Indian ocean dipole: the new one to watch

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CottonInfo climate technical specialist Jon Welsh.

CottonInfo climate technical specialist John Welsh.

PHOTO: Melanie Jenson

In July last year, the Indian Ocean Dipole (IOD) index plunged to a record low, heralding what will long be remembered as one of the wettest springs in eastern Australia’s winter-cropping regions.

According to CottonInfo climate technical specialist Jon Welsh, it is one of the reasons Australia’s grains industry would be advised to pay as much attention to the IOD as it does to its famous Pacific cousin, the El Nino–Southern Oscillation (ENSO).

“Its life cycle is well-aligned with the winter crop and, while we’ve observed the effects of north-west cloud bands moving south-east across Australia for a long time, we’re only now starting to realise how much influence the IOD can have,” Mr Welsh says.

The IOD measures the difference in sea-surface temperatures between the western and eastern parts of the Indian Ocean.

According to a 2015 article by Chaoxia Yuan and Toshio Yamagata, “Impacts of IOD, ENSO and ENSO Modoki on the Australian Winter Wheat Yields in Recent Decades”, the Australian wheat crop can be as much as 28 per cent below the long-term average when the IOD index is positive. In negative years, it can be up to 13 per cent above.

In his address to the March 2017 GRDC Update at Goondiwindi, Mr Welsh showed how the IOD index’s highs were historically tied to drought years for south-eastern Australia, and its lows often coincided with wet ones.

The IOD has most influence on Australian rainfall between June and October and, Mr Welsh says it is worth noting, in combination with climate drivers such as ENSO and the Southern Annular Mode (SAM), which reflects wind movements around Antarctica.

“It’s understanding what the different combinations from these indicators means that will give us a better idea about what is going to have an influence and when, and that can help us better anticipate the boom-and-bust cycle that makes farming so difficult.”

Mr Welsh began studying the IOD in 2005 after approaching CSIRO researcher Dr Mike Pook to try to get more accurate seasonal outlooks. At Dr Pook’s suggestion, Mr Welsh contacted the Japan Agency for Marine-Earth Science and Technology, which discovered the phenomenon. “I was on the family farm in Coolah and wondering why we were selling feedgrain into a depressed market one year, then lopping kurrajongs to feed stock during a drought the next because we couldn’t afford to buy the grain back at three times the price.

“I got talking to Mike in Hobart, and then to the researchers in Japan, and they ran some analysis on the farm rainfall at Coolah to look at what drivers affected our climate.” What Mr Welsh saw was a correlation between a positive IOD, an El Nino and droughts at Coolah in 2002, 2006 and 2009.

In the past decade, some climatologists outside Japan have come to recognise the significance the IOD can have on forecasting for Australia.

The Australian Bureau of Meteorology (BoM) has been publishing the IOD index in fortnightly summaries, along with ENSO commentary, for several years. It already incorporates IOD data, and Mr Welsh says its seasonal forecasting ability is likely to improve when the BoM updates the Predictive Ocean Atmosphere Model for Australia with the higher-resolution ACCESS-S version later this year.

“The Indian Ocean Dipole is certainly well-understood science; it’s a collective view of all the indicators that we need to help join the dots so this science can have a positive influence on agronomic practices.”

Mr Welsh is employed by the Australian Government’s Carbon Farming Extension and Outreach Program and works with CottonInfo to deliver climate information to cotton growers.

Figure 1 Australian rainfall deciles for June–October during the anomalous years (left) 1993 (El Niño and negative IOD) and (right) 2007 (La Niña and positive IOD).
Graphic showing Australian rainfall deciles for June–October during the anomalous years (left) 1993 (El Nino and negative IOD) and right (2007) (La Nina and positive IOD)SOURCE: BoM

More information:

Jon Welsh,
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