13.5: Antarctic Circumpolar Current

The Antarctic Circumpolar Current is an important feature of the ocean’s deep circulation because it transports deep and intermediate water between the Atlantic, Indian, and Pacific Ocean, and because Ekman pumping driven by westerly winds is a major driver of the deep circulation. Because it is so important for understanding the deep circulation in all ocean, let’s look at what is known about this current.

A plot of density across a line of constant longitude in the Drake Passage (figure \(\PageIndex{1}\)) shows three fronts. They are, from north to south: 1) the Subantarctic Front, 2) the Polar Front, and 3) the Southern ACC Front. Each front is continuous around Antarctica (figure \(\PageIndex{2}\)). The plot also shows that the constant-density surfaces slope at all depths, which indicates that the currents extend to the bottom.

Figure \(\PageIndex{1}\): Cross section of neutral density across the Antarctic Circumpolar Current in the Drake Passage from the World Ocean Circulation Experiment section A21 in 1990. The current has three streams associated with the three fronts (dark shading): SF = Southern ACC Front, PF = Polar Front, and SAF = Subantarctic Front. Hydrographic station numbers are given at the top, and transports are relative to 3,000 dbar. Circumpolar deep water is indicated by light shading. Data from Alex Orsi, Texas A&M University.

Figure \(\PageIndex{2}\): Distribution of fronts around Antarctica. STF: Subtrobical Front; SAF: Subantarctic Front; PF: Polar Front; SACC: Southern Antarctic Circumpolar Front. Shaded areas are shallower than 3 km. From Orsi (1995).

Typical current speeds are around 10 cm/s with speeds of up to 50 cm/s near some fronts. Although the currents are slow, they transport much more water than western boundary currents because the flow is deep and wide. Whitworth and Peterson (1985) calculated transport through the Drake Passage using several years of data from an array of 91 current meters on 24 moorings spaced approximately 50 km apart along a line spanning the passage. They also used measurements of bottom pressure measured by gauges on either side of the passage. They found that the average transport through the Drake Passage was \(125 \pm 11 \ \text{Sv}\), and that the transport varied from \(95 \ \text{Sv}\) to \(158 \ \text{Sv}\). The maximum transport tended to occur in late winter and early spring (figure \(\PageIndex{3}\)).

Because the antarctic currents reach the bottom, they are influenced by topographic steering. As the current crosses ridges such as the Kerguelen Plateau, the Pacific-Antarctic Ridge, and the Drake Passage, it is deflected by the ridges.

The core of the current is composed of Circumpolar Deep Water, a mixture of deep water from all oceans. The upper branch of the current contains oxygen-poor water from all oceans. The lower (deeper) branch contains a core of high-salinity water from the Atlantic, including contributions from the North Atlantic deep water mixed with salty Mediterranean Sea water. As the different water masses circulate around Antarctica they mix with other water masses with similar density. In a sense, the current is a giant ‘mix-master’ taking deep water from each ocean, mixing it with deep water from other oceans, and then redistributing it back to each ocean (Garabato et al, 2007).

The coldest, saltiest water in the ocean is produced on the continental shelf around Antarctica in winter, mostly from the shallow Weddell and Ross seas. The cold salty water drains from the shelves, entrains some deep water, and spreads out along the sea floor. Eventually, \(8–10 \ \text{Sv}\) of bottom water are formed (Orsi, Johnson, and Bullister, 1999). This dense water then seeps into all the ocean basins. By definition, this water is too dense to cross through the Drake Passage, so it is not circumpolar water.

The Antarctic currents are wind-driven. Strong west winds with maximum speed near 50\(^{\circ}\)S drive the currents (see figure \(4.2.1\)), and the north-south gradient of wind speed produces convergence and divergence of Ekman transports. Divergence south of the zone of maximum wind speed, south of 50\(^{\circ}\)S leads to upwelling of the Circumpolar Deep Water. Convergence north of the zone of maximum winds leads to downwelling of the Antarctic intermediate water. The surface water is relatively fresh but cold, and when they sink they define characteristics of the Antarctic intermediate water.

The position of the circumpolar current relative to the maximum of the westerly winds influences the meridional overturning circulation and climate. North of the maximum, Ekman transports converge, pushing water downward into the Antarctic Intermediate Water north of the Polar Front. South of the maximum winds, Ekman transports diverge, pulling Circumpolar Atlantic Deep Water to the surface south of the Polar Front, which helps drive the deep circulation (figure \(13.4.5\)). When the maximum winds are further from the pole, less deep water is pulled upward, and the deep circulation is weak, as it was during the last ice age. As the Earth warmed after the ice age, the maximum winds shifted south. The winds were more aligned with the Circumpolar Current, and they pulled more deep water to the surface. Since 1960, the winds have strengthened and shifted southward, further strengthening the Circumpolar Current and the deep circulation Toggweiler and Russell, 2008).

Because wind constantly transfers momentum to the Antarctic Circumpolar Current, causing it to accelerate, the acceleration must be balanced by drag, and we are led to ask: What keeps the flow from accelerating to very high speeds? Munk and Palmen (1951) suggest that form drag dominates. Form drag is due to the current crossing sub-sea ridges, especially at the Drake Passage. Form drag is also the drag of the wind on a fast-moving car. In both cases, the flow is diverted, by the ridge or by your car, creating a low-pressure zone downstream of the ridge or downwind of the car. The low pressure zone transfers momentum into the solid earth, slowing down the current.