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Ben Domensino, 11 Dec 2019, 2:08 AM UTC

Indian Ocean Dipole losing its grip

Indian Ocean Dipole losing its grip

The Indian Ocean Dipole has significantly weakened during the last five weeks, raising hopes that more normal weather patterns could soon be returning to Australia.

In the absence of El Nino and La Nina, the Indian Ocean Dipole (IOD) has been one of the main climate drivers influencing Australia's weather patterns during the last several months.

The IOD is an index that measures the difference in sea surface temperature between the western and eastern sides of the tropical Indian Ocean. Positive IOD events typically cause abnormally warm and dry weather in parts of Australia during winter and spring. They are also associated with a more severe bushfire season in southeastern Australia. 

IOD events usually occur during winter and spring and break down before December. However, strong IOD events can persist into summer, which is what we are seeing this year.

The IOD index has remained above the positive IOD threshold of +0.4 degrees Celsius since the end of July. According to the Bureau of Meteorology, the "overall pattern of sea surface temperatures has remained generally consistent with a positive IOD pattern since late May." During this time, the IOD index reached as high as +2.15 degrees during the week ending on October 13th, which was the strongest positive IOD value in at least 20 years.

Image: Weekly IOD index values for the last five years. Source: Bureau of Meteorology

This year's strong positive IOD underpinned Australia's driest spring and ninth driest winter on record, while both seasons also ranked among Australia's six warmest in 110 years of records based on maximum temperatures. 

The abnormally hot and dry weather during the last two seasons exacerbated drought conditions across large parts of the country, including the Murray Darling Basin. It also provided the fuel for bushfires that have burnt through more than two million hectares of land in NSW since July.

Now, the positive IOD is weakening. The IOD index value for the week ending on December 8th was +0.86 degrees. While this is still comfortably above the positive IOD threshold of +0.4 degrees, it represents a drop of 1.15 degrees during the last five weeks.

The Bureau of Meteorology expects the IOD to remain in a positive phase into the start of 2020, however it is unlikely to persist into the second half of summer.

If the IOD breaks down early next year as expected, Australia could be left without any significant influence from the IOD (Indian Ocean) or El Nino and La Nina (Pacific Ocean). This means these two broad-scale climate drivers are not expected to push the weather in Australia towards unusually wet/dry or hot/cold by late January or February.

It's important to point out that even when Australia's main climate drivers are in 'neutral' phases, average weather patterns are not guaranteed.

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