We’ve got some earlier than normal developments underway across the Indian Ocean basin, and it doesn’t paint the prettiest picture for western Indonesian locations.

As the Eastern Pacific slowly cools down from El Niño and likely transitions to La Niña through late winter/spring, we’ve got a weak cool signal developing in the eastern Indonesian region - immediately north-west of Australia.

This looks to be the start of a positive Indian Ocean Dipole (IOD) event, with it possibly being strong and peaking through winter.

The Indian Ocean Dipole is an index measuring the differences in temperature across the Indian Ocean, varying between positive, neutral, or negative in phase.

A positive IOD event is characterised by cooler waters off Indonesia and warm waters off eastern Africa. This difference between cold and warm drives stronger than normal southeast trade-winds that flow from high pressure across Indonesia, towards low pressure generated by the heating in the west - see image below.

More often than not it becomes a positive feedback loop, with the stronger than normal trade-winds inducing upwelling across the Indonesian region while piling up warmer water to the west. This creates a stronger temperature gradient, and hence even stronger trades.

Under a negative IOD event, warm water forms around Indonesia with cooler waters off eastern Africa, resulting in weaker than normal trade-winds and bouts of north west winds across Sumatra.

In short, positive IOD events limit surfing options away from the more exposed breaks of Indonesia thanks to the relentless southeast wind when otherwise winds are more light and variable. It also brings cooler, greener water (due to the upwelling), which along with the associated wind chill, means extra rubber is needed.

If you're heading to Indo this season, particularly from mid-season onwards, best you pack some rubber and when the trades kick look for protection (Shield)

There are a couple of interesting points regarding the likely early onset of this positive IOD event. Firstly IOD events usually develop through winter and peak in spring, but this one is quite early which is less common.

Also, we generally (but not always) see positive IOD events coupling with El Niño’s and negative IOD events with La Nina’s. The correlation is strongest when the El Niño/Niña signals are strong, but they can exist independent of each other.

The correlation exists thanks to the warm water build up seen in the western Pacific during La Niña’s flowing through to Indonesia, favouring a negative IOD, and vice versa during El Niños.

Looking at the current state of play across the Indian Ocean, and following last year’s positive IOD event, the index only temporarily dipped to neutral during February before pushing back above the positive threshold (+0.4°C) into March, and persisting into the end of April at +0.68°C.

Long-range seasonal forecasts from nearly all meteorological agencies around the world are pointing to a positive IOD event developing over the coming months, peaking through July. As stated above, this is earlier than normal with it expected to start backing off through spring.

Chance of sea surface temperature being higher/lower than normal for June/July/August. Dark blue = 70-100% lower, dark red = 70-100% higher (ECMWF)

The impact to the Australian winter swell season looks neither here nor there, with there being no significant correlation between IOD events and significant wave height across the southwest of the country.

Looking further afield, and the stronger than normal trade-winds across the northern Indian Ocean should favour the Maldives and Sri Lanka, with plentiful amounts of southeast trade-swell likely through winter.

A quick scan of the East Coast reveals no major driving signals thanks to the slow transition between the outgoing El Niño and the likely La Niña during our winter surf season.

//CRAIG BROKENSHA