Lows, Cyclones, Typhoons and Hurricanes: What's in a Name?

Craig Brokensha picture
Craig Brokensha (Craig)
Swellnet Analysis

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.

Comments

tango's picture
tango's picture
tango Thursday, 30 Dec 2021 at 9:04pm

Nice one, Craig.

The interesting thing to me is that the level of professional and amateur analysis on this system is out of all proportion to the intensity and swell it appears to be going to generate. If this one is any indication, the level of froth for the first proper cyclone looks to be highly likely to be completely off-tap....

freeride76's picture
freeride76's picture
freeride76 Friday, 31 Dec 2021 at 7:04am

hahahaha.....it's hard for southern staters to understand how much we love these systems Tango.

Craig's picture
Craig's picture
Craig Friday, 31 Dec 2021 at 7:06am

And then the Joint Typhoon Warning Center throw in the hybrid-low curve-ball..

"THIS SYSTEM IS A HYBRID SYSTEM WITH BOTH SUBTROPICAL AND TROPICAL CHARACTERISTICS"

Refund all bets? Ha.

freeride76's picture
freeride76's picture
freeride76 Friday, 31 Dec 2021 at 7:09am

Just an observation: it seems to me hybrid systems tend to be larger, more expansive storms (more like once they undergo ETT) compared to TC's.

Craig's picture
Craig's picture
Craig Friday, 31 Dec 2021 at 7:09am

Yep, not a tight eye like a true cyclone.

freeride76's picture
freeride76's picture
freeride76 Friday, 31 Dec 2021 at 7:19am

And they have a tendency to stall out between Fraser coast/NENSW coast.
Compared to TC's which accelerate poleward after recurvature.

That may be just co-incidence.

memlasurf's picture
memlasurf's picture
memlasurf Friday, 31 Dec 2021 at 9:34am

I always wondered if NZ gets the benefits from cyclones they are certainly in the firing line on the west coast, however you never hear anything about it or see any vids/photos.

Spuddups's picture
Spuddups's picture
Spuddups Sunday, 2 Jan 2022 at 6:00am

Yep, we get swell from them. Mostly on the NE coast, although some do send NW swell down the west coast as well. NW being an unusual angle lights up some spots that don't break that often. Exciting times.
I think the reason you don't see many photos or video comes down to the relative lack of population over here. You get the same thing with Tasmania. I'm sure there's great waves other than Shipsterns but you very rarely hear about them.

Island Bay's picture
Island Bay's picture
Island Bay Sunday, 2 Jan 2022 at 12:07pm

The supporting/cradling ridge is to the south of a tropical low, meaning the OZ east coast gets the significant swell off the much stronger E/SE fetch (squash zone).

On the N/NE flank the gradient is much weaker, with mostly very little NW swell reaching the west coast of NZ.

Every now and then, though...

Greenfrog's picture
Greenfrog's picture
Greenfrog Friday, 31 Dec 2021 at 9:57am

It seems generally the cyclones that occur in WA don’t produce the swell that they do on the east coast. Pilbara gets about 2 - 3 foot and if they round the Exmouth cape the west coast may get overhead but it only lasts one day if your lucky.

Craig's picture
Craig's picture
Craig Friday, 31 Dec 2021 at 12:13pm

Yeah that's because the ridge is usually on the southern side of the cyclone, so winds aimed to the west and away from WA. The top side of the cyclone is never going to get strong and generate significant west winds.

When there is a ridge more on the eastern side (rare), generating stronger N'th winds, the cyclone usually quickly drops south of even south-west and away from the coast.

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 9:35am

We used to go to Lakeys back before all the forecasting on the net etc early season , and we realised we had cyclone swell when people would turn telling us that there had been another cyclone off WA as the swell was south or SE

Craig's picture
Craig's picture
Craig Saturday, 1 Jan 2022 at 1:01pm

Interesting, was the left running real fat and out to sea?

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 1:17pm

Can't quite remember ,Nungas was biggest no one went out but yeh breaking wide . The peak just got bigger every set to 6+ easy within a few hours and just powerful with boards getting snapped . 2 days later a bloke had an Aussie newspaper with a 4day synoptic and that confirmed it to us was out take . Would of been early March

udo's picture
udo's picture
udo Saturday, 1 Jan 2022 at 1:19pm

What year [s] ?

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 1:22pm

Now that's tricky around maybe 91 , 92 ,93

Craig's picture
Craig's picture
Craig Saturday, 1 Jan 2022 at 1:25pm

Maybe Severe Tropical Cyclone Ian in 91/92 though that track would of still generated S/SW swell..

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 1:32pm

Your onto it most likely was , I just remember clean lines , super consistent lots of waves per sets , sunny skies , no weather whatever from it .

Craig's picture
Craig's picture
Craig Saturday, 1 Jan 2022 at 1:33pm

Noice!

thermalben's picture
thermalben's picture
thermalben Saturday, 1 Jan 2022 at 2:02pm

Of course, there's also the chance that an unrelated, out-of-season Southern Ocean groundswell happened to be pushing through at the same time that STC Ian was meandering off the NW West Oz coast.

Hard to know without charts.. I'll see if I can find any.

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 5:50pm

No worries

john.callahan's picture
john.callahan's picture
john.callahan Friday, 31 Dec 2021 at 12:08pm

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

A low pressure cyclone or tropical low doth not make a swell-producing fetch alone - needs the adjacent high pressure to set up a proper wind field.

freeride76's picture
freeride76's picture
freeride76 Saturday, 1 Jan 2022 at 1:38pm

Not so much in the North Pac.

Lots of intense TC's forming and tracking towards Phillipines lack a good supporting ridge.

perfect track and intense fetches compensate.

Thegrowingtrend.com's picture
Thegrowingtrend.com's picture
Thegrowingtrend.com Friday, 31 Dec 2021 at 2:31pm

i like cheese

scott.kempton's picture
scott.kempton's picture
scott.kempton Saturday, 1 Jan 2022 at 9:26am

Great read

topgeer's picture
topgeer's picture
topgeer Saturday, 1 Jan 2022 at 4:59pm

GFS is showing that high over the Tasman weakening and drifting rapidly SE as the low to the East of NZ intensifies and moves backwards... so not as much fetch and swell generation.
The NZ NE coast is going to receive a solid groundie from it, but the winds are crap.

Fluiddreams's picture
Fluiddreams's picture
Fluiddreams Sunday, 2 Jan 2022 at 2:23pm

The slightest hint of any kind of cyclone = unbelievable crowds.= carpark mayhem. Gotta love the Sunshine Coast.