One of the biggest ever documented Teahupoo swells hit on August 27 2011, and was dubbed the now famous ‘Code Red’ swell. Last week, on August 13 2021 – almost exactly ten years later – another massive swell hit. How did these two swells compare? Where were the storms that generated them, and what was so special about those storms that made the swells so big? What were the differences between them, if any? Let's have a look.
The swells that arrived at Teahupoo were both really impressive. Wave heights just off the coast peaked at around 12-to-15 feet with periods of around 17 to 19 secs. Both swells came from a direction just west of south.
Related: Teahupoo Code Red
As swells propagate away from the storm centre, the energy spreads out over a progressively wider area (see here). Consequently, the further away from the storm centre, the smaller the wave height.
Both the 2011 and 2021 swells were generated by large areas of storm-force winds associated with a low pressure system south of Tahiti. The 2011 storm was further away, so the wave-height reduction due to circumferential spreading would have been greater. However, the storm was more powerful and the wave heights initially generated were bigger. So, by the time the swell reached Tahiti, it was more or less the same size as the 2021 swell.
As swells are generated, the swell propagating in direct line with the wind that produced it is always bigger than the swell that spreads out either side. The greater the angle, the smaller the swell. For example, if the windfield contained predominantly southwest winds, the main bulk of the swell would travel towards the northeast. The swell travelling towards the north-northeast would be smaller, and towards the north it would be even smaller. Once you get to angles approaching 90 degrees, there is usually very little swell propagating in that direction.
Related: Terror Session
In both the 2011 and the 2021 swells, the main bulk of the swell wasn’t pointing directly towards Tahiti. It was heading in a north-easterly direction and would end up hitting Central and South America a couple of days later. The component that reached Tahiti was oblique swell that spread out in a northerly direction away from the storm centre – about 45 degrees away from the main direction.
Now, the above effect is more pronounced if the area containing the swell-generating winds (the windfield, or fetch) is relatively narrow. The broader the windfield, the more swell energy is maintained at angles away from the main direction. In both the 2011 and 2021 swell, the windfield was relatively broad. The broad windfield meant that the northerly component of the still contained a significant amount of energy, which was able to reach Tahiti.
There were, of course, one or two differences between the two swells and the storms that produced them. The 2011 swell was generated by a low that developed way south of Tasmania on August 22. It tracked eastwards, expanding and intensifying but remaining in the far south of the Pacific, just off the ice shelf. As it passed south of New Zealand a huge area of storm-force westerly winds developed on its northern flank, which persisted as the system continued to track east. Then, as it passed way south of Tahiti around August 24, the southwest winds on its western flank increased. Wave heights in the storm centre peaked at over 50 feet.
The system that generated the 2021 swell had a different history behind it. The main distinction was that it developed much further north than the 2011 one. It first appeared as a weak disturbance just west of New Zealand on August 7. It then tracked right over the North Island and began to deepen as it emerged out into the open Pacific on August 9. It continued to intensify and expand, while moving towards the east-southeast. It reached its peak intensity between 10th and 11th August, almost directly south of Tahiti, but significantly nearer than the 2011 system. The swell was generated by an area of storm-force winds on its northwest flank, with open-ocean wave heights peaking at just over 40 feet.
In a growing windsea, the different components of the wave energy inside the ocean interact with each other. As the waves grow, energy is continually flowing from the short-period into the long-period components. The more the waves grow, the more energy there is in the long periods. Therefore, really big swells also contain really long periods. And these periods persist as the swell propagates out, even though the wave heights diminish due to circumferential spreading, as I mentioned above.
The 2021 storm wasn’t quite as powerful as the 2011 one, so it didn’t generate so much energy in the long-period components. As a result, even though the swell ended up about the same size when it got to Tahiti, the long-period waves would have been more significant in the 2011 swell.
Cover shot of Code Red by Jason Corroto for MSW and Friday August 13th by Tim McKenna.