2.2                                   The Ocean Areas

2.2.1                                The surface ocean circulation pattern

The most striking feature of the ocean area around Antarctic is the strong seasonal cycle of sea ice and surface temperature change. The sea ice pack is dynamic being subject to the variable wind stress and ocean currents and plays an interactive role in the heat exchange between the atmosphere and the ocean.

The ocean circulation is primarily influenced by the submarine orography, the oceanic density gradients and the wind stress. The location of the Antarctic ocean surface divergence follows the atmospheric circumpolar trough closely around the Antarctic continent. South of the divergence the mean ocean surface currents flow from east to west (the “East Wind Drift”) over the continental shelf with a strong influence of the submarine orography and also the mean easterly winds. The current speeds tend to be greatest near the edge of the continental shelf. Further north from the ocean surface divergence the prevailing currents are towards the east forming the Antarctic Circumpolar Current (ACC). Typical current speeds are of the order of 0.1 m s^–1 (~9 km per day) but can be very much higher as a result of higher wind speeds.

At a number of locations around the continent where the orography develops a north–south trend the east wind drift tends to turn the flow northwards and connect with the ACC. These occur in the large gyres of the Weddell and Ross Seas but also in a smaller way near the Balleny Islands (~162º E), the 90º E ridge, and near the Riiser–Larsen shelf (~33º E). At other locations the submarine orography contributes to turning the ACC southwards towards the continental shelf, such as the region west of the Antarctic Peninsula (near Marguerite Bay, ~70º W) and west of the Balleny Islands (~157º E). This broad picture of Antarctic ocean mean surface currents shows considerable similarity to the surface geostrophic wind directions from the mean sea level pressure (MSLP) patterns. Although the mean winds contribute to the mean ocean current patterns, the submarine orography also has a strong influence. This raises the question as to why the MSLP field has its observed pattern. The answer is that it is partly influenced by the continental orography and the large–scale circulation, but is also influenced by the surface heat balance over the ocean and sea ice.

In addition to the smooth pattern of ocean surface currents as depicted above there is high variability generated by the continual change in winds associated with the continual development and movement of low–pressure systems around Antarctica. In a similar way there is a continual development and movement of oceanic eddies which have spatial scales of typically several tens of kilometres diameter. These are very common near the Antarctic ocean surface divergence, particularly in high shear regions and also often in association with submarine orographic features.

2.2.2                                The sea ice and its annual changes

In late summer (February to March) only about 3 million km² of the sea ice extent remains from the maximum extent of almost 20 million km² reached in September to October. This remaining sea ice is primarily concentrated in the western Weddell Sea along the Antarctic Peninsula and from the Bellingshausen and Amundsen Seas to the eastern Ross Sea. This sea ice extends about 500 km from the coast near the front of the Ronne Ice Shelf and in the eastern Ross Sea, and about several hundred kilometres from the coast east of the Antarctic Peninsula and in the Bellingshausen and Amundsen Seas. Around the rest of the coastline, mainly East Antarctica, on average only patches of sea ice, generally less than a few tens of kilometres in extent, tend to remain. Figure 2.2.2.1 shows 20–year (1979–98) averages of sea ice concentrations from Special Sensor Microwave/Imager (SSM/I) data derived using the "Bootstrap Algorithm" (Comiso et al., 1997).

Over the period of open water, mainly from January to March, the surface mixed layer of the ocean (to about 200 m depth) tends to warm by about 1 to 2ºC above the ocean surface freezing point (~ –1.8ºC). This warming results primarily from the absorption of solar radiation with the low albedo of the water, compared with that of the sea ice and the mixing to deeper levels by the strong winds.

After the period of the maximum surface temperature, usually reached in January to February, the ocean surface mixed layer tends to cool until freezing point is reached. Starting with the high latitude regions first, most notably in the Ross and Weddell Seas, the sea ice cover gradually spreads outwards. In some locations fast ice attached to the coast extends several tens of kilometres to seaward while in other locations strong offshore winds maintain coastal polynyas with low ice concentration for some time. Beyond the fast ice the pack drifts with the wind and currents primarily towards the east, but with considerable temporal and spatial variation, until the ice extends to the region beyond the Antarctic divergence where the eastwards flowing ACC prevails. In the outer pack, mean sea ice drift rates of 0.2 m s^–1 (~18 km per day) are not uncommon. This means that over a six month winter period the sea ice movement in the outer pack could exceed 3,000 km. There is also a tendency for the ice drift to be directed to the left of the wind (~an average of 25º). This tends to drive the sea ice northwards in the circumpolar current spreading the ice outwards in a divergent manner.

Moreover, around the climatological mean low–pressure centres within the Antarctic circumpolar trough the mean outwards divergence tends to make the ice more open and thinner there than in the surrounding sea ice area. The increased heat flux through the thinner more open ice tends to enhance the mean low–pressure centre. This gives a positive feedback to the strong coupling between the ocean currents, the submarine orography, the sea ice drift and concentration, and the mean wind and pressure fields.

Similarly for the transient low–pressure systems there is strong feedback between the opening and closing of the pack by the winds and the heat fluxes from the ocean to the atmosphere. In the mean, the generally divergent movement of the pack tends to maintain the regions north of the divergence at average ice concentrations of around 80% during winter. This results in continual freezing and relatively thin, dynamic ice. A feedback with a self–regulating tendency, acts to maintain the ice concentration in the pack near to the observed mean values. If the pack opens too much, both the heat fluxes from the ocean and freezing increase, reducing the open water fraction. When the pack closes, or freezing of new ice covers the leads, the mean surface temperature decreases and the direction of the mean sensible heat flux can change to be directed from the atmosphere to the sea ice surface.

Snow cover over the sea ice acts as a strong insulating factor for Antarctic sea ice. In East Antarctica the winter sea ice typically accumulates about 0.3 m of snow thickness. The snow conductivity is ordinarily about an order of magnitude lower than that of the sea ice so that the snow thickness therefore has a disproportionate influence on the heat flux through the pack compared with the ice thickness alone. In some regions, such as the Bellingshausen and Amundsen Seas the snow thickness on average is much greater, but as a consequence, flooding and freezing can take place, reducing the low conductivity snow thickness to a buoyancy limit.

In many locations around the coast persistent polynyas with lower ice concentration occur. These locations tend to be associated with coastal obstructions to the ocean currents in the “East Wind Drift”, and also with strong offshore winds from the continent. These polynyas tend to provide high levels of heat flux to the atmosphere along with high freezing rates, which can reach about an order of magnitude above that of the mean in the surrounding pack. In some of these locations the possibility of increased atmospheric cyclonicity has been proposed. Because of the high variability of the atmospheric systems and the pack ice dynamics in the sea ice zone it is difficult to establish clear mean associations. Nevertheless, modelling simulations indicate an enhancement of the depth of Antarctic low–pressure systems over the sea ice zone with interactive dynamic sea ice or an increased open water fraction.