New research shows increase in extreme IOD events
Regular readers of Swellnet would be familiar with all types of acronyms for weather phenomena. From LWT to SAM, AAO, and IOD, but it's this last one - the Indian Ocean Dipole - which has had a significant impact on the late winter through to early summer 'wet' season across Indonesia, and also affected Australia through our winter and spring.
The IOD is a measure of the difference in water temperature across the tropical Indian Ocean between the north-west (off the African coast) and north-east (off Java-Sumatra).
When in positive mode we see cooler waters off Indonesia, increased upwelling and drier weather locally, while anomalous south-east trades pile up warmer water to the east, off the African coast bringing wetter weather.
When in negative mode, the opposite occurs with warmer waters off Indonesia, anomalous westerly winds along the equator and upwelling leading to cooler water off east Africa.
Regarding surf potential, the positive IOD events bring stronger south-east trade winds across Java and Sumatra and this limits surfing options in between large swell events (as small wave spots become blown out) while also adding discomfort regarding the cooler water temps and wind chill. Nothing a springy can't fix, but if you travel without one, your surfing time will be cut shot.
In negative IOD events, west to north-west winds favour some regions more than others and also open up more exposed breaks which make the most of smaller swells.
Besides affecting the winds across Indonesia, the IOD also effects the climates across eastern Africa, Indonesia and Australia as touched on above. Under positive IOD events the cooler waters off Indonesia suppress convection bringing drier weather which then flows onto Australia with less available moisture from weather systems being brought down from the north-west. This creates hotter and drier than normal weather for central and south-eastern Australia during winter and spring. On the other hand it creates increased rainfall for eastern Africa and under extreme events, flooding.
When negative we see increased convection and moisture across Indonesia which then flows on across Australia bringing wetter and milder weather in winter and spring. Eastern Africa countries then see drier and hotter weather.
Last year we saw one of the strongest positive IOD events on record, bringing strong south-east trades to Indonesia, significantly colder water, drought to much of Australia and torrential rains and flooding to eastern Africa.
Putting this event in comparison to previous events, there have only been three extreme positive IOD events since 1950, that being 1961, 1994 and 1997. These events standout from, say, the 2006 event shown below in that not only were there significant differences in sea surface temperatures across the Indian Ocean but also strong zonal wind anomalies.
A few commenters asked if this extreme event was linked to climate change, or if we could expect more of them in the future, and on further investigation there are a few studies on the effects of global warming and the frequency and intensity of IOD events.
The bottom line is that we're expected to see an increase in extreme positive IOD events, and the more the warming, the greater the frequency.
A paper released in 2018 by Professor Wenju Cai ran thirteen different climate models that can generate extreme IOD events using the optimistic RCP2.6 scenario of keeping global mean temperatures to only 1.5-2 degrees above pre-industrial levels.
What was found is that the frequency of extreme positive IOD events increase linearly with the increase in global mean temperatures, rising from 6.5 events per century (one every 15 years) to 13.4 event per century (one every 7 years).
The reason for this increase in extreme positive IOD events is underpinned by a surface warming pattern which is faster in the western equatorial ocean, compared to the eastern equatorial ocean.
The rate of recent warming of the western Indian Ocean is one of the fastest of any tropical ocean over the last century as discussed here.
With the RCP2.6 scenario being conservative and needing immediate mitigation and reduction of emissions, a more likely and warmer scenario would see the frequency of extreme positive IOD events increase further.
So what does this mean for the surf?
Well, we can expect more troublesome south-east wind seasons across the usually reliable Indo locales, plus cooler water temperatures, and for the Maldives an increase in south-east swell consistency later in the season, but also less favourable local winds. For Australia, more regular bouts of drier and hotter weather are likely through winter and spring, and with flow on effects delaying the onset of the monsoon, a later start to the wet season in the north.
To keep abreast of the IOD signal, keep an eye on the Bureau of Meteorology's climate monitoring portal from the start of winter each year.. IOD Index Time Series.