Remarks From A Most Remarkable Swell

It was the swell that went against the grain.

Here on the East Coast most of our swell arrives from the south. Yet if you were to conduct a straw poll among NSW surfers, there's little doubt the majority would say they prefer swells from the east or northeast.

Yeah, some coastlines really light up in south swells - we're looking at you Cronulla, Shellharbour, and Ulladulla - however the really memorable swells, the ones that lodge in our collective consciousness, are almost always from the east.

May '83, June '91, July '01, June '16 - all struck from 90° or less.

The argument against oversized south swells is two-fold. Firstly, for the majority of NSW, they go to waste. They're great to look at but those long southerly lines mean the options are limited towards the southern ends, which are extremely straight thanks to swell refraction.

Secondly, most southerly swellls are one-day pulses - there one minute, gone the next - as the coast gets brushed by sidebar energy and the bulk of the swell moves off towards Fiji.

However, following four days of waves between six and fifteen feet, NSW surfers may have to re-evaluate their relationship to southerly-angled swell.

For once, the big wave brigade got into a rhythm, picking up where they left off the day before, which allowed the adrenalin and nerves to settle. And even those not chasing the big stuff could pick and choose provided they had somewhere protected nearby.

It was a most remarkable swell.

The swell trace from the Eden waverider buoy shows the wave height topping out at 10.4 metres overnight on Sunday. Incredibly for a south swell, the peak arrived under light wind conditions (BOM)

It all began last Wednesday when the remnants of a storm that, a few days earlier, delivered large surf to Indonesia then traversed the Australian continent and moved into the Tasman Sea.

Its arrival brought a short-lived spike of initial southerly swell, though more importantly it laid the foundation for what was to follow.

As the low lingered in the Tasman Sea it drew in warm, moist air from the northeast while at the same time a mass of polar air was injected from the south. These were the active ingredients for a significant deepening of the low in the southern Tasman Sea.

ASCAT readings show a long-lasting (the readings are two days apart) and broad area of 35 - 45 knot winds - that's gale-force to severe-gale - aimed towards the NSW coastline (NOAA)

It didn't just deepen, however, the early stage of the low was marked by its movement towards the Australian continent - what's called retrograde movement.

Why that's remarkable is the low was positioned at roughly 45° south, firmly ensconsced within the Roaring Forties, the belt of winds that blow relentlessly eastwards, yet rather than being whisked away with them, the low moved back to the west, towards Australia. The reason for that is explained shortly.

This movement created an initial bout of southeast swell, acting on an active sea state, with the retrograde creating a captured fetch scenario - where a storm moves in the same direction and at the same speed as the swell it's generating - which rapidly accelerates swell growth.

The swell front arrived on Sunday, remarkably coinciding with a clear skies and near-zero wind in the afternoon.

Late Sunday on the NSW South Coast and a surfer is dwarfed by the hard-hitting swell (Photo Simon Kotze)

After the inititial southeast burst, the low sat mid-Tasman for days on end, projecting multiple surges of gales up towards the Australian coast.

The unique thing about this setup - we're going to stop saying how remarkable it was - is how stationary the low has been since its development. This is thanks to an upper-level ridge effectively cradling the low between New Zealand and Australia.

The BOM's MSLP charts from 10am Saturday to 10pm Sunday show the low deepening, retrograding, then remaining near stationary. At the time of publication - Friday 21st June - the low was still in the Tasman albeit much weaker (BOM)

Though it's not obvious on basic Mean Sea Level Pressure (MSLP) charts, in simple terms the low is sitting in an area that’s not feeling any influence from the polar jet or westerly storm track, which would otherwise move the low off to the east. Instead the low has been meandering, bumping this way and that, and even retrograding back to the west, bringing favourable winds and at times freezing temperatures across the eastern seaboard.

The MSLP anomaly chart for the past week below shows it clear as day - see image below left. The pressure has been around 16hPa below normal in the southern Tasman Sea, along with the cradling high pressure either side. Meanwhile, the chart on the right shows the sea surface temperature change for the same period. We can see the low has absorbed energy from the Tasman Sea, resulting in a cooling signal off the southern NSW coast.

MSLP anomaly for the past week (left) and change in sea surface temperature for the same period (blue/purple = cooling). (NOAA)

The low produced extra-large surf across Tasmania on the weekend which continued into the early week with the swell marching up the southern NSW coast later Sunday, with various ebbs and pulses seen throughout Monday, Tuesday, and Wednesday.

The wave buoys showed the most significant groundswell since April 2022 but the results on the ground varied.

Some places lit up for the first time in years while other saw the swell marching straight past. As often happens, local bathymetry combining with little tweaks in period and direction sometimes played funny buggers with the waves. Though at some spots that meant better surf with less hype the day after the peak in energy.

Click here to view a photo gallery from the remarkable swell.

// CRAIG BROKENSHA with STU NETTLE