TLDR:

• A minor warming event was recorded in the upper atmosphere during the middle of July
• These warming events result in cold air escaping the polar regions = more frontal systems across the mid-latitudes
• The Indian Ocean and Australian regions may see the increased frontal activity impacting through the middle to end of August
• This bodes well for an active end to winter

Unbeknownst to most, a minor sudden stratospheric warming event was observed through the middle of this month.

The term sudden stratospheric warming (SSW) might ring a bell for regular Swellnet readers, with a major event occurring in September 2019, leading to the horrific black summer fire season as strong frontal systems pushed up, across the country at a time when high pressure systems usually dominate.

The Surf Coast could be a beneficiary of the SSW event (Scanlon)

So what is a SSW and how does it affect the Australian region, and more so our swell climate?

In short, it is as it reads: A sudden increase in the air temperature in the upper atmosphere, called the stratosphere.

The causes of such a warming in the Southern Hemisphere are still not fully understood, but this one looks to have occurred thanks to a strong wind disturbance over the Andes mountain range. 

Much like an obstruction disturbing the regular flow of water down a river, high mountain ranges can do the same for the atmosphere. The obstruction sets off wobbles and eddies downstream.

Regular disturbances aren't powerful enough to break through from the troposphere (the lower atmosphere) to the stratosphere, but on rare occasions it can happen with the turbulence altering the layers of the atmosphere. Cold air will sink through the stratosphere, with the response to this being warm air lifted towards space.

This is a 'warming' event. And it will also cause downstream effects.

Typically, the stratosphere reaches a low point in temperature during winter, then gradually warms during spring. However, around the 14th of July, a rapid increase of 15°C was observed. This rates as a minor event compared to 2019 which saw a 75°C jump in temperature, but the timing of this warming is earlier than normal.

Sudden, 15°C warming registered in the statosphere (JMA)

So what does this all mean?

When there’s a warming in the stratosphere, we see a weakening of the polar vortex  - the strong westerly winds that trap cold air to the poles - and if it’s a significant enough event, the westerly polar winds can even reverse and swing easterly.

We'll see no switch in winds this time, however the weakened polar vortex will allow cold air that would otherwise be trapped towards the pole by to 'escape' further northwards in latitude - i.e we can expect the westerly storm track to shift further north, known as a negative Southern Annular Mode (SAM) event.

The lag time between the warming event and the impact on the surface weather is usually a month or two, but current modelling has a strong negative SAM event travelling down from the upper atmosphere to the surface in the coming weeks to month.

Negative SAM event descending from the upper atmopshere (stratosphere) down to the surface (StratObserve)

What does this mean for the Australian region?

While a negative SAM event has the potential to bring stronger cold fronts up and into Australian waters, it’s not guaranteed. There will be increased activity, yet where in the Southern Hemisphere that activity occurs is hard to predict. For instance, it could happen across the South Pacific and mainly miss us.

Long-range modelling has the activity firstly impacting the Indian Ocean and Western Australia, possibly migrating further east during the middle to end of August.

Nothing is certain yet, and we'll keep watching the numbers closely, but the take-home message is that we could see some interesting developments in the Southern Ocean over the coming month. Watch this space.