A negative phase of the Southern Annular Mode (SAM) has developed in the Southern Hemisphere and it’s going to influence weather patterns in the Southern Hemisphere, including the Australian region, in the opening week of October.

What is the Southern Annular Mode?

The SAM is an index that monitors the position of the westerly winds flowing from west to east between Australia and Antarctica. When the SAM is negative, these westerly winds, and the cold fronts and low pressure systems they carry, are located further north than usual for that time of year. When the SAM is positive, the westerly winds, cold fronts and low pressure systems are located further south than usual for that time of year.

A negative SAM started to emerge towards the end of September and this signal has strengthened over the past week. The graph below shows the clear negative SAM signal at the end of September, with forecasts suggesting that it will persist into the opening week of October.

Image: SAM observations and forecast from the Bureau of Meteorology.

What does a negative SAM mean for Australia?

The impacts of a negative SAM change throughout the year. In winter, a negative SAM typically increases wind and rainfall over southern Australia and causes drier-than-average weather in the nation’s east. In summer, a negative SAM increases the likelihood of drier than average weather over much of eastern and southeastern Australia, while increasing the chance of rain in western Tas.

Image: Typical negative SAM impacts during winter.

As we are still in the first half of spring, this week’s negative SAM will increase the likelihood of rain and wind in southern and southwestern Australia and promote drier weather in the east.

The maps below show how the current negative SAM will influence rain and wind this week. The first map shows the forecast accumulated rainfall during the next seven days, with southern Australia being the focus of this week’s rain. The second map

Image: Forecast accumulated rain during the seven days starting on Tuesday, October 1, 2024.

Image: Forecast wind gusts at 11am AEDT on Sunday October 6, 2024.

The current negative SAM event is expected to be short-lived, with models suggesting a return to neutral SAM by the second week of October.