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Why has it been so windy in Melbourne this winter?

By Nicole Mills, Wednesday August 8, 2018 - 13:48 EST
ABC licensed image
Negative SAM and a long wave trough are to blame for Melbourne's recent windy weather. - ABC licensed

Winter in Melbourne is always windy but this year has been a little unusual.

Strong winds have been flinging car doors open and rattling windows with alarming regularity, but mingled in among the ice-cold blasts has been just a hint of warm air.

Forecaster Ben Matson, who founded surf forecasting website Swellnet, said periods of strong northerly winds in the middle of a Victorian winter did come around "every so often".

He said two factors were combining to create the current conditions — a negative southern annular mode and a long wave trough over Western Australia.

But don't panic. They're not as incomprehensible as they sound. Let us explain in language scrubbed of (almost) all weather boffin speak.

The ups and downs of the long wave trough

A long wave trough is a kind of steering mechanism which sits midway up in the atmosphere.

It directs a lot of the surface features that affect what we feel on the ground day to day.

"The long wave trough has been positioned just off the West Australian coast for about the last month or so and Victoria is on its downward flank," Mr Matson told .

"It's a very, very slow moving feature."

It is causing polar winds to be directed up into Western Australia, across the continent and down into Victoria.

"These long troughs, when they anchor in position for a period of time, can deliver days or weeks or persistent winds from one direction, and in this case it's been mainly out of the north.

"Conversely, if we have the long wave trough closer to Victorian latitudes, or maybe steering up through the Tasman Sea, then we'll get those cold, icy polar winds and that's when we get a different set of weather than what we're seeing at the moment."

Effects of the negative southern annular mode

A negative southern annular mode is known as a negative SAM.

It's the measure of where the storm track — the Antarctic oscillation — is at any given time as it circles Antarctica.

If it's close to Antarctica, it's positive. If it's close to Australia, it's negative.

"When it's negative it's closer to Australia and this means more storms and cold fronts and all of those kinds of things," Mr Matson said.

"It happens from time to time. SAM will go negative, it will go positive.

"But if those two things coincide — the negativity of the southern annular mode and the long wave trough in the right position, which it is at the moment — that's when we get those northerly winds."


© ABC 2018

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