Spring is here, and the outlook is damp. Australians are well acquainted with La Nina – the damn thing just won’t leave – and coming to grips with the rain-bearing negative Indian Ocean Dipole. Now there’s another weather pattern to wrap your head around – the Southern Annular Mode, or SAM.

The Bureau of Meteorology predicts warmer-than-average sea surface temperatures paired with a negative Indian Ocean Dipole and a likely La Nina event will lead to a wetter-than-average spring.

It says the SAM index is likely to be mostly positive for the coming three months, which will have a wetter influence on parts of eastern Victoria and eastern NSW.

But what is the SAM? In the region between Tasmania and Antarctica, strong westerly winds blow almost continuously and are associated with storms and cold fronts that move from west to east, bringing rainfall to southern Australia.

The SAM refers to how far north and south these westerly winds move. It has three phases – neutral, positive and negative. A positive SAM in summer brings an increased chance of rainfall, though this varies greatly by region.

Weatherzone meteorologist Andrew Schmidt says: “When southern Australia had that conveyor belt of cold fronts in winter that never seemed to end, the SAM was in negative mode. At the moment, SAM is more positive, and we aren’t having as many cold fronts.”

Unlike La Nina and the Indian Ocean Dipole (IOD), which are slowly moving weather patterns that can remain for months, SAM events tend to last for one or two weeks, though longer periods may occur.

Professor Nerilie Abram, a climate scientist at Australia National University, says: “It can change quickly because we’re only talking about the atmosphere, whereas La Nina and the IOD also include the ocean temperatures, and they stick around for many months.”