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Why Has the Winter of 2024 Felt So Cold Over Southeastern Australia If There's Been No Cold Fronts?

Thursday July 4, 2024

As yet more cold weather records tumble across Southeastern Australia (Liawenee rebroke the Tasmanian July record this morning falling to -13.5C, after yesterday falling to -12.9C) the question is, why is it so cold if there's been a distinct lack of cold fronts this winter?


It seems counter-intuitive, but cold fronts across southeastern Australia can actually bring warmer temperatures! This is because in winter, the ocean is often warmer than the land because the ocean retains more heat. Currently the sea surface temperatures off Tasmania and Victoria are ranging from 12 to 15C, and to record colder sea surface temperatures (below 10C), you need to travel many hundreds of kilometres further south again. When cold fronts sweep through, they bring Southern Ocean air into southeastern Australia - this airmass (while still cold), is limited by how cold it can get due to the ocean temperatures being comparatively milder. 

BoM SST.png

SSTs for July around Australia - compared to land, ocean temperatures even off Southern Australia tend to be warmer. Source: Bureau of Meteorology.

In the Northern Hemisphere, there are significant landmasses (such as Canada and Russia) that rapidly lose their heat in winter which provides a significant source of cold air. This is why locations of equal latitude in America and Eurasia are frequently colder in winter compared to their Australian equivalents. As an example, Atlanta in the US which is at a similar latitude to Sydney has had a record low of -22C! Even if we take into account that Atlanta is well inland, the Australian all-time record which is in the Snowy Mountains at nearly 2000m is -23C.

If we look at the months of May and June so far for 2024, they've been characterised by a lack of cold fronts. This means no injection of "mild" ocean air, and the air has just been sitting over the same spot. Since land loses its heat more rapidly than the ocean, there's no change in airmass and the only heat is the sun during the day to briefly warm temperatures up. 

A lack of cold fronts often means lighter winds - and light winds contribute to cold nights and drop the overall average temperature (that's the average minimum + maximum temperature). Not to mention cold fronts often bring cloud and rain - further factors that normally keep minimum temperatures warmer at night (though can bring colder temperatures in the daytime).

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The "Wind Drought" across Southeastern Australia has resulted in lower outputs of wind energy for 2024 compared to previous years.

What happened to all the cold fronts?
The frequency of cold fronts is linked to the Southern Annular Mode (SAM). When the SAM is negative, then cold fronts extend further northwards into Australia. When the SAM is positive, they retreat southwards closer to Antarctica. Positive SAM events occur due to unusually high pressures over Antarctica. What causes the higher pressure anomalies to develop is still unknown - but it's likely linked to an overall warmer climate compared to previous decades. There has been a trend towards a higher number of positive SAM events - which may mean less winter cold fronts on average in the future.

Pressures across large parts of Antarctica have been well above average for June so far. This has contributed to the persistent positive SAM phase. Source: NCEP NCAR Reanalysis.

Pressures through the Southern Ocean have also been above average for June due to a lack of large winter lows. However, the Tasman Sea has experienced below average pressure anomalies. This is likely due to the warm water across eastern Australia resulting in successive low pressure systems occurring from upper trough activity. Source: NCEP NCAR Reanalysis.

Does this mean all of winter will be cold?

Not necessarily - and this is where the patterns get complex. The Sun has begun its march back towards the Southern Hemisphere as the earth begins to slowly tilt the other way - by August there'll be a lot more heat radiation hitting northern Australia and if there's no introduction of cold air pushing into these areas, then these areas will become warm quickly. In other words, while the lack of cold fronts can allow for cold air to sit over and recirculate across southeastern Australia, in northwestern Australia, the lack of cold fronts can allow for warm air to sit over and recirculate. This could mean a shorter winter and an earlier start to warmer temperatures. Though this isn't always clear cut because a positive SAM (especially in a potentially cool neutral ENSO phase that we may see this year) can help bring onshore easterly winds over eastern Australia. 

As the climate continues to change it will continue to make climate forecasting more challenging in the coming years as patterns that were true in the 90s and 00s will potentially bring different expected patterns. 

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Increasing SAM trend over time from 1979.

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Average temperature anomaly (min + max combined) for Australia showing below average anomalies across southeastern Australia but above average in Western Australia. Source: Bureau of Meteorology

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