We're just three weeks into the new year, and for most of that time the East Coast has been bathed in swell courtesy of La Niña.
Tropical Cyclone Seth kicked things off, first roaming the warm waters of the Coral Sea before turning extratropical and continuing to bombard the coast with swell as it pushed back towards land.
Seth was followed by Tropical Cyclone Cody which remained at a distance from the Australian continent, moving slowly south through the Coral Sea from a position between Fiji and Vanuatu.
However, just like Seth, Cody transitioned from a tropical cyclone to an extratropical storm, with this secondary intensification providing the largest pulse of swell for the episode. The initial groundswell from Cody made landfall last Thursday, tailing off slowly through Friday before the next pulse arrived through Sunday, peaking Monday. We're now on the backside of this prolonged swell episode with the long-range easterly groundswell finally tailing away.
So that's two cyclones and two extratropical transitions since the start of the year.
But what is an extratropical transition and why are they so good for swell?
Before answering the above question we first have to know how tropical cyclones form, and so a quick refresher.
Tropical cyclones form (and strengthen) by extracting energy from a warm ocean - specifically, sea surface temperatures of 26.5° and above. Before a cyclone forms, however, we see a small surface depression (read: low pressure system) develop, with surface winds rushing into the centre to fill the vacuum created by the rising air. The warm air rises before cooling and condensing into clouds, releasing latent heat energy and consequently warming the air column above the low.
Now, things get complex here yet it's enough to know that a small area of high pressure forms at the head of a cyclone with air flowing out to surrounding areas of lower pressure, in turn creating a positive feedback loop. To read a longer explanation of this process see footnote*.
When fully formed, the cyclone's eye appears, a result of rotation induced by the Coriolis Effect. The cyclone then falls under the influence of upper atmospheric winds and further Coriolis forcing, steering it to the south and left, away from the equator in the Southern Hemisphere. This track is generally south and then south-east once entering the mid-latitudes.
As the cyclone starts to move outside of the tropics it encounters cooler sea surface temperatures and increasing wind shear (strong winds in the upper atmosphere) which generally leads to the breakdown of the cyclone's structure and feeding mechanisms, bringing the eventual weakening of the storm.
There are, however, some scenarios where a cyclone meets favourable upper atmospheric conditions which override the waning latent heat energy from the cooler ocean surface.
When this occurs, it's known as an extratropical transition (ET). ET cyclones become broader in size compared to the initial cyclone while also re-intensifying in strength - which is the key to their increased swell-generating potential.
The dynamics involved are tricky to articulate, but with the warm air infeed dissipating at the surface, the low needs to regain its momentum from the upper atmosphere. That being an upper-level low pressure trough - an area of divergence where the storm can expand and allow the surface low pressure to rise into and deepen. Such upper level troughs can also bring a large amount of wind shear, adding additional wind forcing to the top of the storm, increasing outflow and divergence, helping to lower the surface pressure.
Such winds in the tropics would destroy a tropical cyclone but in the transition phase this aids development.
Another driver is the introduction of cooler, drier air being advected (transported) from the mid-latitudes as the tropical storm heads towards the poles. The mix of warm, moist air from the core of the cyclone with cooler, drier air being advected aloft produces a volatile and explosive combination, fueling the re-intensification of the tropical storm.
When all these pieces of the puzzle come together we see the cyclone making an ET transition, and once complete the storm ends up being what's termed a cold core system.
This is where the swell-generating properties become favourable, as they did with the latter stages of Seth and Cody. With strengthening winds around the low acting on already active sea state, they generate large levels of additional swell after the initial cyclone swell energy. Due to the broader nature of the system, the swell is also farther reaching with a greater radial spread.
Below are some great satellite images depicting the ET transition of Tropical Cyclone Cody. When classified as a tropical cyclone, Cody has a tight, closed, symmetrical circulation with bands of thunderstorms surrounding it. (Source: Zoom.Earth)
The key signs for an ET transition are the breakdown of symmetry, widening of the storms centre, with a dry slot of air feeding in from the north-east flank. This dry slot identifies cooler, drier air originating from southern latitudes, wrapping in around its northern flank. Also, a cirrus edge develops in the poleward outflow followed by warm frontal cloud bands as shown clearly in the satellite image above.
ET cyclones slowly weaken once the cooler air advected into the core gets used up, or the ET cyclone gets absorbed into the jet stream and whisked off to the east.
*A further note on cyclone formation:
Air pressure decreases with height into the atmosphere. However, the rate at which warm air decreases in pressure through the atmopshere is slower than colder air. This means the rising colomn of warm air (generated by latent heat released from cloud formation) creates a small area of relatively high pressure compared to the air around it (because it hasn't dropped in pressure at a similar rate to air around it). Air flows from high to low pressure, so we see the air flowing out of the top of the storm to lower pressure surrounding it, creating divergence. A schematic of this process is shown in the image earlier in this article.
When this divergence at the top of the storm is higher than the inflow at the bottom we get a positive feedback loop and a further drop in surface pressure and a strengthening of surface winds. This whips up more water vapour for the storm and also increases evaporation across the ocean surface, feeding the low further until it forms into a tropical cyclone.