Sudden Stratospheric Warming update
Three months since the Sudden Stratospheric Warming (SSW) event over Antarctica, the verdict is out on its effects on the weather and also surf across the Australian region.
This event was only the second in recorded history (since the phenomena was identified in 1979) with the last event occurring in 2002.
With the 2002 event being the baseline, the Bureau of Meteorology nailed the outlook following this SSW event in early September, that being the forecast of strong negative Southern Annular Mode (SAM) events in the months following.
There's a lot of acronyms flying around above, so here's a brief summary of what's occurred.
The forcing and development of such a rare SSW event is still being discussed, but the very strong positive Indian Ocean Dipole event and cold waters off Indonesia have likely played a part in disturbing the atmosphere (setting up a blocking pattern), and this disturbance propagated from the lower atmosphere (troposphere) up into the upper atmosphere (stratosphere). Additional mountain forcing from the South American Andes looks to also have played a part in setting off the initial disturbance.
This disturbance caused a sinking of cold air in the upper stratosphere, and with all things being equal, a significant warming event occurred, jumping around 75 degrees, hence the term – Sudden Stratospheric Warming.
Such SSW events then disrupt the polar vortex (strong westerlies sitting around the south pole) even causing a reversal which then propagates slowly back down into the lower atmosphere. It took a good while for the SSW event to properly start influencing the surface weather, with it filtering down just over a month after the event.
What this has resulted in is instability in the surface westerlies and zonal flow (west to east) in the form of negative Southern Annular Mode (SAM) events.
Regular readers of Swellnet would be familiar with the SAM, that being under positive SAM events, the westerly storm track retracts closer towards Antarctica, while under negative SAM events the westerly storm track is shifted north towards Australia.
In 2002 strong negative SAM events were seen in the wake of the SSW event, and this year we've seen the same.
The south-east corner of the country has been copping persistent cold fronts, cooler weather and plenty of surf, not typical for this time of year where smaller swells and more favourable winds from the eastern quadrant are usually the norm. Victoria's Surf Coast has offered numerous days of great waves, while there have been some standout days across the South Australian coast closer to Adelaide when November usually goes flat.
The colder air has also provided numerous out-of-season snow dumps to the Victorian and New South Wales high country, but on the other side of the coin, these unusual westerly winds have fuelled and played a big part in this years unprecedented fire season on the East Coast. Instead of moist onshore easterly winds, dry, hot westerly winds have intensified any small fire outbreak.
Monthly climate data for November has just come in and we can see in the below charts, the difference in Mean Sea Level Pressure (MSLP) and surface winds from normal.
It paints a clear picture with two significant negative MSLP anomalies below Tasmania and also in the south-west Indian Ocean (indicating much lower pressure than normal), along with westerly wind anomalies under South Australia, onwards across Victoria and Tasmania, north-west across the East Coast. While smaller southerly swells have prevailed across NSW, the amount of clean, autumn like mornings has been the most noticeable.
One other consequence of the SSW is that the ozone hole is the smallest on record since 1982. This is directly linked to the warming of the stratosphere, resulting in fewer stratospheric clouds forming (the surface of these cloud particles provide the catalyst for ozone depleting chemical reactions).
We're edging towards two months of negative SAM events, but how long the SSW event continues to influence the weather and surf is yet to be seen. It'll likely start to play less of a role into the end of December, but keep an eye on the site and comments below for running commentary.
The strong positive IOD event is slowly weakening, but it's delaying the onset of the moonsoon across northern Australia, and as a result we'll see tropical developments delayed into summer.
On a final note, this SSW event isn't linked to climate change per-se and is a seperate phenomena in itself, though there may be a slight causal link to the strong negative IOD event seen in the Indian Oean.