WITH major climate drivers in the Indian and Pacific Oceans currently at neutral levels the weather scene through July was set up for other climatic factors to have a major say on conditions.
And the Southern Annular Mode (SAM) delivered for growers in southern Australia.
Its first venture into negative territory, correlated with more frontal systems hitting southern parts of the nation, came at the start of July and neatly tied in with improved rainfall across southern regions of WA, SA, Victoria and NSW.
Dale Grey, climate agronomist with Agriculture Victoria, said during winter a positive SAM was consistent with fronts sliding south of Australia.
He said a negative SAM correlated with more fast moving cold fronts hitting land and delivering frequent, relatively light rainfall events.
Felicity Gamble, climatologist with the Bureau of Meteorology (BOM) climate forecasting team, said the SAM was a complex climate driver that had different influences at different times of the year.
“At present, the negative phase is conducive to higher rainfall in southern Australia, but in eastern areas, such as coastal NSW, north-east Tasmania and Gippsland in Victoria a positive SAM can lead to higher rainfall, especially in spring and summer,” Ms Gamble said.
“This is due to it attracting more anomalous, moisture containing easterly flows.”
She said the SAM was a factor on weather right throughout the year.
“Unlike other climate drivers, for instance the Indian Ocean Dipole (IOD), the SAM can have an influence at any time of year, but how it works varies according to the region.”
Ms Gamble said the SAM had much shorter cycles than El Niño / La Niña or the IOD.
“It can change on a fortnightly basis.
She said the recent July negative SAM had led to frontal systems being pushed along in westerly winds.
In terms of its influence on the weather, Ms Gamble said the SAM had more impact this year as the other drivers were not as prominent.
Mr Grey said the change in the SAM from June to July was mirrored by improved rainfall.
“In June, with a positive SAM, the sub tropical ridge moved further south than usual and blocked potentially rain-bringing cold fronts, in July it moved further to the north and that allowed the frontal systems to hit.”