Arctic Ocean

A systematic analysis of future projections for the Arctic Ocean circulation is still lacking. Coarse resolution in global models prevents the proper representation of local processes that are of global importance (such as the convection in the Greenland Sea that affects the deep waters in the Arctic Ocean and the intermediate waters that form overflow waters). The MMD models project a reduction in the MOC in the Atlantic Ocean (see Section 10.3). Correspondingly, the northward oceanic heat transport decreases south of 60°N in the Atlantic. However, the CMIP2 model assessment showed a projected increase in the oceanic heat transport at higher latitudes, associated with a stronger subarctic gyre circulation in the models (Holland and Bitz, 2003). The Atlantic Ocean north of 60°N freshens during the 21st century, in pronounced contrast to the observed development in the late 20th century (Wu et al., 2003).

11.8.2 Antarctic

11.8.2.1 Key Processes

Over Antarctica, there is special interest in changes in snow accumulation expected to accompany global climate change as well as the pattern of temperature change, particularly any differences in warming over the peninsula and the interior of the ice sheet. As in the Arctic, warming of the troposphere is expected to increase precipitation. However, circulation changes in both ocean and atmosphere can alter the pattern of air masses, which would modify both precipitation and temperature patterns substantially over the region.

The dominant patterns controlling the atmospheric seasonal-to-interannual variability of the SH extratropics are the Southern Annular Mode (SAM) and ENSO (see Section 3.6). Signatures of these patterns in the Antarctic have been revealed in many studies (reviews by Carleton, 2003 and Turner, 2004). The positive phase of the SAM is associated with cold anomalies over most of Antarctica and warm anomalies over the Antarctic Peninsula (Kwok and Comiso, 2002a). Over recent decades, a drift towards the positive phase in the SAM is evident (see Section 3.6). Observational studies have presented evidence of pronounced warming over the Antarctic Peninsula, but little change over the rest of the continent during the last half of the 20th century (see Sections 3.6 and 4.6). The response of the SAM in transient warming simulations is a robust positive trend, but the response to the ozone hole in the late 20th century, which is also a positive perturbation to the SAM, makes any simple extrapolation of current trends into the future uncertain (see Section 10.3).

Compared to the SAM, the Southern Oscillation (SO) shows weaker association with surface temperature over Antarctica but the correlation with SST and sea ice variability in the Pacific sector of the Southern Ocean is significant (e.g., Kwok and Comiso, 2002b; Renwick, 2002; Bertler et al., 2004; Yuan, 2004). Correlation between the SO index and antarctic precipitation and accumulation has also been studied but the persistence of the signal is not clear (Bromwich et al., 2000, 2004a; Genthon and Cosme, 2003; Guo et al., 2004; Genthon et al., 2005). Recent work suggests that this intermittence is due to nonlinear interactions between ENSO and SAM that vary on decadal time scales (Fogt and Bromwich, 2006; L’Heureux and Thompson, 2006). The SO index has a negative trend over recent decades (corresponding to a tendency towards more El Niño-like conditions in the equatorial Pacific; see Section 3.6) associated with sea ice cover anomalies in the Pacific sector, namely negative (positive) anomalies in the Ross and Amundsen Seas (Bellingshausen and Weddell Seas) (Kwok and Comiso, 2002a). However, a definitive assessment of ENSO amplitude and frequency changes in the 21st century cannot be made (see Chapter 10).