Remembering XXL Indo 2018: The Session All Others Will be Measured By

Tony Butt

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Updated 18d ago

In this series, MSW forecaster Tony Butt takes a look at some of the most prominent sessions that have left a lasting impact on the surfing and ocean-faring world. Check out the analysis of Hercules, Braer and Hawaii 1969.

One of the biggest, gnarliest swells in recent history hit Indonesia on July 24-26 2018. If you happened to be there at the time you’ll know that Indonesia was in the middle of an incredible run of surf that started a long time before and continued a long time after that swell.

It seemed like one swell merged into another and, at times, it was difficult to tell when one ended and the next began. But most people agree that the 24th to 26th was the most spectacular. With numerous high-profile surfers and photographers on the case, it was one of the most documented big Indo swells ever. Here we are going to have a brief look at the mechanics behind that swell.

Forecast: Uluwatu

Wave height and period for the Indian Ocean July 2018.

Wave height and period for the Indian Ocean July 2018.

The storm responsible for the swell started off as a broad, complex area of low pressure in the mid-south of the Indian Ocean around July 18. At this stage it was nothing unusual. But 24 hours later things started to change. The multi-centred complex system had evolved into a single-centred low and deepened considerably. It was now situated about 1000 miles southeast of the Kerguelen Islands, with a developing area of strong southerly winds on its western flank.

For the next 24 hours it remained almost stationary and intensified, with the central pressure dropping to 932 mb. There was now a massive area of storm-force winds pointing straight towards Indonesia. The windfield persisted in size and strength as it started moving towards the northeast, creating that all-important dynamic fetch I’ve talked about many times before (HERE). After pumping out a massive swell, the system finally began to weaken around July 22, and was eventually absorbed by another complex area of low pressure south of Australia. Open-ocean wave heights in the southeast Indian Ocean off southwest Australia were exceeding 40 feet, and the swell on its way to Indonesia was unstoppable

And by then, open-ocean wave heights in the southeast Indian Ocean off southwest Australia were exceeding 40 feet, and the swell on its way to Indonesia was unstoppable.

The first long-period forerunners started arriving at exposed coastlines in West Java late on 23rd July. They quickly spread further afield and arrived at most other spots within the next few hours, with periods in excess of 20 secs. To the east, at Uluwatu, for example, the swell ramped up overnight and continued to do so all day on 24th. It peaked early on 25th but really didn’t drop much all day, with wave heights a solid 15 feet and periods around 19 secs. The swell continued the next day, and slowly tapered down over the following three days.

Way over to the west, at Nias, a similar pattern emerged. Forerunners with periods of about 23 secs arrived in the early morning of 24th, then the swell gradually filled in and plateaued at a solid 10 to 12 feet all day on 25th. It continued through 26th and then gradually diminished over the next few days.

Almost every winter the Indian Ocean gets storms that look just as spectacular on the charts, but not all of them produce such big surf as this one. Why? Well, one factor was the dynamic fetch, as those storm-force winds travelled in the same direction as the swell they were producing, pumping extra energy into the ocean surface.

But dynamic fetch wasn’t the only factor. The windfield maintained its intensity a long way east in the Indian Ocean, practically all the way to Western Australia – almost directly south of Indonesia. So the swell that arrived on the Indonesian coast was direct, square-on, rather than weaker tangential swell that would have been produced by a system that intensified further west.

It is also interesting to note how the height and period changed as the swell progressed. When the swell arrived, the period was longest right at the beginning and then steadily decreased. This is classic, text-book behaviour for long-travelled swells – the longer, faster waves outpace the shorter, slower ones – a phenomenon called radial dispersion (HERE).

Big Ulus is such a sight, no?

Big Ulus is such a sight, no?

© 2020 - PBdrone.

However, you’ll also notice that the wave height didn’t pick up from nothing to 15 feet in a blink of an eye. Instead, it took a whole day to ramp up to its maximum size, reached a plateau and then ramped down again even more slowly. When a swell like this arrives at the coast from a distant storm, the way the wave height changes relative to the period as is not always the same. It ultimately depends on how the heights and periods are distributed in the storm centre itself.

The DNA of the swell, as it were. This is something that was studied a lot by some of the pioneering scientists, right at the beginning of the development of wave-forecasting models, and is something I’ll talk about in a future article.

Uluwatu forecast for July 2018. The swell first appears at 18:00h on the 23rd and takes until midnight on the 24th to reach its peak.

Uluwatu forecast for July 2018. The swell first appears at 18:00h on the 23rd and takes until midnight on the 24th to reach its peak.