A wide band of westerly winds encircles Antarctica and influences Southern Hemisphere weather patterns throughout the middle and high latitudes. This belt undergoes periodic tightening, in which it shifts farther south, or expansion, when it shifts north. Such movement is known as the Southern Annular Mode (SAM), or the Antarctic Oscillation.
Previous computer modeling studies have suggested that SAM can cause anomalies in sea surface temperatures, which can, in turn, enhance SAM behavior. However, the details of this feedback are largely unknown. In a new study, Xiao et al. provide support for a mechanism by which sea temperature anomalies caused by SAM feed back to the atmosphere.
To examine interactions between the sea and atmosphere in SAM, the researchers analyzed daily weather data from the European Centre for Medium-Range Weather Forecasts, sea surface temperature data from the National Oceanic and Atmospheric Administration, and ocean current data from the Global Ocean Data Assimilation System.
The data support the findings of previous modeling studies that suggested air-sea interactions enhance the persistence of SAM behavior. The results also support a previously proposed mechanism for this positive feedback effect: Sea surface temperature anomalies caused by SAM help maintain temperature anomalies in the lower atmosphere, which cause air eddy behavior that enhances the persistence of SAM movements.
Further analysis also found that this eddy feedback operates by a barotropic mechanism (involving propagation and deformation of air eddy) at midlatitudes and a baroclinic mechanism (involving generation of air eddy) at high latitudes.
On the basis of their results, the authors suggest that air-sea interactions may significantly affect SAM variability at timescales of 70 to 80 days or longer. The findings could help improve predictions of SAM behavior between seasons or within a single season. (Geophysical Research Letters, doi:10.1002/2016GL070255, 2016)
—Sarah Stanley, Freelance Writer