Indian Ocean Dipole turns positive, may prove too late for monsoon 

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Even as monsoon-damaging El Niño is predicted to become established during September-November, nearer home, there has been a significant change in the Indian Ocean Dipole (IOD) index in the past week, according to the Australian Bureau of Meteorology. The IOD index has risen from +0.34°C to +0.79°C, which is in excess of the positive IOD threshold of +0.4 °C. 

IOD refers to the swing in sea-surface temperatures from warm to cool in the Indian Ocean basin to the South of Sri Lanka. Warmer in the western side of the basin is referred to as a positive IOD event, while the reverse indicates a negative event. Depending on the time of its evolution, a positive IOD event has boosted the performance of India’s South-West monsoon.

Above threshold

The Australian Bureau qualified its statement by saying the IOD index must remain above the threshold for a sustained period before it can declare a positive event. It added that all surveyed climate models continue to indicate a positive IOD event during September-October-November, though monsoon will have started withdrawing from North-West India from early September. 

A potential event in 2023 is expected to be much briefer than the 2019 event that developed in May, and drove the India monsoon to excess and extended it into October. If an event develops now, it is unlikely that it would approach the strength of the 2019 event, although some climate models currently indicate a 2023 event could be a relatively strong event.

Strongest in 2019 

The positive IOD event is known to help the South-West monsoon. The 2019 was one of the strongest events on record. For reference, the 2019 event had sustained index values close to +2.0 °C at its peak, whereas forecasts indicate a peak value of around 1.5 °C in 2023. The event may have evolved too late for the monsoon to be of help to deficit States in South India.

But Application Laboratory of the Japanese national forecaster Jamstec said on Tuesday some parts of India and Sri Lanka, along with those of Nepal, Bhutan, East Asia, Central and West Africa, the Horn of Africa as well as southern Alaska, the Caribbean and La Plata, will experience a wetter-than-normal condition during September-October-November. 

Poor Australian monsoon

For Australia, a positive IOD event year has mostly brought low rainfall from October to December, a late monsoon onset and low tropical cyclone activity. If El Nino were to develop in addition to a positive IOD in 2023, these impacts would potentially be exacerbated and extend further into the wet season. Rainfall trend associated with recent positive IOD events, even without concurrent El Nino events, shows below-average rainfall across northern Australia from October to December.