Weather News

The SAM has gone negative ? here's what it means for Australia

Ben Domensino, Tuesday October 1, 2024 - 12:47 AEST


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.


- Weatherzone

© Weatherzone 2024

Site search


Enter a postcode or town name for local weather, or text to search the site. » advanced search

Giant hail and intense rain lash Qld, NSW

16:32 AEDT Severe thunderstorms have formed over parts of Qld and NSW this afternoon, with reports of hail around 7cm in diameter, intense rainfall and flooding and damaging winds.  The combination of a surface and upper-level trough and moisture-laden easterly winds are triggering severe thunderstorms across southeastern Qld and northeastern NSW.  The images below show the severe thunderstorms across the region on Wednesday afternoon.

Help with Farmonline Weather