Understanding the Sudden Stratospheric Warming (SSW) Event and Its Impact on Australia
There's been a lot of discussion around the Sudden Stratospheric Warming (SSW) event currently happening in the Southern Hemisphere. Many might remember hearing about SSW for the first time during 2019, which contributed to devastating bushfires of the 2019 spring and summer across large parts of eastern Australia.
Fortunately however, there are some key differences between the 2019 event and the current one, which are likely to result in different impacts on Australia.
What is Sudden Stratospheric Warming?
Sudden Stratospheric Warming occurs when the stratosphere (a layer of air approximately 12-50km above the surface) warms rapidly. This can happen for various reasons and is much more common in the Northern Hemisphere.
10mb temperature over the polar regions of the Southern Hemisphere. The red line is the observed temperature - the green lines indicate the GFS forecast temperature at this level. Source: NOAA
A diagram of the different levels of the atmosphere. The stratosphere sits above the troposphere which is where the world's weather occurs. Source: ECMWF
The warming results in a weakening, displacement, or elongation of the polar vortex over Antarctica, which is a large region of cold air in the upper atmosphere. When the polar vortex is disrupted, it can increase sea level pressure over Antarctica, causing cold air to move towards lower latitudes. This often triggers a negative Southern Annular Mode (SAM) phase, which leads to more frequent and intense cold fronts over Australia.
10mb temperature anomalies & pressure level. Note the elongation of the polar vortex - which is typical of a SSW event. The elongation or distortion of the polar vortex can cause a more northward positioning of cold fronts and the triggering of a negative SAM event. Source: ECMWF.
Observed (black) and forecast (red) SAM values. Note the sharp decline in the expected SAM values which can occur during SSW events. This can result in more cold fronts extending northwards into Australia and increase the westerly flow. Source: NOAA.
Comparing 2019 and 2024 SSW Events
Timing
The SSW event in 2019 occurred in September, two months later than the current event, resulting in increased cold frontal activity during the second half of spring. By this time, the northern half of Australia had already significantly warmed, so when the cold fronts increased, it resulted in strong and gusty northwest to westerly winds ahead of each cold front, drawing in the hotter air mass from northern Australia. Additionally, 2019 saw a very strong positive Indian Ocean Dipole (IOD), which delayed the monsoon significantly across northern Australia. For reference, the monsoon in the Northern Hemisphere didn't retreat from northwestern India until October 9, more than a month later than usual!
This means the current SSW event, while likely to increase cold frontal activity, should occur earlier—before northern Australia has become as hot. Nonetheless, August and September remain elevated bushfire periods for northern Australia and could still increase bushfire dangers, but not at the same level as in 2019. Also, recent years have remained quite wet for large parts (though not all) of Australia—something that wasn't present in 2019.
IOD comparison between 2019 and 2024. Note the very strong positive IOD during 2019 compared to 2024. Source: BoM
Intensity
The 2019 SSW event was particularly strong, with stratospheric temperatures rising by 50°C. The current event, while still ongoing, has shown a more moderate warming of about 15-25°C (depending on the atmospheric level). The caveat is that the current SSW event is still ongoing—and while some models suggest it could strengthen, as of now, it seems less intense than the 2019 event, which could mean reduced impacts.
Approximate comparison between the 2019 and 2024 SSW events. The 2019 SSW event contained more significant warming and occurred later in the year. Source: JMA
Climatic Patterns
The very strong positive IOD wasn't the only different climatic pattern in 2019. There was a "warm neutral" phase in the Pacific Ocean. A positive IOD often correlates with drier conditions in spring, but a warm neutral phase occurred in the Pacific Ocean as well. A warm neutral phase is when the Pacific Ocean is within the realms of "neutral" (not El Niño but not La Niña), but is leaning towards an El Niño. This year, there is a neutral IOD and the Pacific Ocean, while also technically neutral, is currently in a "cool neutral" state. This means that it's leaning towards a La Niña but not strong enough to be classified as a La Niña.
In summary, overall climatic indicators in 2019 were drier than they are now in 2024.
ENSO comparison between 2019 and 2024. During 2019, patterns were "warm neutral" compared to the expected neutral or "cool neutral" phase for the 2024 spring. Source: BoM
Expected Impacts
SSW events are rarer in the Southern Hemisphere than in the Northern Hemisphere, so their impacts are not as well understood. But currently, we expect the following impacts to occur (noting the current SSW event is still evolving and this could change the downstream impacts):
Southern Areas of Australia - Increased Cold Fronts & Rainfall: More cold fronts are likely during August and September, potentially increasing rainfall and winds across southern parts of Australia (SW WA, southern South Australia, Victoria, Tasmania, and possibly parts of southern NSW).
Northern Areas of Australia - Decreased Rainfall & Elevated Bushfire & Frost Risks: Conversely, cold frontal activity during the cooler months for the northern half of Australia can correlate with drier conditions. Noting that August and September are already normally the driest months of the year for parts of the northern half of the country, this could bring periods of gustier winds which may elevate bushfire conditions (but not to the same degree as in 2019). The intrusion of drier, westerly winds can also bring an extended frost season or below average minimum temperatures.
Eastern Australia - Delayed Storm Season: There is potential for a delayed storm season. Storms require heat, moisture, and sources of cold air coming from the south. An increase in westerly winds may block the easterly winds coming off the Coral Sea for a month or two, delaying the onset of the storm season slightly into October/November (rather than the slow buildup that can occur in September in some years, particularly during cool neutral or La Niña years).
Our team at Weatherwatch will be closely monitoring these developments, ensuring that we keep all our clients and partners prepared and informed of any adverse weather that may occur. Please reach out to our team if you have any questions or concerns!
Forecast temperature and rainfall anomalies from ECMWF for early to mid August. This shows the potential for cooler and wetter conditions over the southern areas of Australia. Source: ECMWF.