Southern Annular Mode and the Australian climate
What is the Southern Annular Mode?
The Southern Annular Mode, or SAM, is a climate driver that can influence rainfall and temperature in Australia. The SAM refers to 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. This belt of westerly winds is also associated with storms and cold fronts that move from west to east, bringing rainfall to southern Australia.
The SAM has three phases: neutral, positive and negative. Each positive or negative SAM event tends to last for around one to two weeks, though longer periods may also occur. The time frame between positive and negative events is quite random, but typically in the range of a week to a few months. The effect that the SAM has on rainfall varies greatly depending on season and region.
SAM in autumn and spring
Climatologically, winter sees the belt of westerly winds at its northernmost position, while summer sees the belt at its southernmost position. In the other two seasons, autumn and spring, the belt of westerly winds is located somewhere in between summer and winter. In autumn, Australia's climate typically sees very little effect from SAM, while in spring, the effect on rainfall resembles a weak summer pattern.
Autumn and spring also tend to be the times of the year when SAM has greatest influence on extreme heat. In autumn and winter, northern and central Australia have double the chance of extreme temperatures occurring during a negative phase of SAM, while in spring, southern Australia has double the chance of extreme temperatures occurring during a negative phase of SAM.
Interaction between SAM and other climate drivers
In some seasons, ENSO events (i.e. La Niña and El Niño) tend to favour a particular phase of SAM. El Niño tends to favour negative SAM during the spring to summer months, while La Niña tends to favour positive SAM during the spring to summer months. Both relationships typically enhance the respective dry and wet signals of El Niño and La Niña.