The Evolution of a Hybrid Cyclone: From Tropical Beginnings to a Baroclinic Twist - Cyclone Alfred

Author: Chris Nitsopoulos

Posted on: Sunday, March 2nd, 2025, 10:25:57 AM

Over the last couple of days you might have heard us using a term you may not have heard before when we referred to TC Alfred. We have used the term a 'hybrid' cyclone when referring to him and many of you have rightly asked us two important questions: 

1 - What is a hybrid Cyclone?

2 - How can a Tropical Cyclone over cooler oceans still be forecast to be a Category 2 (strong system) system into the future? 

Cyclones are among northern Australia's most dramatic weather events. While many of these storms neatly fall into tropical or extratropical categories, some defy easy classification. In this article, we explore a fascinating case study as it's unfolding right now: a cyclone that begins as a classic tropical system with a deep, warm-core structure, only to transition into a hybrid form under the influence of an upper LOWand corresponding sub tropical jet stream and wind shear. This process not only alters the storm's intensity, wind fields and dynamics but also reshapes its identity—leading to an intriguing blend of tropical compactness and extratropical asymmetry.

Phase 1: The Tropical Genesis

Overview:
The journey begins over warm waters, where the cyclone develops a robust, warm-core structure. At this stage, the system exhibits a tight wind field and well-organized convection from the surface up to the top of the troposphere. Satellite imagery shows a classic tropical cyclone with symmetric convection and a clearly defined eye. The image below shows TC Alfred as a Severe Cat 4 TC. 

When we take a look at the cross section below thanks to https://tropicaltidbits.com of TC Alfred at the same time as that satellite image was taken we can see he has a really tall column of warm air that extends from his core all the way to the Tropopause (denoted by the red shadings) This is a sign of a really strong, well structured and fully Tropical Cyclone. 

Phase 2: The Impact of Wind Shear

Overview:
Next, the storm encounters vertical wind shear. This shear weakens the cyclone by disrupting the vertical alignment of the convection. The system’s core begins to tilt, and the once-symmetric wind field starts to break down. At this stage the Cyclone structure remains purely tropical. This is the stage we were at last night on the 1st March 2025 and during today we have seen a terribly sheared TC Alfred struggle to develop any deep convection.

Vertical wind shear (shown by the red arrows) distorts the tropical cyclone’s structure, initiating the transition towards a hybrid state. Image thanks to CIMSS 

Phase 3: The Cold air intrusion that initiates hybrid transition

Overview:
As the cyclone moves over waters with slightly cooler temperatures (around 24-25°C), it begins to weaken until a cold pool at the 200 hPa level begins to interact with the warm surface to 500hPa and even up to 300hPa core. This interaction, where a cold upper low or sub tropical jet stream “pushes” over or even to the north of the storm’s warm core, triggers explosive convective development and some possible re-intensification despite the cooler waters. The wind field contracts again, but the system now shows hybrid characteristics—a nearly tropical tight wind field overlaid by extratropical influences. Note a hybrid Tropical Cyclone is different to an Extra Tropical Cyclone which is reason for a whole other blog article when the time comes.  

 A cross-sectional view thanks to https://tropicaltidbits.com of Cyclone Alfred as expected on Tuesday morning illustrating the warm core beneath a cold uppe level intrusion as he transitions away from being a 'Tropical Cyclone'.

Phase 4: Transition to a Hybrid State

Overview:
After its explosive redevelopment of convection and re-intensification, the cyclone begins to move into warmer waters. It retains the warm-core structure from the surface to 500 hPa, but the lingering influence of the cold mid-level remains evident. This results in a storm with a tighter, tropical-like wind field yet displaying asymmetry—often lopsided to the south as extratropical forces become more dominant. It is important to understand that if this cold air aloft begins to warm while Alfred is still not in an environment with water temperatures in excess of 26.5 degrees celsius, he will begin to rapidly weaken. If Alfred moves over waters in excess of 26.5 degrees and he loses that cold air aloft he can then do something that we only very rarely see, and that's transition back from a hybrid cyclone into a Tropical Cyclone - this only happens on very rare occasions, approximately once or twice a year globally. 

The Weather IQ Weather Centre image depicts a hybrid cyclone Alfred on Wednesday night which now exhibits an asymmetric wind field with stronger winds on one side, a hallmark of mixed tropical-extratropical influence. You can also see that while his strongest winds are still tapped tightly around its core, we also see winds just as strong as those near the core well away from it associated with deep convective cells that have developed on the edges of the cyclone due to the cold air aloft well away from the circulation. 

Understanding Hybrid Cyclones

Hybrid cyclones defy simple categorization. They start as classic tropical cyclones but evolve under external influences—such as wind shear and middle and/or upper tropospheric cold air intrusion—to adopt characteristics of extratropical systems. The result is a storm that is intense, with a compact wind field reminiscent of tropical cyclones, yet asymmetric and baroclinically influenced like extratropical systems.

Such systems are not confined to just our region. With examples abound in the North Atlantic, the NW Pacific and even the Mediterranean (medicanes), similar hybrid transitions can occur in other parts of the world. In the Southern Coral Sea near SE Queensland and NE New South Wales, tropical cyclones sometimes undergo these transitions as they interact with cooler upper tropospheric air and shear upon moving poleward, while also being able to maintain that warm moist air advection from the Tropica parts of the South Pacific Ocean. The more common transformation of Coral Sea Tropical Cyclones is what we call an Extra Tropical Transition and that's what commonly happens to the multitude of Tropical Cyclones that head to the South Pacific Graveyard. But once in a while we get a difficult case that doesn't do that and as we have seen over the past two weeks Tropical Cyclone Alfred is certainly an example of a 'difficult case'.

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