Global climate impacts from changes in Antarctic Intermediate Water

Observations suggest that properties of Antarctic Intermediate Water (AAIW) are changing. The impact of these variations is explored using a series of idealised perturbation experiments. 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 10-200S in the Atlantic, Pacific and Indian oceans separately in a coupled climate model, HadCM3. Potential temperature was changed by ±1 DC, 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 when the anomalies surface at higher latitudes (>300). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean. Resulting changes in ocean circulation cause responses to opposite perturbations to be nonlinear. In the Southern Ocean, changes in the meridional density gradient lead to changes in Antarctic Circumpolar Current transport. The North Atlantic is particularly sensitive, with density anomalies causing changes in the meridional overturning circulation of up to 1 Sv. Surfacing anomalies and changes in meridional ocean heat transport 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. 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). Long-term trends in the ITF are caused by bottom pressure anomalies in the Pacific.