Tropical low or cyclone, what's the difference and how do each differ in regards to swell production?

With the tropical activity currently firing up across the north of the country, there's still some conjecture on whether we're looking at a tropical low or tropical cyclone forming.

Currently the storm falls under the tropical low category, with wind speeds around its centre being below gale force (35kts, 69km/h). Once wind speeds reach the gale-force threshold across two quadrants (which it's forecast to) it becomes classified as a tropical cyclone and given a name by the appropriate regional meteorological service. In Australian waters this is done by the Bureau of Meteorology. The next tropical cyclone name in line is Seth.

It's interesting to note that the wind strength threshold depends on the overseeing entity. In the Pacific and Atlantic Oceans tropical cyclones are known as typhoons or hurricanes. NOAA's National Hurricane Center requires higher wind strengths before they are named - that being an aptly named hurricane-force (64kts, 119km/h). This means that Cat 1 hurricanes are stronger than Cat 1 tropical cyclones.

Tropical cyclones also need sea surface temperatures of at least 26.5°, providing a continues supply of energy from the ocean into the atmosphere.

Current tropical low strengthening in the Coral Sea (Zoom.Earth)

Regarding swell production, it's not the size/intensity of the storm but the presence of a supporting high pressure ridge that really matters.

Put simply it's the difference in pressure between the high and the centre of the tropical storm that forms the basis of how strong the swell generating fetch will be and over what area. The greater the pressure difference the stronger the winds, and the broader/wider the area the high takes up the broader/wider the fetch of swell generating winds.

If there's no supporting high for the storm to squeeze against, even if it's a severe tropical cyclone, the severe winds around it will be very localised and only generate a very directional, short-lived swell event (shown below).

Dual tropical lows with no supporting ridge to the south, leading to localised strong winds

Counter this with a setup where the tropical storm squeezes against a strong, supporting high pressure ridge, and we see a much broader, elongated area of swell producing winds generated along with the stronger winds around the core of the storm acting on an energised sea state. This produces a prolonged, wider reaching swell event.

Whether or not the current storm reaches cyclone strength or not is a moot point, with all the other ingredients falling into place for an extended swell episode.

Right now we've got a good, broad supporting high sitting across the Tasman Sea, with the tropical storm due to track south-east, setting up a tightening pressure gradient and broad 20-35kt fetch. This alone would generate a healthy, moderate sized easterly trade-swell event, but with the tropical system strengthening while travelling further south, we should see additional gales producing a larger pulse of groundswell for most of the East Coast.

A much more favourable setup with the tropical low squeezing against a high pressure system, generating a broader, stronger fetch

The high is forecast to move slowly east across New Zealand as the tropical low drifts south, allowing it to stall while slowly weakening due east of the Port Macquarie region during early next week. In most cases the ridge stays stationary and tropical storms slide off its eastern flank, away from Australia.

What's fascinating to observe is the relaxing pressure gradient and weakening winds around the low as the high moves eastward, resulting in a slowly easing swell in both period and size.

There's one final type of low to discuss, that being an extratropical low. An extratropical low is a tropical low that makes an extratropical transition. This occurs when the tropical system moves into the mid-latitudes (away from the equator), interacting with cooler upper atmopsheric air resulting in the transformation from a warm core system to a broader, more intense cold core low.

These extratropical lows produce the strongest winds on the poleward side of the low, which for the East Coast generates favourable east to south-east swells.

The current tropical low isn't forecast to make an extratropical transition due to the lack of cold air feeding in from the south but it's one to watch out for as we head into autumn.