Global climate impacts from changes in Antarctic Intermediate Water
Observations suggest that properties of Antarctic Intermediate Water (AAIW) are changing. An understanding of how changes in the ocean may interact with the atmosphere is crucial for understanding how Earth’s climate will evolve in the future. The impact of variations in AAIW is explored using a series of idealised perturbation experiments in a coupled climate model, HadCM3. Two sets of ensembles have been used. The first varied initial atmospheric states; the second varied initial states in the ocean and atmosphere. The ensemble simulations were integrated over 120 and 100 years, respectively, altering AAIW from 10-20°S in the Atlantic, Pacific and Indian oceans separately. Potential temperature was changed by ±1°C, along with corresponding changes in salinity, maintaining constant potential density. There is a surface response to changes in AAIW in each of the three major ocean basins. When the water mass surfaces in the equatorial regions, there is no significant change in sea surface temperature (SST). However, there is a SST response for each experiment when the anomalies surface at higher latitudes (> 30°). The spatial pattern of SST anomalies in the North Pacific resembles the Pacific Decadal Oscillation. Heat and salt distribution in the Indian Ocean is influenced by the Indonesian Through-Flow (ITF), with long-term trends in transport caused by bottom pressure anomalies in the Pacific. Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in non-linear responses to opposite perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing a reduction in the meridional overturning circulation of up to 1 Sv. Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basin-wide changes in the surface ocean and overlying atmosphere on multi-decadal timescales. Cooling in the North Atlantic Current may be self-sustaining as it leads to high pressure anomalies in the overlying atmosphere, and increased wind stress over the sub-polar gyre. All these experiments indicate that the response to cooler, fresher AAIW would be both greater and more significant than that for warmer, saltier AAIW.