Shivering in Sumatra
A few years ago we introduced you to the Indian Ocean Dipole (IOD), an index monitoring the difference in water temperature across the Indian Ocean Basin. The talking point was a strong positive event in 2015, bringing poor and strong S/SE winds to the Mentawais while providing an excellent late season in the Maldives owing to the persistent sizey south-east swell.
Another positive IOD event has occurred this year, but the strength is almost twice that of 2015, and reports coming back from the Sumatran region are of cold water, very strong and limiting S/SE winds along with much drier weather.
This from a regular Swellnet contributor on the ground..
“Everyone is a bit stunned by the water temps. Apparently they are way down on normal. About five weeks ago was meant to be even colder. With a plague of stinging jellyfish. When I first got here three weeks ago I was forced out of the water from wind chill with the long waits between sets. Many long term visitors reckon its unprecedented .
I saw a local Indo fella in his diving steamer and hood the other day, but that was an aberration, most crew are just in neoprene vests. I’m just right in a spring suit. Luckily I met a bloke who was leaving who sold me a spring suit.
I reckon the water is 21-22 but the wind makes it seem colder if you’re sitting around for a while.
The water has been green mostly and then brown with the big swells. Lots of foam on the water and thick fogs due to temperature differences.
Every day has strong SSE winds, which are supposed to be dying or glassy this time of year.
Locals are saying it’s very much drier than normal as well. Should be rains most nights but have only had one or two brief showers.
Swells have been relentless though. Everyone is saying they’ve never seen such a prolonged run of large swell , particularly this time of year. The point here has been 6-8 plus ( plus, plus ) for a week or so.”
So what's the cause and how significant is this year's event?
The Indian Ocean Dipole is a measure of the difference in water temperature between the north-western Indian Ocean (off the African coast) and north-eastern Indian Ocean (off Indonesia).
In a positive phase, the waters off Indonesia and northern Western Australia are colder, with warmer water off the African coast, and opposite under a negative phase. This plays a big part in Australia's climate as well as affecting the winds and weather across the Indian Ocean basin.
When in a positive phase, (with colder waters off north-west Australia) there's less moisture available to weather systems pushing down and across Australia, resulting in a drier and hotter weather through winter and spring. When negative, the warmer waters off Australia bring increased rainfall to the country and lower maximum temperatures.
The triggers for IOD events are still unknown, some believe they are triggered from the western Pacific Ocean and linked to El Nino/La Nina events but others believe it is triggered closer to Indonesia.
When looking at positive events like we're in now, an initial upwelling event off the Sumatran coast at the start of winter results in a greater heat differential between the ocean and land, and stronger sea breezes out of the S/SE, which then again creating more upwelling and a positive feedback loop.
Equatorial westerly winds also weaken and even reverse, pilling up warmer water to the east towards Africa and enhancing the upwelling further off Indonesia. Similar to upwelling events off the Australian coast, the increase in nutrients and photosynthesis has also resulted in darker, greener waters which can be picked up by satellite by looking at the chlorophyll concentration.
This year's event has seen the IOD index push just over 2, (2.06 to be exact) and this is the strongest since 1997, some two decades ago, and likely the second strongest in the last 37 years of records.
This years event was strong early on but continued to strengthen the last two months and is now peaking.
Satellite observations show the evolution of the upwelling event and we can also see that sea surface temperatures off South Sumatra have dropped over 4 degrees from normal. Add this drop in temperature with long waits between sets and strong S/SE winds and it makes for a much colder than normal Indonesia surf trip. It's also limited surfing options in between swell events, with the usually reliable southerly swell magnets blown out. There have been more than a few reports of people getting skunked on their Indo trips this year.
Looking at the wind anomalies (difference from climate average) for the Sumatran and Mentawai region you can clearly see S/SE winds have been stronger than normal, identified by the long drawn out arrows.
Some may wonder if this strong positive IOD event is linked to the Sudden Stratospheric Warming event, and in short no it isn't but this later phenomena is likely linked to the extended run of large swell late season.
IOD events usually weaken and break down through spring and don't influence the summer months across Australia, but because of the strength of this current event, it's likely to linger into the start of Summer, keeping Australia hotter and drier than average and the winds blowing across Sumatra and the Mentawais.