The Weddell Sea sector in the instrumental period

This page is part of the topic The Southern Ocean in the instrumental period

The Weddell Sea hosts a subpolar gyre (e.g. Treshnikov, 1964), the Weddell Gyre (Figure 1.10), that brings relatively warm, salty circumpolar water (Warm Deep Water, WDW) south towards the Antarctic continent, and transports colder, fresher waters northward (Weddell Sea Deep and Bottom Waters, WSDW and WSBW, as well as surface waters). The transformation of this source water mass is one of the major climate-relevant processes in the Southern Hemisphere, affecting and involving ocean, atmosphere and cryosphere and a major contribution to the deep meridional overturning cell (Figure 1.9).

In respect to climate change, the most relevant property is the change in the intensity of the overturning circulation, i.e. the transport of shallow source water masses into the formation areas and the newly formed water masses leaving the Antarctic Systems into the world oceans. However, since these transport differences are relative small in comparison to the gyre scale circulation, the latter must be known to distinguish between recirculation and net transformation.

The cyclonic Weddell Gyre is bounded to the west by the Antarctic Peninsula, to the south by the Antarctic continent and to the north by a chain of roughly zonal ridges at ~60ºS. The eastern boundary is less well defined but is generally agreed to extend as far east as ~30º E (Gouretski and Danilov, 1993). The flow associated with the Antarctic Slope Front, a boundary current tied to the steep topography of the Antarctic continental slope, contributes a major proportion of the gyre transport. The Weddell Gyre is primarily driven by the cyclonic wind field (Gordon et al., 1981), leading to a doming of isopycnals in the centre of the gyre. The volume transport is still an ongoing topic of research, partly because it is dominated by the barotropic component (Fahrbach et al., 1991) so it is difficult to measure. Integrating the wind field in a Sverdrup calculation revealed a volume transport of 76 Sv (Gordon et al., 1981) while the first attempts to reference the geostrophic shear to current meters yielded a transport of 97 Sv (Carmack and Foster, 1975). For a section across the Weddell Gyre from the tip of the Antarctic Peninsula to Kapp Norvegia, transports have, uniquely, been referenced to a long time series current meter array, and this has produced lower transport estimates of 20-56 Sv (Fahrbach et al., 1991; Fahrbach et al. 1994). At the Greenwich meridian, larger transports of the order of 60 Sv were obtained from referencing shear to shipboard ADCP (Schröder and Fahrbach, 1999). Current meter arrays subsequently yielded 45–56 Sv (Klatt et al., 2005). The larger values at the Greenwich meridian may be due to recirculations within the central gyre not being captured by the Kapp Norgvegia section. However, a recent high resolution section at the Antarctic Peninsula also yielded ~46 Sv (Thompson and Heywood, 2008), and it is suggested that referencing the geostrophic shear across the steep Antarctic continental slope to a relatively widely spaced current meter array may have underestimated the barotropic contribution to the total volume transport.

WSBW is formed on the Antarctic continental shelves where they are wide. In the Weddell Sea, the Filchner-Ronne Ice shelf is one such source region (e.g. Foldvik et al., 2004), and recent evidence suggests formation also on the eastern side of the Peninsula near the Larsen Ice Shelf. The water on the Antarctic continental shelves is typically fresher than its warmer, salty source to the north; it is believed to freshen by the addition of sea ice melt, glacial ice melt from the Antarctic ice sheet and floating ice shelves, and from precipitation. This freshening is a necessary precursor to the bottom water formation process, which involves salinification from brine rejection during sea ice formation, together with cooling. One contributing process to WSBW involves mixing with Ice Shelf Water, and the other process involves mixing with HSSW. The WSBW descends the continental slope and entrains ambient water as it goes (Baines and Condie, 1998). The descent of the WSBW affects the structure of a series of fronts along the rim of the Weddell Gyre, in particular the Antarctic Slope Front and the Weddell Front.

The WSBW is too dense to be able to escape the Weddell Sea. It can only escape by mixing with the water above it, becoming warmer, saltier and less dense, and forming WSDW, which is sufficiently shallow to flow out through the passages in the topography surrounding the Weddell Sea. WSDW outside the Weddell Sea has the water mass properties of Antarctic Bottom Water, of which it is believed to be the major contributor. Estimates of the proportion of Antarctic Bottom Water originating in the Weddell Sea range from 50 to 90% (Orsi et al., 1999). An inflow of water of WSDW properties from the east has been documented (Meredith et al., 2000) that may form in the region of the Prydz Bay Gyre, or may even originate in the Australian Antarctic Basin and enter the Weddell Sea through the Princess Elizabeth Trough (Heywood et al., 1999).

The export of WSDW to the world ocean is of the order of 10±4 Sv (Naveira Garabato et al., 2002). This can escape through gaps in the ridges to the north and east of the Weddell Sea, and subsequently invades all ocean basins.

Measurements of water mass transports exist in relation to climate time scales only as snapshots and not over a long enough times to be able to detect changes. Therefore, one has to measure water mass properties and derive from their variations conclusions about changes of transport and formation rates. Carefully validated models play a significant role here. Even for water mass properties observations in sufficient spatial resolution and accuracy last only over one to two decades. Therefore it is still not possible to unambiguously distinguish between a trend and decadal to multidecadal variation. In spite of the fact that the variability in the deep water masses seems to be relatively small in comparison to those of the surface water masses, it is importance, because the large volume of the deep waters can store significant heat quantities or dissolved substances even if the changes in property is only minor.

Considering its remote and inhospitable location, the Weddell Sea was well observed during WOCE and subsequently through CLIVAR, with (largely summer-time) hydrographic sections across the Weddell Gyre onto the Antarctic continental shelf, and with arrays of moorings. These sections indicate a number of decadal-scale changes in water mass properties. The WDW warmed by some 0.04º C during the 1990s (Robertson et al., 2002; Fahrbach et al., 2004) and has subsequently cooled (Fahrbach et al., 2004). This was accompanied by a salinification of about 0.004 (note salinity does not have any units), just detectable over the decade. A quasi-meridional section across the Weddell Gyre occupied in 1973 and 1995 revealed a warming of the WDW in the southern limb of the gyre by 0.2º C accompanied by a small increase in salinity, whereas there was no discernible change in the northern limb of the gyre (Heywood and King, 2002). There has been debate as to whether these changes to the warm inflow to the gyre are caused by advection of warmer circumpolar waters, and/or by changes in the wind field (Fahrbach et al., 2004, 2006; Smedsrud, 2005).

During the 1970s a persistent gap in the sea ice, the Weddell Polynya, occurred for several winters. The ocean lost a great deal of heat to the atmosphere during these events. The polynya strongly affected the mode of overturning in the Weddell Sea, shifting it from shelf and slope processes to open ocean convection (Gordon, 1978) with consequences for the overturning rates. After this one event the large polynya did not show up again, but weak ice cover was observed in the Maud Rise area several times and interpreted as a sign of possible emergence of a new polynya, though no such polynya emerged. In a recent study, Gordon et al. (2007) attribute the occurrence of the Weddell Polynya to variations in the SAM.

Weddell Sea Bottom Water is observed on the western continental slope and within the basin up to the Greenwich Meridian. Whereas the bottom water was warming and getting more saline in the basin (Fahrbach et al., 2004) it became colder and fresher on the slope in the western Weddell Sea (Heywood et al, in preparation). It is a matter of debate if such regional changes in phase are caused by the time lag of 5-10 years involved when freshly formed bottom water spreads across the basin.

Because much of the Weddell Sea is covered with sea ice for much of the year, there is a lack of observations on the continental shelf and slope, especially in winter. This was a priority area during IPY and moored arrays were deployed together with hydrographic sections to fill the observational gaps. Measurements beneath the sea ice in the Weddell Sea were obtained for the first time by acoustically tracked floats and by instruments carried by marine mammals such as elephant seals.