Topic Brief: Stalling Extratropical Cyclones

What are Stalling Extratropical Cyclones, and why do they matter?

Nothing is stronger than tropical cyclones, so why should we care about extratropical cyclones?

Heat is a formidable source of energy. Tropical cyclones harness energy stemming from the temperature difference between the warm tropical oceans and the cooler upper atmosphere; they become very strong, with high wind speeds and heavy rainfall. Extratropical cyclones tap into the energy stored in the atmosphere by heat; it is the warmth of the tropical atmosphere relative to the cold polar air that drives their development. Most extratropical cyclones produce weaker winds and lower rainfall amounts, but some of them can still produce strong rainfall, hail and winds.

Tropical cyclones are weather systems that form in the tropics. The tropics are a very warm and humid region, and all that stored heat and humidity can provide a lot of energy to power tropical cyclones. They are stronger, and generally also smaller, than extratropical cyclones.

Extratropical cyclones are also weather systems, specifically the low-pressure systems on weather charts. While they are responsible for most of the rainfall in the extratropics, an individual extratropical cyclone usually does not bring the extreme wind and rainfall amounts that tropical cyclones bring.

These weather systems are larger than tropical cyclones and can take several days to move over a given region. Nonetheless, strong extratropical cyclones can produce strong winds and heavy rainfall, and if they remain stationary over a given region, they have a high chance of causing widespread flooding. One notable example of extratropical cyclones causing extreme weather is East Coast Lows, yet a new study shows that focusing exclusively on East Coast Lows may cause us to overlook important hazards.

What is the difference between Tropical Cyclones and Extratropical Cyclones?

The fundamental physical difference between the two begins with latitude.

The tropics are warm and humid, with only small temperature differences from one place to another. The ocean surface is very warm, and the air just above the surface is very humid. Here, temperature and humidity form a reservoir of available energy; meanwhile, higher in the atmosphere, the air is cold and dry. This difference between the surface and the upper atmosphere causes vertical motion of warm and humid air parcels because nature tends towards equilibrium and balance. As a consequence, thunderstorms develop. They are driven by convection, which involves strong upward motion and the release of latent heat via the condensation of moisture. If those thunderstorms become strong and start moving away from the equator, they will start to rotate (clockwise in the Southern Hemisphere) due to Earth’s rotation and become tropical cyclones.

Outside the tropics, beyond roughly 25 to 30 degrees latitude, the ocean surface is cooler, and the temperature difference between the surface and the atmosphere is smaller. However, sharp temperature contrasts exist throughout the atmosphere between regions near the equator and those closer to the poles. Just as before, the temperature difference serves as an energy reservoir for storms, but this time it is extratropical cyclones that form to equalise the temperature differences between the equator and the poles.

Another difference from tropical cyclones is that due to Earth’s spherical shape, the effect of Earth’s rotation is stronger at higher latitudes (farther from the tropics), an effect called the Coriolis effect. The Coriolis effect prevents pressure and temperature from reaching equilibrium quickly, as any air parcel that tries to move from high-pressure to low-pressure will be deflected to the left in the Southern Hemisphere, and forced to circle around low-pressure centres (clockwise in the Southern Hemisphere) at scales of thousands of kilometres; an extratropical cyclone is born.

The equator-to-pole temperature difference is present at all longitudes. This means that extratropical cyclones can form anywhere in the midlatitudes and are therefore quite abundant, with typically five to seven extratropical cyclones present across the Southern Hemisphere at any given time. By contrast, tropical cyclones are smaller because their driving mechanism, convection, acts on small scales and they are relatively rare because it is harder for initial disturbances to form.

Stalling cyclones are responsible for large-scale flooding along the East Coast

Stalling cyclones share these characteristics: they occur outside the tropics, are slow-moving and lead to catastrophic rainfall. Researchers analysed these storms and found a surprising global symmetry in their distribution: they all occur on east coasts.

Research has found that the atmospheric conditions that form these storms had a strong flow of warm, moist air; they were trapped by high-pressure systems, which leads to their persistence, or ‘stalling’. The regions where they occur most often are also regions of warm ocean temperatures, potentially linking the storms to the global ocean circulation.

How are extratropical cyclones changing in a warming world?

Extratropical cyclones are expected to become less frequent, but produce more rainfall when they do occur. However, we don’t know if stalling cyclones follow the same logic. Climate models struggle to produce these storms, meaning our confidence in future projections is low. Probably the best way to establish how they might change is by trying to understand the mechanisms that produce and sustain them.

How are we studying them?

Climate models represent normal states well, but they do particularly poorly in regions now identified as hotspots for slow-moving, rain-bearing weather systems. The systems we must understand better are the least well-represented.

We believe that one reason for the poor representation of stalling cyclones in climate models is their resolution: Extratropical cyclones are large and well resolved by climate models, but they also produce smaller thunderstorms that are embedded in the cloud bands of extratropical cyclones. We know that at current resolutions of climate models, these smaller storm cells are not resolved. This is one reason why there are currently worldwide efforts underway to produce high-resolution global climate models. These are weather models that run over the entire globe rather than just small regions.

This is a major effort on all fronts, including high-performance computing and storage, adapting code to work on GPUs (Graphics Processing Units) rather than CPUs (Central Processing Units), replacing components with machine learning, and even adding new physical processes into the models as they start being resolved by these higher resolutions.

For instance, if warm ocean currents are an important mechanism, we might research how ocean currents change in the future to infer how stalling might change. If large-scale atmospheric circulation is important, we might infer changes to stalling extratropical cyclones from changes in the winds and temperatures across the atmosphere. If the storms stall because they produce a high-pressure system that blocks their path, the change in water vapour and resulting availability of latent heat in the storms could be the key. Whatever the mechanism we find dominates might give us an idea of how stalling extratropical cyclones might change in the future.

A key roadblock: CSIRO job cuts

Developing the next generation of climate models requires a major commitment in hardware, software, and scientific and technical expertise. CSIRO is a driving force behind Australia’s climate science, providing world-class expertise, infrastructure, and long-term research essential to developing and maintaining climate projections – the key data source we use to understand future climate change in Australia.

As the primary developer of the ACCESS climate model, CSIRO ensures that Australia has locally relevant, up-to-date data crucial for understanding and responding to climate change. ACCESS is a computer-based model that simulates a virtual Earth, using fundamental physics, chemistry and mathematics of the atmosphere, oceans, land and ice to build simulations of the future. These projections are essential to understand how Australia will change under various greenhouse gas emission scenarios.

This work cannot be replicated or outsourced, as it draws on deep knowledge of Australia’s unique environment and weather risks. Without CSIRO’s leadership and sustained investment, Australia risks relying on outdated and incomplete overseas information, undermining national efforts to mitigate and adapt to climate threats.

At a time when climate science capabilities are already at risk in some parts of the world, recent CSIRO cuts to climate science and model development put Australia on the back foot in preparing for current and future flood risk and in adapting effectively to climate change.

Find out more

To read more about the latest academic research into stalling extratropical cyclones, you can access Global Hotspots of Stalling Extratropical Cyclones in the American Geophysical Union journal Geophysical Research Letters.

Download a PDF copy of the topic brief here.

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