Why the winds were bad in the Mentawais this season

Craig Brokensha
Swellnet Analysis

Aside from being one of the most consistent places for swell on the planet, the Mentawais is also situated in a fortunate location for winds. Positioned between 1 and 3 degrees south of the equator, in a region often referred to as the Doldrums, the Mentawais rarely receives any gradient winds. It's an area of mostly slack breezes, often localised in nature and sensitive to passing storms.

Yet anyone who's visited the Mentawai Islands this year or kept an eye on our forecast notes would of noticed that from the middle to the end of this season unfavourable south-southeast winds have plagued the islands.

“The winds have been funky this year for sure,” says Aaron Sherman, Director of Operations at Macaronis Resort. Aaron claims the persistent south winds have limited the range of waves that can be surfed – notwithstanding Maccas' ability to shirk off a bit of southerly breeze.

It's also made travelling difficult, says Aaron. “I've heard stories from friends on charter boats about having to turn back from attempted crossings of the Sipora strait and also having to turn around on trips down towards the deep south Mentawai.”

An easy conclusion is that these vexing winds are linked to the current strong El Nino. But this isn't entirely correct; it's actually linked to another climate phenomena known as the Indian Ocean Dipole (IOD).

The Indian Ocean usually displays a difference in sea surface temperature across its basin. During a positive IOD event, warm water piles up in the western Indian Ocean (off the African coast) with cooler water in the eastern Indian Ocean (off Indonesia and Australia's North West coast). A negative IOD is the opposite with warm water off Indonesia and North West WA, and cooler water off Africa.

rao_cloud_p.jpg

During positive IOD events - with cooler waters off Sumatra and Java - there's an increase in south- easterly trade winds feeding towards the increased convection and low pressure to the west.

These trades also increase upwelling off the Sumatran and Java coast, reducing water temps further, creating a positive feedback loop that is usually strongest from August to October before weakening.

Now, a positive IOD - as has been happening this season - can occur independently of El Nino, but more often than not a positive IOD phase will be linked to an El Nino event. When a positive IOD and El Nino event coincide though, the affect on the local winds across the Mentawais is strongest and least favourable.

The link between the positive IOD and bad winds fit with what some longtime Mentawai guides have reported. Mark from Macaronis Resort reports that “2006, 2008, and 2012 were all south wind years” and that syncs up with the IOD index from the past decade.

     

While this doesn't spell the best outcome for surfers travelling to the Mentawais during these years there is one winner: the Maldives.

The prime swell window for the Maldives is to the south of Sumatra where broad and sustained fetches of south-east winds can sit anchored, aimed perfectly at the islands. Under a positive IOD phase, these south-east trades are stronger than normal, producing larger swells and bumper seasons, as has occurred this year.

The only problem for those looking to take advantage of this climate phenomena is that the IOD phase doesn't become apparent until the season is well under way. For instance, this year it wasn't until late autumn that the IOD was shown to be positive.

As forecasting improves it may become possible to predict the IOD from months out so surfers can choose their destination accordingly. At present it benefits late season decisions only. //CRAIG BROKENSHA

IOD Index chart courtesy of ESSO-INCOIS
Positive Dipole Mode diagram courtesy of JAMSTEC

Comments

thermalben's picture
thermalben's picture
thermalben commented Wednesday, 14 Oct 2015 at 9:54am

Follows on nicely from Craig's article back in August: "Epic week of waves inbound for Maldives" and the following recap from Stu: "Maldives swell of the season - photos and videos" - all the result of what Craig described at the end of the article above.

tonybarber's picture
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tonybarber commented Wednesday, 14 Oct 2015 at 10:12am

Interesting article. A bit confused - you state the positive IOD can occur independent of El Niño but can be linked. Which is it ? Seems we don't know the 'link' but it would be handy given that we tend to know when the El Niño develops. It is assumed that the Maldives had favourable winds.

Craig's picture
Craig's picture
Craig commented Wednesday, 14 Oct 2015 at 10:16am

Yes correct, positive IOD is only slightly correlated to El Nino (research being done in this area).

But.. when a positive IOD does coincide with El Nino, this is when we see the greatest affect on the winds off Indonesia (strongest SE anomalies).

Here's a list of the El Nino/La Nina/Neutral years along with IOD phase.. http://www.marine.csiro.au/~mcintosh/Research_ENSO_IOD_years.htm

donweather's picture
donweather's picture
donweather commented Wednesday, 14 Oct 2015 at 8:36pm

Craig wrote:

Yes correct, positive IOD is only slightly correlated to El Nino (research being done in this area).

But.. when a positive IOD does coincide with El Nino, this is when we see the greatest affect on the winds off Indonesia (strongest SE anomalies).

Here's a list of the El Nino/La Nina/Neutral years along with IOD phase.. http://www.marine.csiro.au/~mcintosh/Research_ENSO_IOD_years.htm


Craig that link only goes up to 2008. Any other info on more recent years. In particular 2014.
Craig's picture
Craig's picture
Craig commented Wednesday, 14 Oct 2015 at 9:02pm

Don, IOD index from above, and SOI..

kaiser's picture
kaiser's picture
kaiser commented Wednesday, 14 Oct 2015 at 12:07pm

I was in The Maldives in mid Sep and it was effing windy. Mostly Westerly which makes it offshore (or near enough) for Male and central atolls, but the strength was the main factor. Was there for a week and it never got below 15-20 knots, and got up to about 35 knots. Rained a shitload also

lost's picture
lost's picture
lost commented Wednesday, 14 Oct 2015 at 10:29am

Hey Guys,

So that was the season....what about the off-season. I'm on a 2 week boat trip to ments first two weeks in December. First time I have not gone in July. The operator sold a pretty good picture of that time of year but want to keep my expectation in check. What does the wind and swell typically do in December ? Anyone else been in December. Many thanks

knB

Craig's picture
Craig's picture
Craig commented Wednesday, 14 Oct 2015 at 10:50am

Haven't kept a close eye on the off season for the Ments (others would have a better idea), but in general less swell. One good thing is that positive IOD events usually weaken after their peak through Sep/Oct/Nov, so with this the winds should also improve for you.

Blob's picture
Blob's picture
Blob commented Saturday, 17 Oct 2015 at 2:30pm

I did 10 days in December on the Pelagic a few years ago. Dad's and sons trip. I'd do it again. Less spot options but a mellow shoulder to head high every day with soft cross to offshore winds. ...and we didn't even get 'a swell'.
We only saw 2 other boats, one of which scored Lances at 6' with 6 guys out just before we got there.
Don't tell anyone though.

mantown's picture
mantown's picture
mantown commented Wednesday, 14 Oct 2015 at 1:04pm

I went to Lagundri in 2005, the winds were constant south so we got a fishing boat to ASU. I stayed for 7 weeks it was offshore ALL DAY, EVERY DAY the whole time I was there. No surfcamps they'd fallen down in the earth quake, not one charter boat turned up and the most amount of surfers on the island at one time was 7 people. Memorable trip...

donweather's picture
donweather's picture
donweather commented Wednesday, 14 Oct 2015 at 8:40pm

mantown wrote: I went to Lagundri in 2005, the winds were constant south so we got a fishing boat to ASU. I stayed for 7 weeks it was offshore ALL DAY, EVERY DAY the whole time I was there. No surfcamps they'd fallen down in the earth quake, not one charter boat turned up and the most amount of surfers on the island at one time was 7 people. Memorable trip...

Hmmmm IOD was neutral to negative in 2005?
blindboy's picture
blindboy's picture
blindboy commented Wednesday, 14 Oct 2015 at 3:38pm

What about the water temperatures at G-Land? I went there one year and it was 17!

Craig's picture
Craig's picture
Craig commented Wednesday, 14 Oct 2015 at 3:47pm

Yep, G-Land was reported to be really cold this year as well. As the shelf is so close it seems to suffer more from the cold water upwelling compared to other regions.

blindboy's picture
blindboy's picture
blindboy commented Wednesday, 14 Oct 2015 at 6:50pm

Straight up from the bottom of the Java Trench!

gibbsy's picture
gibbsy's picture
gibbsy commented Thursday, 15 Oct 2015 at 6:54am

Yeàh this year was freezing, and really green from upwelling. However real strong ese trades in late Sept after a period of more s wind, especially through July and august. Lotta guys in steamers!

caml's picture
caml's picture
caml commented Wednesday, 14 Oct 2015 at 4:05pm

lots of wetsuit worn in 97 after june onwards. Cold as & south winds / variable / no trades . Asu is the spot agreed

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Wednesday, 14 Oct 2015 at 5:00pm

Good article, so it is more than just the Elnino.

I actually thought the water was cooler this year not cold but more refreshing than normal.

southey's picture
southey's picture
southey commented Thursday, 15 Oct 2015 at 1:26am

They are linked , but the causal is tenuous .
The main and easiest effect is a strong ElNino will invariably drag the IOD towards positive . Only if the IOD is set for a strong negative will an El Niño struggle to neutral the IOD and rarer than hens teeth an El Niño will couple with a negative IOD .
A strong La Niña will also give higher chance of a proceeding negative IOD but mostly when the IOD is in a neutral state .
IOD 's uncertainty period is from May -June . It usually becomes entrenched by July , when an impending Pacific certainty is looming , then sometimes like this season the swing can be quite late in August .
The IOD will then remain entrenched until the Monsoon drops down into the Sth Indo Archaepalogo in Late Nov/Dec . From then until April -May the symptoms of its state are benign as seasonal monsoon /MJO pulses take over .
There is a reasonably successful telltale of an impending independent IOD state , which is correlated with the presence of a major tropical disturbance / hurricane /typhoon in the Bay of Bengal in May -June . When I say successful it has about a 80 % chance of pre emptying the IOD forecast .
The other that is often cited is the waters directly to the west of the Irian Jaya . This is said to lead the Austral maritime continents tropical waters by 14 mths and inverted . Ie it currently just went extra cool and triggered a sudden late cooling in the Eastern node of the IOD and then also countered back into the western node of the ENSO ( Pacific - Coral Sea ) so that being said I will have to read back through some of Bob Tisdales blogs . He is currently the most learned in these and many other oceanic teleconnections .
For basic correlation on a broader scale , the western node of the ENSO shares influence with the Eastern Node of the IOD . And both systems are opposite . Ie positive IOD's correlate to how a La Niña works . Their effects on Australasia contrast , in that they counteract .
So it's like two magnets with opposing poles .
But unlike magnets , like pokes attract . The rare phenom discussed above is inverse of but the same result among as repelling poles on a magnet .

" SA's Reserve Capacity "

Craig's picture
Craig's picture
Craig commented Thursday, 15 Oct 2015 at 12:03pm

Thanks Southey, and reading literature now.

southey's picture
southey's picture
southey commented Thursday, 15 Oct 2015 at 1:26am

" SA's Reserve Capacity "

Craig's picture
Craig's picture
Craig commented Thursday, 7 Jul 2016 at 9:28am

Here's a great video put together by the BOM explaining the IOD and more info here: http://www.bom.gov.au/climate/iod/

FYI we're in a negative phase and it is clear to see in these observations..

caml's picture
caml's picture
caml commented Thursday, 7 Jul 2016 at 9:37am

At last someone has explained this & might have a clue as to what southey & co are talking about . If I just watch it again & learn it , so does that explain why g.land gets cold water ? cheers craig great info

Craig's picture
Craig's picture
Craig commented Thursday, 7 Jul 2016 at 9:41am

Yeah, you'd be looking at a positive IOD and also stronger SE trades further inducing upwelling like last year.

cycd's picture
cycd's picture
cycd commented Thursday, 7 Jul 2016 at 9:51am

Just reading between the lines .... are you indicating the same wind pattern this year? heading there n a week so curious. Cheers

Craig's picture
Craig's picture
Craig commented Thursday, 7 Jul 2016 at 9:52am

No, this year is negative IOD, last year was positive :)

groundswell's picture
groundswell's picture
groundswell commented Thursday, 7 Jul 2016 at 11:12am

Does this effect nusa tengarra as well?

Craig's picture
Craig's picture
Craig commented Thursday, 7 Jul 2016 at 11:13am

I would say so, but possibly to a lesser extent? Not sure.

groundswell's picture
groundswell's picture
groundswell commented Thursday, 7 Jul 2016 at 11:35am

Thanks Craig and good article. Also Southeys comments. Good info.

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 7 Jul 2016 at 1:25pm

Good Vid.

And makes sense last year when i was in Sumatra there was south winds almost everyday with little rain (remember the fires)

This year during the month i was in the Mentawais and Telos the winds were mostly from the North West with heaps of rain.

Craig any obvious reason for such a big jump from Positive to Negative? as the years before don't have that bigger changes.

Craig's picture
Craig's picture
Craig commented Thursday, 7 Jul 2016 at 1:39pm

I'm guessing due to one of the strongest El Nino's on record, the rebound had to be significant?

Southey might bet better answering that?

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 7 Jul 2016 at 1:25pm

Not that i got to surf anyway :(

barrel_pig's picture
barrel_pig's picture
barrel_pig commented Thursday, 29 Aug 2019 at 5:41pm

She's back. Just had my dream trip tarnished a little due to dreaded SSE winds.

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Thursday, 29 Aug 2019 at 5:55pm

Yeah pass month south winds every day all the way up to Aceh sucks for rights.

freeride76's picture
freeride76's picture
freeride76 commented Thursday, 29 Aug 2019 at 6:05pm

lots last season too.

Craig's picture
Craig's picture
Craig commented Friday, 30 Aug 2019 at 11:26am

Yep, we've got another strong +ve, similar to when this article was first created.

You can see the anomalies here, cold off southern Sumatra and Java..

Max Wax's picture
Max Wax's picture
Max Wax commented Sunday, 6 Oct 2019 at 6:54pm

hey craig, do you think this +ve will last through all of november early december? its seems stronger than the 2015 event?

Craig's picture
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Craig commented Sunday, 6 Oct 2019 at 7:39pm

Yeah it'll start to dissipate but with it being so strong it's effects will still likely be felt.

Craig's picture
Craig's picture
Craig commented Sunday, 6 Oct 2019 at 7:43pm

Here's the FC for Nov, still 1.29..

Blowin's picture
Blowin's picture
Blowin commented Sunday, 6 Oct 2019 at 7:50pm

Craig ....does this mean the water is currently colder than average off South Sumatra ? How cold exactly?

More southerly winds ?

Craig's picture
Craig's picture
Craig commented Sunday, 6 Oct 2019 at 7:56pm

Yeah it's quite cold!

Looks 3deg+ cooler than normal.

And at a closer look here you can see the temps are 24-25?


Max Wax's picture
Max Wax's picture
Max Wax commented Sunday, 6 Oct 2019 at 9:26pm

Thanks guys, i'm heading over in mid nov, might pack a wetty hahaha

Blowin's picture
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Blowin commented Sunday, 6 Oct 2019 at 9:49pm

You might be joking , but as the boys have said , I’ve been to G land and wished I’d had a wetsuit.

Surely it doesn’t get that cold in Sumatra, does it ?

Max Wax's picture
Max Wax's picture
Max Wax commented Sunday, 6 Oct 2019 at 9:55pm

nah semi joke semi serious, i've heard the same about g land, and whenever i go to sumatra its significantly warmer than g land and bali

Blowin's picture
Blowin's picture
Blowin commented Sunday, 6 Oct 2019 at 7:59pm

24-25 sounds delightful.

Cheers for that.