BOM, 13 Oct 2015
While the 2015 El Niño is the strongest seen since 1997, its equivalent in the Indian Ocean—the positive Indian Ocean Dipole (IOD)—is now at levels not seen since late 2006. The strong El Niño is expected to last until at least the end of the year before declining in the first quarter of 2016, however the positive Indian Ocean Dipole is expected to decay earlier, in November 2015.
Sea surface temperatures (SSTs) in the central to eastern tropical Pacific continue to warm, further entrenching El Niño, while waters south of Indonesia have cooled, strengthening the positive IOD. Likewise, waters to the north of Australia have also cooled over the past three weeks, which may further contribute to drier conditions.
Most international climate models surveyed by the Bureau of Meteorology indicate the anomalous warmth in the tropical Pacific Ocean is likely to peak around the end of 2015. Typically, El Niño peaks during the late austral spring or early summer, and weakens during late summer to autumn.
Four out of five international climate models suggest the strong positive IOD event will persist into November, but decline rapidly as the monsoon trough shifts south over the IOD region, changing the wind patterns. This change means the IOD pattern is no longer able to form in the ocean.
El Niño is usually associated with below-average spring rainfall over eastern Australia, and increased spring and summer daytime temperatures in Australia south of the tropics. A positive IOD typically reinforces the drying pattern, particularly in the southeast.
Indian Ocean Dipole
The positive Indian Ocean Dipole (IOD) event persists, with the weekly index value to 11 October strongly positive at +1.18 °C. This is the highest weekly value since the very strong positive IOD event of 2006.
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 water off the coast of the Indonesian island of Sumatra (see About the Indian Ocean Dipole). Cool anomalies in this area 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.
Until recently, very warm sea surface temperatures across much of the Indian Ocean were likely moderating the influence of these two climate drivers on Australian climate. See the updated Climate Outlook for further discussion of the interplay between all three influences.
Four of the five surveyed international climate models indicate this event is likely to persist through to the end of spring and decaying by early summer.