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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |