12-year analysis of Teahupoo's April and May swell patterns

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SURFLINE HISTORICAL SWELL ANALYSIS: TEAHUPOO [PT 1]

By: Sean Collins

March 23, 2010

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Surfline was asked to research the swell climate for Teahupoo, Tahiti to determine the possible best dates during April and May for an ASP World Tour event. The event traditionally has been scheduled for a 10 day waiting period in May, and was held during the May 9-20th period in 2009. Unfortunately, in the last few years the Teahupoo event has struggled with less than optimum surf, which prompted the exploration of a better time period to run the event. In this analysis our goal will be to help choose an improved future 10 day swell window during the months of April and May.

Historical swell patterns for the south shore of Tahiti were reviewed in this analysis. All significant swells for this area are generated by storms in the South Pacific, and surface pressure and swell analysis charts were reviewed during the April-May 1997-2009 periods to study large scale high pressure and low pressure patterns. These pressure patterns control all storm development and possible swell generation for Teahupoo, Tahiti.

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SURFLINE HISTORICAL SWELL ANALYSIS: TEAHUPOO [PT 1]

Now this graph is for swells 10 feet or greater between 1997-2009. A deep water 10 foot swell can grow to become a mutant of a wave with a 15-20 foot face barreling into 3 feet of water at Teahupoo. Notice the graphing of the days slides a little to the left if there were no 10 foot or greater waves during the period.

Additionally, Surfline LOLA and Wavewatch III archived swell data between 1997 and 2009 was used to develop a statistical foundation for our expectations of future swell patterns.

The final conclusion of this report is that the best time period for significant swell for Teahupoo, Tahiti during the months of April and May, are during the last two weeks of April and the first week of May.

WEATHER ANALYSIS OF SURFACE PRESSURE AND WAVE HEIGHT CHARTS - April-May, 1997-2009

The April to May period in the southern hemisphere tropics is Fall, when water and air temperatures are generally the warmest after the long Summer. Tropical cyclone activity has typically winded down somewhat but there is still the possibility of late season tropical activity. The jet streams near Antarctica and the Roaring 50's latitude region begin to shift north in April and May, also bringing cooler air masses farther north to mix with warmer air closer to the tropics. The mixing of the cold air from Antarctica and the warmer tropical air create atmospheric instability and deepening low pressure systems between 30 - 65 degrees South latitude. The developing low pressure systems in this region break the typical southern hemisphere west to east flow of winds and seas , to develop more south to north flow of winds and seas resulting in increased swell for Tahiti.

The ideal swell window for Teahupoo, Tahiti is displayed in the first chart in the slide show above. This swell window is the region that extends from just east of New Zealand to directly south of Tahiti, between 140-180 degrees West Longitude. As storms move underneath New Zealand, the storm fetch will extend northward throughout this region with strong winds and seas aimed at Tahiti. During April and May, a prevalent high pressure system is typically located over New Zealand and will extend eastward at variable times. Typically this high pressure system will help to generate strong winds on the back side of the storm with lots of swell.

Strong high pressure systems follow storms around the globe and can occasionally lock themselves down in specific areas for a month or more. If one of these strong high pressure systems happens to lock down in the middle of the swell window for Tahiti as per the example in the chart at the right, it will block swell generation by forcing the storm tracks to stay far to the south in a west to east flow of winds and swell. This is exactly the poor swell situation that happened in 2009 when a strong high pressure system blocked the storm track during the waiting period of May 9-20th.

The primary goal in extended long range forecasting is to calculate where the high pressure systems will lock down because their location will also dictate the storm tracking patterns. The April and May period in the South Pacific is a transitional period when the jet streams, high pressure systems, and storm tracks will be adjusting from their Summer to Winter patterns. The high pressure location over New Zealand is fairly consistent during April and May, and the eastern boundary of that high pressure system will bulge eastward with the ebb and flow of the passing storms.

Historically, April through May are good months for South Pacific swell activity due to the increased mixing of cold and warm air masses, and a receded ice pack off Antarctica also allows more swell generating fetch in the storms. After reviewing the charts archives for 1997-2009, there is a noticeable pattern of a sub-transitional period in May when the New Zealand high pressure will extend eastward as another high pressure system develops in the mid South Pacific. As this situation develops the Tahiti swell window may be temporarily blocked, and the time period might vary between a few days or a few weeks depending on the strength and location of the high pressure systems.

Historically, April through May are good months for South Pacific swell activity due to the increased mixing of cold and warm air masses, and a receded ice pack off Antarctica also allows more swell generating fetch in the storms.

-- Sean Collins

It's really impossible with today's technology to consistently forecast a year in advance when the New Zealand high pressure system might extend eastward to block the storm track resulting in smaller swell for Tahiti. But we can identify general periods when the odds are more likely for this not to happen and where there would be more swell for Tahiti. The analysis of the historical swell height charts indicate that mid April through June are the most consistent periods of swell for Tahiti, but very difficult to pin down the 1-2 week transitional period that seems to occur in May when the New Zealand high pressure system may extend out to block swell.

HISTORICAL ANALYSIS OF SWELL DATA FOR TEAHUPOO, TAHITI - April-May, 1997-2009

Wave data was retrieved for the swell model point located just southwest of Teahupoo, Tahiti. The first step was to eliminate all wave data that did not fall into the parameters to generate possible good surf for Teahupoo, so we filtered out any North, West, and East swells, Southeast trade wind swell, and any other local short period wind swell. Additionally, we wanted to focus on only the GOOD swells that would create optimum contestable surf for an ASP World Tour event at Teahupoo. The target data for this project was: all swells 6 feet of deep water swell height or greater; AND only swells with periods of 12 seconds or greater; AND only swells within a very specific directional range that is favorable for the specific reef at Teahupoo. Using these parameters all potentially good April and May swell events for Teahupoo were isolated throughout the 1997 through 2009 period.

The swell data was displayed in a series of graphs to allow easier analysis and presentation within this report, and additional filters were added to gain more resolution into the data. A shorter time period between 2005 and 2009 also displays a more near-term trend over the last 5 years versus the entire 13 year period to avoid long term skewed data. Swell heights were filtered from 6, 8, and 10 feet - with 12, 14, and 16 second periods to help isolate the larger swells. The source of swells for Tahiti is usually within 1,500 miles, so the longer period filters are the most significant to review and would also contain the longer 16-20 second period energy of the largest swells.

The first graph in Slide #3 above displays the overall swell trend for April and May throughout the entire 1997-2009 period, and the number of days of deep water swell 6 feet or greater, 8 feet or greater, and 10 feet or greater - but only when the period is 12 seconds or greater and the swell within the directional parameters for Teahupoo. This data indicates very good years of swell during April and May for 1999 through 2004, and 2007. There were generally poor years of swell during April and May in 1997, 1998, and 2005 through 2009 with the exception of 2007.

When comparing these graphs of swell model data to the historical Weather Analysis charts covered in the previous section, the swell results in Tahiti are very clearly affected by the location of the high pressure system over New Zealand, and whether it was blocking the swell window or allowing the swell window to be open.

The swell graphs that follow in the slides above display two views of the data: the first view showing the actual maximum swell heights for each day during April and May, and the second view showing the overall percentage of time that a specific day was over 6 feet, 8 feet, or 10 feet.

The graphs that most strongly reflect the detail to determine the best historical period of waves in the months of April and May, are the 8 foot swell charts. These swells are significant swell events. The view of the 8 foot or larger swell data between 1997-2009, and especially over the last five years between 2005-2009, strongly indicates that the best periods for significant swell for Teahupoo have been during the last two weeks in April leading into the first week of May.

An additional glaring result from the swell data shows how poor the recent waiting periods of May 9-20th have been during the last 5-year period of 2005-2009 for the Teahupoo event. As discussed in the previous Weather Analysis section, the sub transitional period in May when the high pressure over New Zealand shifts into Tahiti's swell window, may occur more often during the second week in May as reflected by the data.

Additional data reviewed and compared for analysis in this report included local Tahiti moon phases, local tides, El Nino and La Nina oscillations, and even sunspot historical data, to explore what other variables might possibly exist that could have additional effects on the swell. At this time, none of these other factors showed significant possibilities to add valuable input for future long term forecasts. Although there did seem to be more consistent swell during the transition from New Moon to Full moon phases for some reason. Hmmmm...

Using our historical LOLA Swell Model and Wavewatch III data, this report is an example of the long term capability we now have to determine the optimum surf, swell, and wind conditions for any location in the world. Not only will this analysis be valuable to help plan surfing events in the long term future, but also for surfers to identify the optimum period for surf travel anywhere in the world, far in advance.

As always, we continue to learn more and more the deeper we dive into the science and art of wave forecasting in our efforts to score the best possible surf.

There's always a perfect wave going off somewhere. We hope to help you find it!

See ya out there, Sean Collins...

Next up: Part 2. A long term swell analysis for Teahupoo in August and September, when the 2010 ASP Billabong Pro will take place.

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