Analysing The 2022 Christmas Swell

The largest, strongest groundswell of the year impacted the southern states over the last couple of days, and the irony was not lost on South Australian and Victorian surfers.

With such a poor winter for large, quality groundswells, to have the one of the most pure, largest groundswells hitting in the depth of summer is quite remarkable.

While it's not uncommon to see large swells during summer, to have one with such a large period (20s) undisturbed by any other local noise is rare.

We touched on this swell in the country wide Christmas forecast last week, while the regional Forecaster Notes delved a little deeper into the source of this significant swell event.

That source being a 'bombing' low that developed to the south-west of Western Australia last Friday. A 'bombing' low is a low pressure system that deepens rapidly, having to drop at least 24hPa in central pressure over a 24 hour period.

This is quite a significant intensification and only occurs a handful of times through the year, if that in the Southern Ocean.

With 'bombing' systems being so few and far between, there has to be a catalyst for such rapid intensification. In this case it was a tropical depression sitting just south of Madagascar and east of South Africa being swept south-east into the westerly storm track. An upper level cold outbreak was responsible for capturing the tropical depression, and this combination of tropical moisture and cold air resulted in the rapid deepening of the tropical depression as it was dragged down to the furious 50s.

Explosive cyclogenesis occurred between Western Australia and the Heard/Kerguelen Islands, resulting in wind speeds around the core of the low reaching 50-60kts (93-111km/h). These winds speeds were maintained for just under 24 hours as the low pushed east and broadened.

The track and eastward speed of the low was similar to that of the swell is was creating, known as a captured fetch. Larger than normal wave growth is seen under such scenarios, as well as the development of a defined open ocean swell front.

This was the case with this 'bombing' low, with it generating an extra-large, long-period swell front that produced step-ladder sets (rising set by set) when impacting the exposed coasts from Western Australia to Tasmania.

The steep swell front can be seen on all the buoys scattered across the country, with a unique J-curve increase in size right after the peak-period of 20-22s hits.

J-curve signal across Western Australian, South Australian and Victorian wave buoys

Reports from various surf zones were all of the same nature, ranging from “I've never seen the swell pick up so quickly” to “This is the biggest I've seen this spot breaking” and close-out sets across bays that shouldn't close-out.

The very small end of the scale. Note the long-period lines stretching across the whole embayment (Luke Greaves)

Below is a reverse trajectory plot of two seperate air parcels from the centre of the low. One sitting at sea level (red line) and the other 5,500m into the troposphere (blue line).

The end point is the 'bombing' low south of Australia, but going back to the west (towards the origin of each parcel), we can see the red, surface parcel originated south of Madagascar (sub-tropical in nature), while the blue lines shows the origins of the cold, upper atmospheric air, that being polar latitudes, tracking north-east before dragging the tropical instability south-east and into the Southern Ocean.

Reverse trajectory plot showing origins of air parcels at the centre of the 'bombing' low (NOAA // HYSPLIT)

The fairly quick transition of the low to the east and pronounced initial swell front leads to a faster than normal easing trend, and that's being seen across all locations into today (yesterday Western Australia).

Finally here's an incredible satellite grab of the 'bombing' low on Christmas Eve, note the scope of the system with warm, pre-frontal cloud bands followed by speckled, cold polar air.

Satellite capture of the 'bombing' low. December 24th. 9pm (ZoomEarth)