Picture: Storm and bioluminescence over Jervis Bay. Credit: Trevor McKinnon (Unsplash).

The Southern Annular Mode (SAM) is the name given to a hemisphere-wide non-periodic shift in the Southern Hemisphere pressure field.

When the pressure is relatively high over the Antarctic SAM is said to be in its negative phase, whereas when the pressure is relatively low over the Antarctic SAM is said to be in its positive phase. Transitions from one phase to the other can take weeks or months.

The SAM is an important idea widely used in climate science. It is commonly used to describe changes in the jet stream and the storm track, and it is sometimes linked to changes in temperature and precipitation in the midlatitudes.

Research by CLEX scientists and colleagues re-examines some of the basic assumptions and interpretations in the theory. In particular, they show the SAM cannot be interpreted as a descriptor of mid-latitude variability and it has little imprint on the weather of the storm track.

Instead, they argue that SAM is really a measure of how strongly the Antarctic couples with the midlatitudes. The study calls for caution in relating mid-latitude weather to SAM.