Eastern Australia is seeing widespread rain and Australia’s ski resorts are struggling without natural snow as a rare atmospheric event unfolds in the Southern Hemisphere.

The Southern Annular Mode (SAM) has entered an exceptionally strong positive phase over the past week, with the SAM index crossing +5.0 for the first time in three years. This rare atmospheric event is having a noticeable impact on Australia's weather.

What is the Southern Annular Mode?

The SAM is an index that reflects changes in the strength and position of a westerly wind belt encircling Antarctica. When the westerly wind belt strengthens and contracts south towards Antarctica, the SAM is in a positive phase, and when the westerlies slacken and expand further north towards Australia, the SAM is in a negative phase.

The state of the SAM influences weather in Australia by affecting pressure patterns in the Australian region. For example, when a positive SAM occurs in winter, the cold fronts and low pressure systems that typically bring rain and snow to southern Australia move further south, and high pressure becomes more prominent near eastern Australia. These changes typically cause below average rain and snow in southern Australia and above average rain in parts of eastern Australia.

Strong positive SAM underway

Sustained SAM index values above +1 represent a positive SAM event, while sustained values below -1 reflect a negative SAM event. Late last week, the SAM index reached +5.06 on Friday, June 26. This exceptionally strong positive SAM value was the highest since May 2023 and one of the highest SAM values in modern records.

The ongoing positive SAM is having a clear influence on Australia’s weather:

Two rain bands are delivering widespread rain to Australia’s eastern and southeast inland this week. This wet weather is due in part to moisture-laden easterly winds flowing over eastern Australia, which is associated with a positive SAM.

The Australian alpine region is also being starved of early-winter snow due to the ongoing positive SAM. Australia’s ski resorts are currently relying on artificially made snow in the absence of any decent natural falls.

What will the SAM do in July?

The SAM is difficult to predict more than a few days ahead. However, most computer model guidance suggests that the positive SAM will weaken in the first half of July.

If this does happen, it will increase the likelihood of rain and snow in southern Australia heading in the coming weeks. There are already signs that a bout of fresh snow will fall in the Alps later this week, between late Thursday and Saturday.

It is important to point out that the SAM is not the only climate index that influences Australia's weather, and other factors will play a role this winter. Most notably, the Bureau of Meteorology recently declared that El Niño is underway in the Pacific Ocean, which typically brings reduced rainfall and warmer-than-average daytime temperatures to much of Australia.

The interaction between El Niño and the SAM will be key factor shaping Australia’s weather in the coming months.