Sudden Stratospheric Warming is bringing weird weather to parts of Australia

29 July 2024

El Niño and La Niña are global metrological events most people are probably familiar with. In Australia, the influence of one or other seems more pronounced over the summer months. El Niño marks periods when ocean temperatures in parts of the Pacific Ocean rise by a certain amount, while La Niña events refer to occasions when these temperatures fall by a certain amount.

While ocean temperatures may affect how many people decide to go for a swim, depending how warm or cool the water is, these variations in ocean temperatures can have a significant, and far reaching, impact on the weather. For instance, parts of Australia may experience higher than normal temperatures during an El Niño event. La Niña’s on the other hand, may instead result in below average temperatures in the same regions.

But El Niño and La Niña are not the only metrological phenomena that influence weather and climate in Australia. The Southern Annular Mode (SAM) is another. The SAM is an indicator of westerly wind belts, and their proximity to the southern coast of Australia. A negative SAM for instance, sees these westerly wind belts, and their associated rain fronts, come much closer to southern Australia, bringing higher rainfall with them.

Then there’s the Indian Ocean Dipole (IOD), sometimes known as the Indian Niño, an indicator of temperature differences between the eastern and western regions of the Indian Ocean. A positive IOD reading can result in periods of low rain fall and drought, particularly in southeast Australia, while negative readings bring higher rainfall to effected regions.

But in trying to determine how the weather may play out over the coming months, an eye should also be kept on Sudden Stratospheric Warming (SSW) events, of which one occurred a few weeks ago, albeit thousands of kilometres from Australia. SSW, as the name suggests, refers to temperature increases in the stratosphere, a layer of the Earth’s atmosphere. The stratosphere is situated anywhere from ten to twenty kilometre’s above the planet’s surface. Higher in equatorial regions; lower near the poles.

But SSW events, which usually occur in the northern hemisphere, can see temperatures rise markedly, generally in the order of about twenty-five degrees Celsius. However, the recent SSW event, over Antarctica — which, in July especially, is unusual to begin with — saw temperature rise by about fifty-degrees Celsius. Fifty-degrees. Remember though, this happened some twenty kilometres above Antarctica, and not on the ground.

One can only imagine the impact of a snap surface temperature increase of fifty degrees in Antarctica, or anywhere for that matter, were that ever to happen. That’s not to say nothing at all, weather wise, will happen though. The previous time a SSW event occurred over Antarctica was in September 2019, resulting in warm, dry weather, across much of Australia in the months that followed. Coupled with an extended period of drought, the event precipitated the tragic Black Summer bush fires of 2019 and 2020.

At this stage meteorologists anticipate more westerly winds for southern parts of Australia over the coming weeks. A negative SAM index reading, then? This has already resulted in heavy snow in some places, but temperatures nudging past twenty-degrees Celsius on parts of the southeast coast, unusual for the middle of winter. As the SSW event occurred in July, and large parts of Australia have been drought-free for some time, it is expected, hopefully, there will not be a repeat of the widespread bush fires of four or five years ago.