An Ill Wind Blows For Indonesia
As the La Niña signal persists throughout the Pacific Ocean, a mirroring event is happening over the Indian Ocean.
In recent weeks there's been an intense cooling of waters off East Africa as warm water sits snuggly around Indonesia and north-west Australia.
This water temperature signal is the Indian Ocean's version of El Niño Southern Oscillation (ENSO), known as the Indian Ocean Dipole (IOD) and is clearly visible in the below chart.
When cooler than normal water surfaces to the north-west of the Indian Ocean and warmer water pools to the north-east this is known as a negative IOD event, with positive events seeing colder water off Indonesia, warmer around East Africa.
We've reached the threshold for a negative Indian Ocean Dipole event (forecast earlier in the year) and we're set to see it strengthen further over the coming month ahead of a peak in September.
Similar to the ENSO signal, the warm water build up to our north-west results in a warming of the air immediately above it along with the transfer of moisture, resulting in increased convection (read: storms) and rainfall. This moisture laden air is then dragged south-east across the country by the jetstream, bringing increased rainfall to central and south-eastern regions of Australia.
The yin to this yang is that the cooler water off East Africa leads to moisture deficits and increased likelihood of drought and failing crops in the region. This is currently playing out across the region for the second year running thanks to back to back negative Indian Ocean Dipole events.
Much like ENSO, it's a self-perpetuating positive feedback loop. Lower than normal pressure develops around Indonesia from the warm, rising air, with higher relative pressure to the west. Winds blow from high to low pressure, strengthening from the west which upwells colder water off the East African coast and piles yet more warmer water across Indonesia.
For surfers, this also brings less favourable north-west winds to the northern and central Sumatran region, with winds tending variable to further south, and weaker trades across eastern Indonesia.
One such run of poor winds and westerly swell is due across Northern Indonesia into next week as a tropical depression deepens into a tropical low south of Sri Lanka, generating more size than any background south-west energy late next week.
In the opposite phase - that is, during positive IOD events - stronger than normal south-east trades are often felt across Indonesia, feeding towards the warmer waters in East Africa. This brings floods and flourishing crops to Africa but drought and bitterly cold waters in Indonesia.
The IOD signal usually peaks through spring before fading quickly into the end of the season, so with the current setup strengthening, we can expect less favourable winds and conditions to impact the Sumatran region through into the late season.
Swell-wise, the mix of La Niña and negative IOD signal doesn't bode well for the late season with higher than normal pressure forecast to dominate the main Southern Ocean engine room (between South Africa and Australia), with smaller mid-periods swell becoming more than likely.
This is shown in the forecast diagram below which shows the probability of pressure being higher or lower for the period October, November, December. Dark blue is 70-100% chance of lower than normal pressure, and dark red 70-100% higher than normal.
On the positive side, savvy surfers might note the north-west winds opening up options across the more exposed south-east flanks of islands and reefs in the Sumatran region.
Across to the Maldives, and we'll see less southerly groundswell energy but plenty of mid-period south-east trade-swell along with gusty offshore westerly winds.
Further east, with the La Niña signal in the Pacific Ocean due to hold into the coming summer, we're staring down three consecutive years of the cold water signal across the equator - a so called triple-dip. This will result in even more easterly trade-swell and less southerly energy pushing up through the Tasman Sea from the Southern Ocean.