Early summer thermohaline characteristics and mixing in the western Weddell Sea

Abstract

The Ice Station POLarstern (ISPOL) cruise revisited the western Weddell Sea in late 2004 and obtained a comprehensive set of conductivity–temperature–depth (CTD) data. This study describes the thermohaline structure and diapycnal mixing environment observed in 2004 and compares them with conditions observed more than a decade earlier. Hydrographic conditions on the central western Weddell Sea continental slope, off Larsen C Ice Shelf, in late winter/early spring of 2004/2005 can be described as a well-stratified environment with upper layers evidencing relict structures from intense winter near-surface vertical fluxes, an intermediate depth temperature maximum, and a cold near-bottom layer marked by patchy property distributions. A well-developed surface mixed layer, isolated from the underlying Warm Deep Water (WDW) by a pronounced pycnocline and characterized by lack of warming and by minimal sea-ice basal melting, supports the assumption that upper ocean winter conditions persisted during most of the ISPOL experiment. Much of the western Weddell Sea water column has remained essentially unchanged since 1992; however, significant differences were observed in two of the regional water masses. The first, Modified Weddell Deep Water (MWDW), comprises the permanent pycnocline and was less saline than a decade earlier, whereas Weddell Sea Bottom Water (WSBW) was horizontally patchier and colder. Near-bottom temperatures observed in 2004 were the coldest on record for the western Weddell Sea over the continental slope. Minimum temperatures were ∼0.4 and ∼0.3 °C colder than during 1992–1993, respectively. The 2004 near-bottom temperature/salinity characteristics revealed the presence of two different WSBW types, whereby a warm, fresh layer overlays a colder, saltier layer (both formed in the western Weddell Sea). The deeper layer may have formed locally as high salinity shelf water (HSSW) that flowed intermittently down the continental slope, which is consistent with the observed horizontal patchiness. The latter can be associated with the near-bottom variability found in Powell Basin with consequences for the deep water outflow from the Weddell Sea.

Introduction

The Weddell Sea in the Atlantic sector of the Southern Ocean (Fig. 1) is a major site, perhaps the largest, for deep- and bottom-water production. It has been estimated to contribute to more than 50% of the total volume of Antarctic Bottom Water (AABW) (Carmack, 1977; Orsi et al., 1999). Other significant sites are the southwestern Ross Sea, most recently documented by Gordon et al. (2004), and the Adélie Land coastal region (e.g., Rintoul, 1998; Marsland et al., 2004). The total contribution of AABW from such sources surrounding Antarctica provides primary forcing for the meridional overturning circulation (MOC) (Orsi et al., 2001).

Acquisition of new field data, powered by international programs (e.g., ROPEX, Ice Station Weddell, DOVETAIL), has improved estimates of the relative contributions of AABW from different regions within the Weddell Sea. However, as more information is gathered, greater variability is revealed in the structure and characteristics of the deep water column, understanding of which necessitates better coverage of regions still undersampled. This is especially true for the western Weddell Sea, where high perennial sea-ice concentration (Heygster et al., 1996) severely limits access. In previous years, this region was only once comprehensively covered; during the Ice Station Weddell (ISW) experiment of February to June 1992 (Gordon, 1998).

Schröder et al. (2002) indicate that ocean-ice shelf interactions at the Filchner–Ronne Ice Shelf (FRIS) alone could not account for the variability observed in the northwestern Weddell Sea. The southeastern Weddell Sea, east of FRIS, is unlikely to make a significant contribution since the continental shelf there is narrow enough to permit Warm Deep Water (WDW) to reach the ice shelves, thus preventing the formation of saline, dense deep water (Fahrbach et al., 1994). Alternative and possibly intermittent sources are therefore needed to explain the volume and variable characteristics observed downstream. Such sources are believed to be located along the relatively unexplored western Weddell Sea continental margin (Gordon et al., 1993; Schröder et al., 2002; Schodlok et al., 2002). The latter is dominated by a strongly barotropic, slope-trapped northward boundary current that consists primarily of the western limb of the wind-driven Weddell Gyre, underlain by a near-bottom flow of dense waters formed mainly further south (Muench and Gordon, 1995). This deep flow of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW) behaves as a density flow whose pathway is controlled by the regional bathymetry. Considerable disagreement remains among available bathymetric charts, and the lack of information about bottom topography in the western Weddell Sea constitutes a great impediment to our understanding of near-bottom flow dynamics. While estimates of the relative contributions to AABW from various regions have improved with the acquisition of new field data, the seasonal, interannual, and longer-term variability in the different AABW outflows are neither well documented nor understood.

The Ice Station POLarstern (ISPOL) expedition (Hellmer et al., 2008) provided an opportunity for resampling the portion of the western Weddell Sea that was studied during the 1992 ISW experiment. Moored to a drifting ice floe for 36 days, ISPOL obtained a large hydrographic data set from the western limb of the Weddell Gyre, roughly adjacent to the Larsen C Ice Shelf (Fig. 1). The sampled area covers ∼14,000 km² within the primary northward path for water masses modified and produced in the Weddell Sea. Among the more significant of these products are recently formed deep and bottom waters that, due to their level in the water column, are able to cross the South Scotia Ridge and contribute to AABW in the global ocean basins. The western Weddell Sea also provides a source of upper ocean water for the southeast Pacific by means of a poorly defined coastal and shelf flow westward around the tip of the Antarctic Peninsula (v. Gyldenfeldt et al., 2002; Heywood et al., 2004). Interest in this sector of Antarctica has increased during the last decade in response to regional changes, possibly climate-related, that it has endured, e.g., the disintegration of Larsen A and B Ice Shelves (Skvarca et al., 1999), and pronounced atmospheric warming over the Antarctic Peninsula (King and Comiso, 2003).

This study describes the thermohaline structure and diapycnal mixing environment observed in the western Weddell Sea during ISPOL. It compares this environment with previous conditions, especially those observed in 1992 during ISW, and discusses the comparison within the context of regional processes.