Mentawai historical swell data by month

As a Surfline premium member, I get access to crazy amounts of data.  Over the last year or so I have been manually collecting data for the Mentawai/Sumatra area day by day from Surfline's LOLA model.

To the best of my knowledge, this data for Sumatra isn't based on anything collected by scientific instruments in Indonesian waters or human observation.  It's just the output from a complex proprietary Surfline algorithm for predicting swell height, period, and direction based on limited, but real wind speed, direction, and sea state data collected by weather stations, buoys, and US government satellites.  As such, it needs to be taken with a grain of salt.

This narrow swell-focused view does not take into account wind or tides, but I feel these are reasonable to ignore.  With the smaller tidal range west of Sumatra most spots work acceptably on most tides and tide flow directions.  With respect to wind, Sumatra is noted for light and variable wind.  Strong, consistent winds from one direction do happen (like the S/SE pattern July-Sept 2006) but such patterns are relatively rare until you go south of the Mentawais on mainland Sumatra. 

So on to the numbers - what I have here is the average "Swell Energy" for each month from July 07-February 09.  I use quotes around swell energy because if Surfline said a given day was 3-4 ft at 15 seconds, I just add the smaller height number to the period to get Swell Energy.  So a pumping, big-as-it-gets swell of 9 ft 17 sec would be a 26 whereas some 3 ft 12 sec junk would be a 15.  This works because wave energy is a function of both swell height squared and swell period squared.  To some degree, the two are fairly interchangeable.  This isn't to say that 3 ft 17 sec will look just like 6 ft 14 seconds at your local break, but that level of complexity is spot-specific and beyond what I can possibly figure out.

July 2007 Average 21.1

August 2007 Average 21.6

September 2007 Average 20.3

October 2007 Average 18.4

November 2007 Average 16.9

December 2007 Average 15.1

January 2008 Average 16.1

February 2008 Average 16.8

March 2008 Average 17.9

April 2008 Average 18.9

May 2008 Average 19.9

June 2008 Average 19.8

July 2008 Average 20.6

August 2008 Average 19.9

September 2008 Average 19.7

October 2008 Average 17.2

November 2008 Average 13.6

December 2008 Average 15.2

January 2008 Average 16.5

February 2008 Average 14.9

I know there are a lot of sites out there that crunch historical data and give it away for free like this one - but there are differences.  Since magicseaweed has to cover all sorts of crappy unreliable spots in Florida, the Caribbean, Brazil, etc, their bar for "ridable" is a lot lower than the average Mentawai boat trip punter.  To say that Macaroni's is "rideable" between 62% and 90% in January might be true in the Florida sense, but if you pulled up there and the swell was 4 ft 11 sec, the guide wouldn't even drop the anchor before pulling a U-turn and heading for Thunder's.

From what I have seen swell energy of 19 or 20 should be enough juice to make swell magnets go overhead+ and get all but the neediest spots into the shoulder-to-head high range.  This is the swell that gives you the session of the trip, everyone's best wave of the trip, all the keeper photos and video clips, and sticks in your mind and makes you determined to come back next year.  In peak season, you can see the islands off Sumatra do that pretty consistently.