BOM, 29 September 2015
The tropical Pacific ocean and atmosphere are reinforcing each other, maintaining a strong El Niño that is likely to persist into early 2016. Tropical Pacific sea surface temperatures are more than 2 °C above average, exceeding El Niño thresholds by well over 1 °C, and at levels not seen since the 1997–98 event. In the atmosphere, tropical cloudiness has shifted east, trade winds have been consistently weaker than normal, and the Southern Oscillation Index (SOI) is strongly negative.
Most international climate models surveyed by the Bureau of Meteorology indicate El Niño is likely to peak towards the end of 2015. Typically, El Niño is strongest during the late austral spring or early summer, and weakens during late summer to autumn.
The Indian Ocean Dipole (IOD) is in a positive phase, having exceeded the +0.4 °C threshold for the past 8 weeks. Recent values of the IOD index have been at levels not seen since the strong 2006 positive IOD event. Conversely, the Indian Ocean remains very warm on the broader scale.
Four out of five international models suggest the 2015 positive IOD event will persist until November, when it typically breaks down due to monsoon development.
El Niño is usually associated with below-average spring rainfall over eastern Australia, and increased spring and summer temperatures for southern and eastern Australia. A positive IOD typically reinforces the drying pattern, particularly in the southeast. However, sea surface temperatures across the whole Indian Ocean basin have been at record warm levels, and appear to be off-setting the influence of these two climate drivers in some areas.
Southern Oscillation Index
During the past two weeks the Southern Oscillation Index (SOI) has remained strongly negative. The 30-day SOI value to 27 September is −18.1.
Sustained positive values of the SOI above +7 typically indicate La Niña, while sustained negative values below −7 typically indicate El Niño. Values of between about +7 and −7 generally indicate neutral conditions.
Most of the eight international climate models surveyed by the Bureau indicate that the central Pacific Ocean is likely to reach its peak warm value towards the end of 2015. All surveyed models indicate that NINO3.4 will remain above the El Niño threshold through the first quarter of 2016. The surveyed models indicate values of NINO3.4 are likely to begin to decline during early to mid-summer, but remain above the threshold value until at least early autumn.
Indian Ocean Dipole
Values of the Indian Ocean Dipole (IOD) index have been at or above the threshold level of +0.4 °C for eight weeks, indicating a positive IOD event. For the week ending 20 September, the IOD index measured +1.1 °C, the highest value since the strong 2006 event. The most recent weekly value is +0.8 °C for the week ending 27 September.
Sea surface temperatures (SSTs) in the Indian Ocean are warmer than average over much of the basin, and the southern Indian Ocean as a whole has been at record temperatures in recent months (see timeseries of winter anomalies). A positive IOD event is typically characterised by cooler-than-average waters off the coast of the Indonesian island of Sumatra (see About the Indian Ocean Dipole), and this can be seen in the Sea surface temperature section. 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 broader Indian Ocean basin also affect Australia’s climate—it’s likely that the widespread warm anomalies observed across the Indian Ocean have moderated the influence of these two climate drivers in some areas (see Climate Outlook).
Four of the five surveyed international climate models indicate this event is likely to persist through to the end of spring.