Water Mass Characteristics of the Antarctic Margins and the Production and Seasonality of Dense Shelf Water

Abstract

Conductivity-temperature-depth data from instrumented seals of the Marine Mammals Exploring Oceans from Pole to Pole program are analyzed to characterize the water masses and the seasonality of the marginal seas. Bottom temperatures are found to be in a cold regime in Dense Shelf Water (DSW) producing regions, identified in this study as the southern Weddell Sea, Cape Darnley, Prydz Bay, Adélie Coast, and the western Ross Sea. DSW occupies the bottom of the Weddell and Ross Sea continental shelves throughout the year: Production of DSW and vertical overturning occur only during the winter. In the other DSW producing regions, salinity is reduced more markedly during the summer. We identify the Princess Martha Coast, Leopold and Astrid Coast, and the Knox Coast as Low Salinity Shelf Water producing regions, where modified Circumpolar Deep Water (CDW) intrudes onto the continental shelf, reaching areas close to the ice shelves keeping the bottom temperatures in an intermediate regime.The Prince Harald Coast, the Amundsen Sea, and the Bellingshausen Sea experience more intense CDW intrusion, which keeps them in a warm regime year-round. CDW layer thicknesses correlate with the meridional winds over the shelf sea, and with the zonal winds at the slope, while DSW layer thicknesses correlate with the meridional winds over the shelf seas and the curl of the wind stress over the slope. Locations of DSW on the continental shelf coincide with an absence of warmer CDW near the ice shelves.

Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica

“…Unlike the Totten region, both mCDW and DSW are found in Vincennes Bay. Although that could indicate that where and when DSW is present in the bay there is no mCDW (Narayanan et al., 2019), the interplay in the polynyas suggests otherwise. Both DSW and mCDW were found to be accessing the local ice shelf cavities and driving basal melt.…”

“…Silvano et al. (2017) showed that meltwater input to the Dalton Polynya results in increased stratification and impedes winter convection near the Totten Glacier, which allowed mCDW intrusions to persist on the shelf (Narayanan et al., 2019). Our observations in Vincennes Bay show that mCDW intrusions are driving local glacial melt, which increases stratification making it harder for polynyas to generate deep convection.…”

Antarctica and the Southern Ocean

“…The 2004-17 MEOP data (Roquet et al 2014Treasure et al 2017) offer a means to investigate Antarctic shelf water masses. Narayanan et al (2019) showed that the presence of dense shelf waters coincides with an absence of warmer Circumpolar Deep Water. As part of that study, mean temperatures were quantified at depths ranging from 250 m to the bottom, and from the coast or ice shelf edge (75 km and beyond) to the continental shelf and slope (shallower than 3000 m).…”

Supercooled Southern Ocean Waters

“…Most (68%) "ice-shelf" supercooled profiles are shallower than 600 m. Only 14% of the "ice-shelf" profiles have potential supercooling above 200 m, and in 10% of the cases, it occurs above the permanent pycnocline (Figure 4h). The rather deep signal of potential supercooling in these profiles is largely associated with DSW in the four locations identified as source regions for AABW: the Weddell Sea, Ross Sea, Adélie Coast, and Prydz Bay (Figure 4e; Foldvik et al, 2004;Narayanan et al, 2019). While the "ice-shelf" criteria capture 10.1029/2020GL090242 supercooled water at depth, ISW that rises toward the surface will likely fall into our category characterized by the presence of potential supercooling over the entire water column or a maximum degree of supercooling at depth and at the surface (section 2.3).…”

“…Thus, the resulting Ice Shelf Water (ISW) would be potentially supercooled relative to a corresponding surface freezing point of about −1.85°C. Depending on its density, ISW either sinks as potentially supercooled Dense Shelf Water (DSW) and may leave the continental shelf to form Deep Waters and Antarctic Bottom Water (AABW; Foldvik et al., 2004; Narayanan et al., 2019) or rises along the bottom of the ice shelf (Countryman, 1970; Lewis & Perkin, 1986, Figure 1). If ISW plumes rise, their local freezing point may increase above the water temperature due to the decreasing pressure, leading to in situ supercooling.…”