Southern Ocean currents and climate

The Antarctic Circumpolar Current (ACC) carries about 130 to 150 x 106 m3 S-l along a 20 000 km path circling Antarctica, making it the largest current in the world ocean. The flow of the ACC connects the ocean basins, allowing water, heat, and other properties to be carried from one basin to another. The interbasin connection provided by the ACC is a key link in a global ocean circulation, sometimes called the "great ocean conveyor", which strongly influences the climate of the Earth on time-scales of years to centuries. Unlike most other regions of the ocean, fluctuations of the currents play a central role in the heat and momentum budget of the Southern Ocean. The fluctuations carry momentum, supplied by the strong winds, down to where pressure forces against seafloor topography can compensate the wind forcing, and also carry heat poleward to balance the heat lost by the ocean to the cold atmosphere south of the ACC. While the circumpolar current is the dominant circulation feature of the Southern Ocean, there are important flows in the north-south and vertical planes. Deep water shoals as it spreads south across the Southern Ocean, ultimately reaching the sea surface near Antarctica. Strong interactions with the atmosphere and sea-ice modify the upwelled water where it reaches the surface: some water is made lighter by warming md freshening due to rainfall and sea-ice melt, while some is made more dense by cooling and addition of salt rejected during freezing of sea-ice. The water mass transformations driven by air-sea exchange in the Southern Ocean allow deep water to be converted to lighter intermediate water, as required to complete the loop of the global conveyor

Bottom water formation and polynyas in Adelie Land, Antarctica

Antarctic Bottom Water is the coldest and densest water found in the global ocean. It spreads into all the major ocean basins, carrying the cold water towards the equatorial regions, and is a central component of the global thermo-haline circulation. However, the mechanisms of bottom water formation are not well established; its geographical distribution and rate of formation have yet to be fully quantified. Polynyas, which are large persistent openings in sea-ice that form during the winter near the Antarctic Coast, playa central role in the formation or Antarctic Bottom Water. This paper describes the bottom water formation around the Antarctic continental margin with particular emphasis on the processes and mechanisms of the Adelie Land Bottom Water formation near Dumont D'Urville south of Tasmania

Change in dense shelf water and Adelie land bottom water precipitated by iceberg calving

Antarctic Bottom Water supplies the deep limb of the global overturning circulation and ventilates the abyssal ocean. Antarctic Bottom Water has warmed, freshened, and contracted in recent decades, but the causes remain poorly understood. We use unique multiyear observations from the continental shelf and deep ocean near the Mertz Polynya to examine the sensitivity of this bottom water formation region to changes on the continental shelf, including the calving of a large iceberg. Postcalving, the seasonal cycle of Dense Shelf Water (DSW) density almost halved in amplitude and the volume of DSW available for export reduced. In the deep ocean, the density and volume of Adelie Land Bottom Water decreased sharply after calving, while oxygen concentrations remained high, indicating continued ventilation by DSW. This natural experiment illustrates how local changes in forcing over the Antarctic continental shelf can drive large and rapid changes in the abyssal ocean

Distribution of water masses and meltwater on the continental shelf near the Totten and Moscow University ice shelves

Warm waters flood the continental shelf of the Amundsen and Bellingshausen seas in West Antarctica, driving rapid basal melt of ice shelves. In contrast, waters on the continental shelf in East Antarctica are cooler and ice shelves experience relatively low rates of basal melt. An exception is provided by the Totten and Moscow University ice shelves on the Sabrina Coast, where satellite-derived basal melt rates are comparable to West Antarctica. Recent oceanographic observations have revealed that relatively warm (∼−0.4°C) modified Circumpolar Deep Water (mCDW) enters the cavity beneath the Totten Ice Shelf through a 1100 m deep trough, delivering sufficient heat to drive rapid basal melt. Here we use observations from a recent summer survey to show that mCDW is widespread on the continental shelf of the Sabrina Coast, forming a warm (up to 0.3°C) and saline (34.5–34.6) bottom layer overlaid by cold (∼freezing point) and fresh (salinity ∼34.3) Winter Water. Dense Shelf Water is not observed. A 1000 deep m trough allows water at −1.3°C to reach the Moscow University ice-shelf cavity to drive basal melt. Freshening by addition of glacial meltwater is widespread on the southern shelf at depths above 300–400 m, with maximum meltwater concentrations up to 4–5 ml L−1 observed in outflows from the ice-shelf cavities. Our observations indicate that the ocean properties on the Sabrina Coast more resemble those found on the continental shelf of the Amundsen and Bellingshausen seas than those typical of East Antarctica

Seasonality of warm water intrusions onto the continental shelf near the Totten Glacier

Warm Modified Circumpolar Deep Water (MCDW) from the Southern Ocean drives rapid basal melt of the Totten Ice Shelf on the Sabrina Coast (East Antarctica), affecting the mass balance of the grounded Totten Glacier. Recent observations show that MCDW intrudes onto the continental shelf through a depression at the shelf break. Here we investigate such intrusions by combining (1) new oceanographic and bathymetric observations collected for two consecutive years by profiling floats in the depression south of the shelf break, (2) oceanographic measurements collected by conductivity‐temperature‐depth‐instrumented seals on continental slope, and (3) an ocean model. The depression provides a pathway for persistent inflow of warm (0‐1°C) MCDW to the inner shelf. In austral autumn and early winter MCDW intrusions were up to 0.5°C warmer and were are ~75 m thicker than in spring and summer. The seasonality of the flow on the continental slope explains the seasonality of the intrusions. The MCDW layer on the continental slope is warmer and thicker to the east of the depression than to the west. In autumn and early winter a strong, top‐to‐bottom westward current (Antarctic Slope Current) transports the warmer and thicker MCDW layer along the slope and is diverted poleward at the eastern entrance of the depression. A bottom‐intensified eastward current (Antarctic Slope Undercurrent) develops in other months, allowing cooler and thinner intrusions to enter the depression from the west. Our study illustrates how circulation on the Antarctic slope regulates the ocean heat delivery to the continental shelf and ultimately to the ice shelves