MaudNESS is aimed at understanding the mechanisms by which deep ocean convection in the Weddell Sea can become well enough established to remove an existing ice cover in late winter, and to possibly instigate an alternative mode of air-sea-ice interaction (Gordon 1991), where formation of sea ice is ephemeral even in winter, and where transfer of heat from deep ocean to atmosphere is much more intense than at present.
Why is it important to understand deep ocean convection in the Weddell Sea? During the late 1970s a large expanse of open water (or low concentration sea ice) persisted for several seasons well within the confines of the winter sea ice limits of the Weddell Sea. The feature formed first near Maud Rise, a bathymetric highland centered near 65oS, 2oE, then drifted slowly westward into the central basin of the Weddell Sea (e.g., Carsey 1980). Gordon (1991) interpreted the feature, which has become known as the Weddell Polynya, as manifestation of deep ocean convection far removed from the continental margin, a view bolstered by large changes observed in Weddell Deep Water (WDW) properties in the central Weddell (Gordon 1982; Foldvik et al. 1985). Gordon suggested that the persistent polynya indicated a mode of air-sea-ice interaction different from the present, one in which ice formation is relatively infrequent even in winter.
The impact of Weddell Polynya convection on heat content of WDW in the central Weddell Sea is illustrated in Fig. 1A (adapted from McPhee 2003). Temperature and salinity profiles (meeting minimum vertical resolution and other quality criteria) were obtained from the NODC database in the time span 1965 to 1996, in the central Weddell Sea over the deep basin, away from the direct influence of both Maud Rise and the continental slopes (delimited by 70o S to 64oS and 40o W to 20o W). While coverage in this remote locale is relatively sparse, the impact of the Weddell Polynya in the late 1970s is obvious. Since the polynya years there has also been a shoaling trend in the depth.
The amount of energy involved in water temperature changes of the magnitude indicated by Fig. 1A is enormous, as shown by comparing a temperature profile observed in 1977, with nearby temperature profiles observed in later years (Fig. 1B). The integrated change in sensible heat from 1977 to 1996 of the water column from 150 m depth (below the summer seasonal mixed layer) to 3000 m, is about 4.6 GJ m-2. The heat extraction needed to cool the 1977 profile from Tmax values observed a few year earlier is thus roughly equivalent to the amount of heat required to melt 18 m of sea ice. If the cooling was mostly from vertical exchange (cooling and ventilation of WDW), it is little wonder that the Weddell Polynya was ice free in winter. Although hydrographic data are too sparse to assess the horizontal extent of WDW cooling in the late 1970s, satellite imagery (Carsey 1980) suggested that the polynya occupied at least 2-3x105 km2 in the years 1974-1976. The interannual persistence of the polynya indicates that once convection is established, it tends to maintain itself.
Widespread reversion to the type of convection apparently prevalent during the polynya years could have significant climate impact. The Southern Ocean, particularly the Weddell Sea, plays a leading role in maintaining the deep and bottom waters of the global ocean (Broecker et al. 1998). Even localized, persistent deep convection significantly alters the formation rate for dense water that can then be exported to lower latitudes. Paleoceanographic evidence suggests that during the last glacial maximum, the abyssal waters of the world ocean were dominated by waters of Southern Ocean origin, far saltier and colder than at present (Adkins et al. 2002), indicative of a much more energetic convective mixing regime in the Antarctic. Air temperatures inferred from Greenland and Antarctic ice cores during the large climate swings of the Dansgaard-Oescher cycles of the last glaciation (with typical periods of about 1,500 y) show temperatures in regions south and downwind of the South Atlantic to be in antiphase with temperature elsewhere on the globe (Alley et al. 1999). There appears to be a lag of about four centuries between cooling in the north and warming in the south (R. Alley, pers. comm.). One can conjecture that atmospheric warming in the south resulted from cooling and ventilation of the Southern Ocean (a mode change). As deep convection in the North Atlantic slowed or stopped, deep waters of the world ocean would warm, with the signal propagating southward, eventually increasing the contrast between near surface and deep water in the Weddell. As described below, this temperature contrast is an important element in setting the stage for thermobarically aided convection. Thus in this scenario, the Weddell Polynya could represent a microcosm of the response of the Southern Ocean to abrupt climate change, one which occurred in recent times, and may under the right conditions, reappear. Our goal is a realistic assessment of conditions that would again lead to widespread deep convection in the Weddell Sea.