The Southern Annular Mode (or Antarctic Oscillation)
The weather can be defined as the end product of the interaction among atmosphere, ocean, biosphere, land surface and cryosphere exchanging mass, momentum and energy.
The focus of this piece will be on the investigation of the Southern Annular Mode (or Antarctic Oscillation). This mode indicates "the (non-seasonal) north-south movement of the strong westerly winds that blow almost continuously in the mid- to high-latitudes of the southern hemisphere" (Commonwealth of Australia 2019) resulting in storms and rainfalls over Australia.
The cycle of SAM forecasts three phases: positive, neutral and negative, both the former and latter of length between 2 weeks or longer. The intervals between the phases can vary between a few weeks to months.
The strongest impact of SAM is experienced in the winter. At this time, negative SAM brings the westerlies North therefore dragging colder winds and rainfalls to Southern Australia, at the same time also blocking tropical storms to bring wet weather to the Eastern part. This condition can also bring snowfall to the Australian Alps.
In the case of positive SAM, the westerlies are drown further South, therefore creating a drier climate in the Southern Australia and allowing the tropical storms to reach the Eastern part. When the event occurs in the same period of La Nina in the Pacific Ocean, the moisture-filled air on the coasts of Eastern Australia is accentuated.
During the summer, positive SAM has the opposite effect as the weaker westerlies, which are further away from the coastline as a consequence of the Earth inclination, do not cause such strong rainfall.
However, in the case of summer negative SAM, the winds shift Northbound and more drier wind blows on the continent which therefore causes dry weather on the Coast and below average rainfall.
The Southern Annular Mode is usually also related to ENSO appearance, being more negative in La Nina and more positive in El Nino, which can cause reinforcements of extreme events.
Has SAM changed in recent years? What has been the impact?
The historical performance of SAM has been analysed with the use of several proxies, such as ice cores, lake sediment cores in Patagonia and glacier behaviour.
In recent decades there has been a trend of more positive SAM events which fall outside the range of natural climate variability. The weather system, associated with stratospheric ozone loss, has shifted southwards because of an increased discrepancy in the surface pressure between Antarctic, where it declined, and mid-latitudes, where it increased, which results in incremented Westerlies over the South Ocean. "The impact of ozone depletion on the SAM in the stratosphere is readily explained by the reduction in absorption of incoming shortwave radiation by stratospheric polar ozone, increasing the meridional temperature gradient" (Randel and Wu, 1999).
How might SAM change in the future?
The forecast of SAM variability is all dependant on the global warming pathways.
In the scenario where we will be able to minimize the climate change to ~1.5°C with a gradual rehabilitation of the ozone hole and reduction of GHG emissions, the Southern Annular Oscillations would return to the Twentieth Century trends (Mayewski, P. A. et al., 2009).
In the scenario where global warming will consistently increase, it will drive the SAM poleward with enhancement of the events in all seasons.
References:
Knutson, T.R., J.L. McBride, J. Chan, K. Emanuel, G, Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava, and M. Sugi 'Tropical cyclones and climate change', Nature geoscience 3(3) 2010, pp.157–163.
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