Australia: The Land Where Time Began

A biography of the Australian continent 

Southern Annular Mode - or Antarctic Oscillation

In the Southern Hemisphere the Southern Annual Mode (SAM), aka the Antarctic Oscillation, is the dominant decadal climate mode (Thompson & Wallace, 2000). According to the authors1 it subsumes various climate models for the Southern Hemisphere that were described in the 1990s. The SAM index is the amplitude of an empirical orthogonal function (EOF) of the geopotential anomaly of the atmosphere, similar to dynamic height). The centre of pressure of the SAM pattern is of 1 sign over Antarctica and in a ring, an annulus, at 40-50oS, the opposite sign. The amplitude reaches its greatest level in the Ross Sea region where it is of opposite sign to sign in the southwest Pacific and the central Indian Ocean. The south-north pressure difference is large when the SAM index is positive and high, indicating the westerly winds are more intense, and there is also a shift of the maximum westerlies to the south. Its sea surface temperature (SST) pattern is more complex, and is related to changes in both the zonal and meridional winds. Stronger northward Ekman transport in the Antarctic Circumpolar Current (ACC) is caused by stronger westerlies and pattern changes in the upwelling around Antarctica (Hall & Visbeck, 2002).

For the past 40 years the SAM index has been rising noisily after which it began to decline. The positive SAM trend has been related to anthropogenic change (Thompson & Solomon, 2002; Marshall, 2003). A rise in SAM has been related to a shift to the south and strengthening of the ACC as well as to an increase in subtropical circulation in the western section of the South Pacific (Roemmich et al., 2007), and incursion of warmer water at depth on the northern side of the ACC has resulted from the southward shift. At a depth of 900 m just such a warming has been observed over the past 70 years (Gille, 2002; Fyfe, 2006). The authors1 suggest an increase in eddy activity, which could increase the transport of heat towards the pole, could have been another result of the change in the ACC.

Sources & Further reading

  1. Talley, Lynne D., Pickard, George L., Emery, William J., and Swift, James H., 2011, Descriptive Physical Oceanography: An Introduction 6th ed.., Academic Press.
Author: M. H. Monroe
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