Chapter 13 - Deep Circulation in the Ocean

Chapter 13 Contents

13.4 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 it contributes to the deep circulation in all basins. Because it is so important for understanding the deep circulation in all oceans, 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 13.12) shows three fronts. They are, from north to south:

  1. The Sub-antarctic Front,
  2. The Polar Front, and
  3. The Southern ACC Front.

Each front is continuous around Antarctica (Figure 13.13). The plot also shows that the constant-density surfaces slope at all depths, which indicates that the currents extend to the bottom.

Figure 13.12 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 = Sub-antarctic Front. Hydrographic station numbers are given at the top, and transports are relative to 3,000dbar. Circumpolar deep water is indicated by light shading. From Orsi (2000).

Figure 13.13 Distribution of the Sub-antarctic and Polar Fronts and associated currents in the Antarctic. 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 11 Sv, and that the transport varied from 95 Sv to 158 Sv. The maximum transport tended to occur in late winter and early spring (Figure 13.14).

Figure 13.14 Variability of the transport in the Antarctic Circumpolar Current as measured by an array of current meters deployed across the Drake Passage. The heavier line is smoothed, time-averaged transport. From Whitworth (1988).

Because the antarctic currents extend all the way to 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 defected 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.

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 seafloor. Eventually, 8-10Sv 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°S drive the currents (see Figure 4.4), 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°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 de ne characteristics of the Antarctic intermediate water.

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 form drag dominates. Form drag is due to the current crossing subsea 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 down wind of the car. The low pressure zone transfers momentum into the solid Earth, slowing down the current.

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