El Niño strengthens but a warm Indian Ocean
BOM, 1 Sep 2015
The 2015 El Niño is now the strongest El Niño since 1997–98. The tropical Pacific Ocean and atmosphere are fully coupled, with sea surface temperatures well above El Niño thresholds, consistently weak trade winds, and a strongly negative Southern Oscillation Index. Weekly tropical Pacific Ocean temperature anomalies (i.e. difference from normal) in the central Pacific are now at their highest values since 1997–98, though still remain more than half a degree below the peak observed during 1997–98.
Most international climate models surveyed by the Bureau of Meteorology indicate the tropical Pacific will continue to warm, with the largest anomalies occurring later in the year. Typically, El Niño peaks during the late austral spring or early summer, and weakens during late summer to autumn. The 2015 event has, so far, been following a normal El Niño life cycle.
While the Indian Ocean as a whole has been at near-record temperatures, the Indian Ocean Dipole (IOD) index has been at or above +0.4 °C for the past four weeks. To be considered a positive event, the IOD would need to remain at or above +0.4 °C through September. Three of the five international models surveyed by the Bureau of Meteorology indicate a positive IOD event is likely during spring.
El Niño is usually associated with below-average winter–spring rainfall over eastern Australia, and a positive IOD typically reinforces this pattern over central and southeast Australia. However, sea surface temperatures to the north of Australia and more broadly across the Indian Ocean basin, also affect Australia’s climate and are likely to be moderating the influence of these two climate drivers in some locations.
Indian Ocean Dipole
Values of the Indian Ocean Dipole (IOD) index have been at or above the threshold level of +0.4 °C for four weeks. The weekly value of the IOD index to 30 August was +0.57 °C.
Sea surface temperatures (SSTs) in the Indian Ocean are warmer than average over much of the basin, and the Indian Ocean as a whole has been at near-record temperatures in recent months (see timeseries of July monthly anomalies). Typically, a positive IOD event is characterised by cooler-than-average water off the coast of the Indonesian island of Sumatra (see About the Indian Ocean Dipole).
Positive IOD events are often associated with lower rainfall in parts of central and southeastern Australia. Positive IOD events are more likely to occur during El Niño, which also is typically associated with a reduction in winter–spring rainfall in eastern Australia.
However, sea surface temperatures in the Indian Ocean basin also affect Australia’s climate—it’s likely that the widespread warm anomalies have moderated the influence of these two climate drivers (see Climate Outlook).
Three of the five surveyed international climate models indicate a positive IOD event is likely during the southern hemisphere spring. If the current values of the IOD index above +0.4 °C are maintained until late September, 2015 will be considered a positive IOD year.