Marine pelagic ecosystems: the West Antarctic Peninsula

3. Physical oceanography

The most voluminous source of ocean heat and nutrients in the Southern Ocean, Circumpolar Deep Water (CDW), is transported by the Antarctic Circumpolar Current (ACC). Research in the PAL-LTER (Martinson et al. in press) and throughout the Southern Ocean (Orsi et al. 1995) show that the climatological southern edge or boundary of the ACC (SBACC, defined in Orsi et al. 1995 as the southern limit of Upper CDW (UCDW) characteristics) lies along the continental shelf break in the WAP region. To the north is the SACCF (the southernmost current core of the ACC). The close proximity of the ACC to the broad continental shelves of the WAP (including the shelves of the Amundsen and Bellingshausen seas at the base of the WAP to the southwest) makes this region oceanographically unique in the Antarctic.

Fundamental to the WAP is the relationship of shelf water masses to those of the ACC. Key water masses as they appear in the WAP austral summer have been analysed and discussed in detail by Martinson et al. (in press)Gordon (1971) distinguishes between UCDW and Lower CDW (LCDW), noting that these are distinguished by temperature (UCDW) and salinity (LCDW) core layer maxima; the latter is absent over the shelf in the WAP. Martinson et al. (in press), wishing to relate shelf waters to those delivered to the region by the ACC, restricts the definition of UCDW to that as it occurs in the ACC immediately offshore of the WAP (hereafter referred to as ‘ACC-core UCDW’). When ACC-core UCDW is swept onto the shelf, mixing cools it to form modified UCDW (M-UCDW). Unmodified UCDW incursions occasionally survive short distances on the shelf (figure 4). Incursions most consistently move onto the shelf at the northern end of the large cross-shelf channel (Marguerite Trough) at the 300 cross-shelf line (figures 1 and ​and4).4). Incursions of UCDW are consistent with the dynamic topography (circulation), indicating interactions of the ACC with shelf bathymetry as the key physical mechanism driving the appearance of UCDW on the shelf.

Figure 4

Annual dynamic topography as blue contours superimposed on a tripartite grey-scale bathymetry (dark grey≤450 m; 450 m<light grey≤750 m bathymetry and white>750 m). Locations where ACC-core UCDW appears anywhere in water column shown by red triangles. Note frequent occurrence of UCDW on shelf at the 300 line, in mouth of trough west of Adelaide Island (Marguerite Trough).

Winter water is prevalent throughout the Antarctic polar waters. This water is formed at or very near the freezing point—being the remnant winter mixed layer water—but here the summer values are well above freezing due to vertical mixing with the warmer waters above and below (Klinck 1998; Smith et al. 1999a; Martinson et al. in press). The most conspicuously absent Antarctic water masses on the WAP shelf are the low- and high-salinity shelf waters (LSSW, HSSW) found at depth in numerous shelf locations around the continent (Carmack 1977). These waters, near the freezing point, with 34.6 salinity delimiting LSSW from HSSW, are notable for their role in deep and bottom water formation (Gill 1973). This absence is consistent with the notion that bottom waters do not form in the WAP region today. LCDW is not commonly seen on the WAP shelf.

UCDW is quickly modified (cooled by mixing) as it moves across the shelf, cooling approximately linearly with distance from the slope (source) of the ACC-core UCDW. The significance of the cooling of this relatively warm water (3–4°C above the freezing point) on the continental shelf is that the heat is passed from the water either to the atmosphere through leads and other openings or to the underside of ice (both sea ice and marine glaciers) thus melting it. This is important given the role of glacial ice melt to rising sea level, and the ocean heat is the only source of enough heat to melt this ice (the heat content of water is 1000 times larger than that of a comparable volume of air at the same temperature above freezing). Recent research in the Pal-LTER region, using two different approaches for estimating the ocean heat flux suggests that the heat flux from the ocean has resulted in a substantial increase in the water temperature and associated heat flux beginning in the 1990s (figure 5; comparable with a number of other changes documented throughout the region for sea ice and other climate variables, Stammerjohn et al. in press a). Figure 5 shows that the increase in heat flux since 1990 is sufficient to cause a ∼0.7°C warming of the upper 300 m of the water column below the winter mixed layer—and indicates that the warming noted by Meredith &amp; King (2005) extends well below the surface layer. There was a further jump in the heat flux after 1998, with an increasing trend since then (figure 5b). This increase is a profound change in the physical environment and underlines the role of ocean circulation as the principal driver translating climate warming into ecosystem changes on the WAP shelf. The heat flux is also a proxy for nutrient fluxes because UCDW is the primary imported source of these as well as heat; see §4a.