2.3                                   The role of the Antarctic in the Global Climate System

The Antarctic region plays an important role in the global climate system in two main areas. Firstly, in the global heat balance the Antarctic and the Arctic represent the main regions of net heat loss of the Earth to space. Secondly, the Antarctic plays a particularly important role in the net water budget that is then redistributed by the ocean circulation but for part of which the Antarctic Ice Sheet acts as a long–term storage system. Since the moisture transport also forms part of the heat balance these two main roles are closely connected. Each are also involved in the atmosphere and ocean circulation and dynamics which form part of the global circulations of the atmosphere and ocean both of which have particularly strong interactions in the Southern Hemisphere mid–latitudes.

2.3.1                                The heat balance

The mean net radiative heat loss to space of the Earth’s domain south of about 40º S needs to be compensated by the atmospheric and oceanic transport from the net gain regions further north. Most (about 3/4) of the total transport is provided by the atmosphere and by the edge of the Antarctic continent the atmosphere is responsible for all of the net heat transport further south. All of the ocean heat transport into the Antarctic domain is released as heat flux at the surface into the atmosphere. The sea ice not only plays a pivotal role in the heat exchange between the atmosphere and ocean through the annual cycle, but also, through the ice movement, plays a key role in the annual net surface heat flux and the north–south heat transport. If the sea ice just grew in winter and melted in summer without movement there would be an annual cycle of heat storage by the ice but no net heat transport. The ice dynamics transports sea ice from the generally more southerly regions of net freezing to the outer pack regions of net melting. This results in vertical net fluxes to the atmosphere and an effective net latent heat transport directed towards the south. Of a total of about 60 Wm^–2 net annual heat loss through the top of the atmosphere over the domain of the sea ice about 15 Wm^–2 comes from the net heat flux from the ocean surface and about 5 Wm^–2 from the net sea ice transport.

For the atmospheric heat transport the synoptic scale eddies play a dominant role. Due to the Earth’s rotation, and the north–south temperature gradient in the atmosphere, the mean troposphere winds in mid–latitudes become westerlies. This means that it is difficult to obtain very much north–south heat flux from the mean flow. Consequently the bulk of the net meridional heat flux results from a combination of the stationary and transient long waves in the flow and the synoptic and smaller scale eddies right down to the scales of atmospheric diffusion. Of these systems the synoptic scale eddies are the most important. The mean tropospheric winds have a maximum in the westerlies around 45º S. North of this region, on average, anticyclonic eddies are dominant whereas southwards of the mid–latitudes the cyclonic eddies dominate the latitude band around the Antarctic maximum sea ice zone represents the most concentrated domain of cyclonic systems on the globe. These systems generally extend through a large part of the troposphere and provide a dominant part of the meridional heat transport via the horizontal air movement. Once the Antarctic continent is reached, the high and steep orography of the continent impairs much of the lower tropospheric airflow, except for the large embayments and the ice shelves. As a consequence the synoptic scale eddies play a very important role around the coast of Antarctica but not so strongly in the interior. The upper–level flow into Antarctica is not so restricted and supplies a continual transport of air inwards to the continent to compensate for the net outflow at lower levels due to the katabatic winds. This means that there is a strong net vertical circulation also involved in the net meridional heat transport of the atmosphere to the Antarctic. The larger part of the net atmospheric heat transport to the Antarctic is from the sensible heat, but part also comes from the moisture transport contributing the net positive accumulation of snow over the region.

2.3.2                                The moisture and water balance

South of about 45º S there is a net positive balance of precipitation over evaporation (P–E). This net positive P–E is compensated (as a long term balance) by the ocean flow and the flow of the Antarctic ice sheet, with some basal melting, and the calving of icebergs at the margins.

Part of the oceanic net transport is accomplished by the horizontal circulation of the surface ocean. In the regions around the Antarctic where the large gyres direct the ocean flow to the north, fresher water is transported to the lower latitudes. In the ACC, Ekman drift also plays a role in the northward surface transport. In addition, the mixing due to ocean eddies provides an effective mean diffusion of ocean properties resulting in a meridional northward net flux of fresh water.

The most important role of the Antarctic region in the ocean circulation concerns the vertical meridional thermohaline circulation. Saltier water is brought to the Antarctic Ocean from the ocean basins further north, particularly the Atlantic Ocean. As this water circulates around the Antarctic continent and impinges on the continental shelf, it becomes capped by the fresher water from the positive P–E balance. Even with the lower surface temperature, decreasing to freezing point in winter, this surface water would still be too fresh and light to sink. The freezing of sea ice over the continental shelf can increase the surface water density enough to produce deep mixing below the warmer saltier circumpolar deep water. In some locations this densification is sufficient to form Antarctic bottom water. This (bottom) water then spreads around the Antarctic in the deep layers and mixes northwards to replenish bottom water in the other major ocean basins. This deep circulation is also important for the transport of dissolved gases (e.g. O₂ and CO₂) and nutrients around the global oceans.

At the surface, the melting of the sea ice, adds to the positive P–E and ice flux from the Antarctic to produce the fresh upper ocean water mass which spreads around the Antarctic ocean and drifts northwards to the Antarctic convergence (near 50º–55º S) where it sinks below the warmer saltier water from the other ocean basins drifting south. This flow of ocean water tends to maintain the strong meridional sea surface temperature gradients in the Southern Ocean.

The longer transport and storage times of the ocean and Antarctic ice sheet compared to that of the atmosphere means that climatic changes can result in significant imbalances over shorter time scales. These imbalances can also occur with inter–annual climatic fluctuations. For the Antarctic region one of the most prominent of these internal changes is the Antarctic Circumpolar Wave (ACW). This involves interaction between the atmosphere, ocean and sea ice with large anomalies of the scale of wave number 2 to 3 moving around the Antarctic continent with about the speed of the mean ocean flow. The ACW can therefore interact with the similar large inter–annual atmosphere–ocean anomalies associated with the other large ocean basins such as the El Niño in the Pacific Ocean. As a result, over longer periods, occasions occur where these interactions make significant contributions to the inter–annual variability of the atmosphere–ocean sea ice system around Antarctica.