobs & Giulivi, 2010; Russo et al., 2011; Rignot et al., 2013; Schmidtko et al., 2014). Time series that resolve the seasonal cycle, which is large in comparison to the longer-term trends in many locations, e.g., within the Weddell Sea (Gordon et al., 2010) and the Ross Sea (Bergamasco et al., 2003, 2004; Gordon et al., 2004, 2009a; Budillon et al., 2011) is required to relate changes in AABW to climate change forcing.

Varied forms of Ross Sea shelf water were observed during the 2003-2004 Anslope program (Gordon et al., 2009b) and the Climate Long-term Interaction of the Mass balance of Antarctica program (Budillon et al., 2002) escaping from the continental shelf and descending as a gravity current that is 300-400 m thick over the continental slope (Gordon et al., 2009a; Visbeck & Thurnherr, 2009; Budillon et al., 2011), and at grater depths tending to geostrophic flow, and at 1,500 – 2,000 m isobaths off Cape Adare. In the Ross Sea the most energetic gravity currents that have been observed over the continental slope are relatively salty, up to 34.82, that are derived from export of HSSW (S=34.7), much of which is produced by air-sea processes within the Terra Nova Polynya (Fusco et al., 2009; Cappelletti et al., 2010; Rusdano et al., 2013). To the east of Pennell Bank in the Glomar Challenger Basin, does not allow access to the slope of shelf water that has a salinity above 34.72, while an escape route for HSSW with a salinity of up to 34.78 is offered by the Joides Basin (Budillon et al., 2002, 2011; Orsi & Wiederwohl, 2009). During the austral summer-autumn period, December – May, The Anslope 2003-2004 mooring time series (Gordon et al., 2009a) revealed HSSW downslope bottom currents of more than 1 metre per second, which correlates with northwards shifts of the shelf-slope front, as was observed by Anslope moorings over the outer continental shelf.

A western boundary pathway for dense water exported from the central and western topographic troughs of the Ross Sea is provided by Cape Adare. The northward transport of benthic water that was observed off Cape Adare in the austral summer of 2004 was about 1.75 Sv, composed of about 25 % of HSSW (Gordon et al., 2009a). Additional water into the deep ocean from the Ross Sea, which is composed mostly of low-salinity shelf water (LSSW; including Ice Shelf Water) and modified Circumpolar Deep Water (mCDW), was found to occur to the east of Iselin Bank (Gordon et al., 2009a; Orsi & Wiederwohl, 2009).

There were 2 moorings, one at 1,753 m sea floor depth, and the other at 1,920 m, were deployed off Cape Adare (Cape Adare Long-term Moorings; CALM) from January 2008 to January 2011 in order to build a time series of temperature, salinity and velocity data within the lower 500 m of the water column, to monitor the export of dense bottom water from the western Ross Sea. A single mooring was deployed at the deeper site from February 2997 to January 2008.

Conclusions

Nearly continuous observations of the outflow of AABW that was generated within the western part of the Ross Sea over the 4 years from 2007 through to 2010 is represented by the CALM moored data set. The resolution of the seasonal variability and the revelation of interannual changes were allowed by the length of the time series. The maximum bottom speeds occur in the late austral summer into autumn, the time of the coldest and saltiest events marking a HSSW source. It has also been reported for the Weddell Sea that export of Weddell Shelf water occurs in late summer (Gordon et al., 2010; McKee et al., 2010). In October-November a secondary cold bottom water event, that is not coupled to high salinity, was recorded, which suggests a LSSW origin. In both January and July the warmest and freshest bottom water occurs, which represents the presence of mCDW at the sea floor. Gordon et al. attributed the seasonality of the bottom water reaching the CALM moorings to the seasonal cycle of the polar easterlies.

Over the 4 year CALM time series it is evident there is a salinity decline. The trend, however, is 10 times the decadal regional decrease in salinity of the HSSW, and it is suggested there is a more local, dynamic effect: resulting from a strengthening of the polar easterlies the contribution of HSSW from the western Ross Sea is reduced.

Gordon et al. found that an important factor in governing the shelf water that is exported within the Ekman benthic layer into the gravity currents of the continental shelf is the magnitude of the polar easterlies along the shelf break, which is consistent with the results published (Stewart & Thompson, 2012, 2013, 2015) and an overview that was presented (Heywood et al., 2014). The densest shelf water, HSSW, is shifted polewards by increased polar easterlies which allows the shelf water that is less dense, e.g., LSSW and CDW, to feed into the gravity currents. Gordon et al. speculate that the role of the margins of Antarctica to deep ocean overturning in our future climate is dependent on the strength of the polar easterlies, as is suggested by the model based research of Stewart & Thompson (2012, 2013), as well as the air-sea buoyancy flux.

Sources & Further reading

1. Gordon, A. L., B. A. Huber and J. Busecke (2015). "Bottom water export from the western Ross Sea, 2007 through 2010." Geophysical Research Letters 42(13): 2015GL064457.