Updated: May 25

This has been a very interesting driver this year, it has lead to above average rainfall through the late Autumn, through early Winter, then went quiet for the back half of winter, stirred a little through September and is expected by most climate agencies outside of Australia to peak in the coming 4-6 weeks.

The overall status of the Indian Ocean by the WMO set of global members tilts it into a NEGATIVE PHASE. This is expected to peak in the coming 4 weeks before waning through November and resolved by December which is the right call.

But the proof is always in the observed weather out there and the influence in real time, so how much of an impact will the negative IOD have on the temperature and rainfall output across the nation?

DATA - Refer to the video for more details

GFS 12Z - Surface Pressure Pattern and Rainfall Distribution - Next 2 Weeks

Westerly wind belt sitting further north and keeping the subtropical ridge further north, allowing for dry and stable air throughout the nation - through the Basin, there is a bit of drier air that is in place as well, so things staying quiet for a while. This is not uncommon even in a negative IOD phase.

GFS 12Z - Precipitable Water Values - Next 2 Weeks

Note the deep moisture values over the Basin connected to a low noted by the red shading, the drier air over the west of Australia may be overrun by mid month as the pattern begins to change.

GFS 12Z - Precipitable Water Anomalies - Next 2 Weeks

There is a bit of drier air through the basin thanks to a deep low sitting in between Australia and Africa, this is helping to draw in drier air. The west of the nation is dry for a while this week under a persistent ridge and southwest winds, this deflecting the moisture away from the continent. That may change towards mid month.

GFS 12Z - 500mb Wind/Flow Pattern - Next 2 Weeks

Westerly wind belt is sitting further north, the SAM trending neutral to negative again assisting in this forecast, but there has been some members suggesting the SAM turns positive, as with Spring, watch the forecasts closely as they will chop and change.

This graphic is explained more in the video - but clearly for any influence of the IOD in the coming 2 weeks, is linked to moisture escaping through a weakness in the subtropical ridge and then being lifted by a cold front and propelled towards the western parts of the nation.

Note the quiet basin and little impact away from equatorial areas in terms of precip, thanks to the westerly winds sitting further north and the subtropical ridge suppressing the amount of moisture being able to deviate south. But will keep watch.

Sea Surface Temperature Anomalies - Next Week - Northwest WA.

Increasing temperatures offshore the NT which is seasonal but that warmer water will be heading west in coming 2-4 weeks in line with the peak of the negative phase.

Sea Surface Temperature Anomalies - Next Week - Outer Northwest WA Waters

Waters are above normal throughout the basin, sufficient for influence, but the synoptic pattern is not quite in phase with drawing in the higher moisture content, instead the upper level winds are more easterly with the westerly wind belt sitting further north, deflecting moisture into the Central Basin.

Sea Surface Temperature Anomalies - Next Week - Coral Sea

This is the key for rainfall into November and December with the La Nina chances increasing through the Pacific pushing the warmer SSTs towards Australia's east coast. That process is underway.

We are seeing the Coral Sea playing a part in the rainfall chances coming up for QLD and NSW and extending into the NT.

The moisture over the Coral Sea again being projected to sweep southeast and east through the upper level winds and then dragged south and southeast thanks to troughs coming through WA and SA and this lifting rainfall chances mainly for the north and east.

Indian Ocean Dipole Forecast - October

A peak is expected later this month.

Indian Ocean Dipole Forecast - November The phase begins to wane which is expected.

Indian Ocean Dipole Forecast - December

Resolution of the phase with a neutral basin normal through our summer.

So we are coming to the peak period, I think we will see moisture run through the jet stream into Australia mid to late month which may spark more inland rainfall, but now the focus is shifting, here anyway, to the Coral Sea and the Pacific Ocean where a La Nina may be forming in the coming 6-8 weeks. Swings and round a bouts. More soon

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