SAM Trends Positive
Climate projections will have some Australian surfers rejoicing, and others not so
SAM Trends Positive
What if I told you that the last few years provides a glimpse into the future for Australian surfers? One where we live with positive SAM.
No, I'm not talking about an upbeat friend, instead we're discussing the Southern Annular Mode (SAM) - occasionally called the Antarctic Oscillation (AAO).
Regular readers would be aware of SAM, and it's impact to the surf across our wide-ranging country whether in positive or negative mode.
In layman's terms, it's an index that provides an idea on how far the westerly storm track is from normal. When in a positive mode, the westerly storm track retreats south to polar latitudes and away from Australia, although it strengthens, resulting in higher pressure across the country.
When in a negative mode the westerly storm track pushes further north, bringing more swell to the southern states albeit with associated cold fronts and local wind.
With the recent triple La Niña, lower than normal pressure across northern Australia has resulted in the sub-tropical high pressure belt moving much further south than normal, and in turn pushing the westerly storm track further south towards the South Pole.
This has resulted in a persistent positive SAM mode since 2020. In fact, 2022 equaled the record amount of positive SAM days, which was set in 1998, that being 278/365 (76% of the year).
The impact of a shift in the sub-tropical high and westerly storm track further south is still fresh in most Australian surfers' minds. The East Coast sees more easterly swell action thanks to the movement of the sub-tropical high south, exposing NSW to easterly trade-wind swells, while in the southern states the retraction of the westerly storm track to the south results in smaller swells arriving more from the south, yet more favourable winds for the exposed coasts.
Plotted below is the SAM index for the past forty years, along with a trend line which shows a shift to more positive SAM events during the past few decades.
There are two causes linked to this positive SAM trend, the first being the ozone hole (which is now slowly recovering) and the second being greenhouse gas emissions.
Lets first look at the ozone forcing.
The release of human-made, ozone-depleting chemicals (chlorofluorocarbons etc) into the atmosphere has resulted in the depletion of ozone in the stratosphere since the 1970's. After realising the damage being done, the 1987 Montreal Protocol was introduced, banning the production and emission of these harmful aerosols.
There's a natural cycle of ozone in the stratosphere, with it decreasing through the start of spring thanks to the formation of polar stratospheric clouds. These clouds contain ozone-eating chlorine and bromine compounds, which are released with the arrival of spring time sunlight.
The emission of CFCs added extra chlorine-containing compounds into the atmosphere, hence accelerating the breakdown of ozone, leading to the formation of the ozone hole.
Because ozone absorbs the sun's heat, a lack of ozone leads to a cooling of the stratosphere and intensification of the polar vortex. A stronger polar vortex equals stronger than normal westerly winds around Antarctica - ie a positive SAM mode. This is evident in SAM observations from about the 1970's onwards.
Thanks to the Montreal Protocol we're seeing a slow repairing of the ozone hole and climate models have a weak negative SAM forcing during summer for the coming decades as a result. This is offset by the positive forcing imposed by increasing greenhouse gas emissions.
With the warming atmosphere and warming tropical oceans, we're seeing an expanding of the tropics. This is leading to a more permanent shift in the sub-tropical high southward, ie - positive SAM mode.
We're also seeing a positive feedback loop taking place, where the more that positive SAM events develop, the more we see strong polar westerly winds. These winds promote upwelling of (relatively) warm circumpolar deep water along the Antarctic shelf, leading to ice melt.
The greenhouse forcing is the main contributor to the positive trend seen through winter, and this is the concern for the southern states.
Winter is their prime surf season and any contraction of the westerly storm track - even if it's stronger - equates to a reduction in the number and size of large groundswell events along with less north-west to south-west frontal progressions.
Under this pattern more exposed locations - the Mornington Peninsula for instance - will benefit as winds become lighter and more favourable, but for spots like the Surf Coast, this means a shorter, more sporadic swell season.
On the East Coast we can expect more easterly swell energy as a general rule, owing to a southerly shift and strengthening of the mid-latitude jet, resulting in stronger mid-latitude lows and extratropical cyclones.
Western Australia is a tricky one, with a reduction in strong polar frontal systems in winter likely to be replaced by an increase in mid-latitude lows which aren't of much benefit to the Margaret River region but produce fun surf for more metropolitan locations.