In the Southern Hemisphere, a zonally oriented high pressure belt extends around the globe in the subtropics and middle latitudes, while lower pressure covers the high latitudes. The pressure in these two regions fluctuates. The Southern Annular Mode (SAM) reflects such fluctuation, which shows a "seesaw" phenomenon of pressure variation in the mid–high latitudes of the Southern Hemisphere. It is called an "annular mode" because of the belt-shaped pressure anomaly in the middle latitudes, and its structure is zonally symmetric. However, some studies have shown some SAM events to be characterized by zonal asymmetries, the structure of which has been revealed in a recent study published in Atmospheric and Oceanic Science Letters.
