CLIMATE - INDIAN OCEAN DIPOLE - WHICH WAY IS IT LOOKING TO TREND FOR WINTER?

A quick look at the Indian Ocean Dipole as we move through Autumn and into Winter. The weather is looking to remain dominated by the La Nina as we enter into Autumn but that impact will wane moving through to the cooler season.


The Indian Ocean is where we turn our attention to for rainfall and temperature guidance, in combination with the SAM. As we have seen from previous years, it is a very tricky driver to forecast well, ahead of time. The negative IOD forecasts from some local global models have been poor in recent years, so a lot of the data I show you, you have to interpret through the lens of this driver is not an easy one to pin down.


IMPACTS - HOW IT WORKS.

Indian Ocean Dipole - Influences when both NEGATIVE and POSITVE.

I have spoken at length about the impacts this year with the current drivers in place for this coming season of winter, spring and into summer, but this chart below shows the average impacts from a negative or positive IOD phase.


Note that the impacts do favour those areas from northwest to southeast Australia, looking at the timing bar underneath, from May through September. The changes in season and the warming of the SSTs off the northern parts of the nation with the seasonal shift to the monsoon override the impact of the IOD influence and therefore it generally breaks down through November and is inactive through summer.


That said, it does not mean that elevated SSTs does not influence the rainfall through northern and inland Australia during summer, but given that the SSTs generally are warming up through this period and the climate has shifted to a summer pattern, the IOD just does not have the same impacts than through the late Autumn through Spring.

A negative IOD phase - Average impacts across the nation.

This phase is very important when coming out of drought as it enhances the rainfall across the nation and starts to repair deficits from previous dry seasons/years through an El Nino or more impactful, a positive IOD.


More moisture is generated through the Indian Ocean Basin, increased convection through convergence sits further south, generally over Indonesia and south of the archipelago allowing highly anomalous rainfall to develop through the region. This convection shears of elevated precipitable water values or moisture through the upper atmosphere via the fast moving jet stream and thus drags that moisture south and east over the nation during the winter and spring.


That moisture generally can float across the nation consistently with out too much fanfare, appearing as high and mid level clouds with not much rainfall in the absence of a trigger such as a cold front or trough.


But when the cold fronts are more active thanks to a negative SAM phase, the we see that moisture being lifted into widespread areas of rainfall. We have seen this happen multiple times this year so far.


Frontal weather interacting with a deep layer of moisture surging southeast from the Indian Ocean producing widespread rainfall.

The impact of the southern ocean influence begins to wane through later September with the retreat of the winter westerlies, and the elevated moisture that is then pooling of the nation generally builds up to very high levels, inland troughs returning thanks to the build up of hot air, then generates more inland rainfall and storm outbreaks and the rainfall then increases for areas such as QLD and NSW, inland SA and northern and eastern VIC before the influence wanes further on closer to summer.

In context we have seen the impacts play out over WA numerous times this year with above average winter rainfall and that has spread through to the southeast a few times. But that frequency does increase as we get into August through October as the phase peaks.


I will say that the one key ingredient for rainfall to be widespread and above average is connected to the SAM being persistently negative and allowing the winter fronts to be frequent, taller and move through large parts of the inland. We are seeing that to a degree this week the frontal weather could be further north than this for more widespread falls.


But what does the IOD look like when it is positive and how does that influence the nations weather


The Positive Indian Ocean Dipole - Average Impacts across the nation.

The inverse is true with the positive IOD phase where the cooler than average temperatures over the Indian Ocean mean there is a lack of moisture that spreads across the nation via the jet stream, leading to lower than average rainfall for extensive areas of the nation.


It also leads to a warmer winter and an early start to the build up temperatures over the northwest of Australia through the heat engine, not the build up to rainfall, but scorching temperatures. We have seen that in years go by, that with a positive IOD, parts of inland WA can reach 40C very early on in the September and this transported down into the south and southeast with very warm temperatures.


Back in 2009 we saw some locations hit 30C through southeast Australia in the late May and early August with fire weather warnings issued in both of those months and a very brief respite from the fire dangers, but lack of rainfall lead to a dangerous fire year that year over southeast Australia.


The positive IOD also leads to below average rainfall across northern Australia with a prolonged build up and higher than average day time temperatures with more sunshine hours.


The positive IOD usually follow periods of negative IODs but that rule is not fixed in. They will happen again, and they usually have a stronger impact on rainfall than the El Nino for WA, SA and parts of the NT.

Where the IOD goes in 2022 remains to be seen but the signals are that we could have some impacts in the winter if some of the International Models are correct.


DATA SETS

March 2022

The IOD has minimal impacts through March but the overall trend is that with tropical weather to be enhanced through February and March in the region, this may lead to a below average spell of SSTs in the region. But that is very normal, so marginally positive values through March is not unheard of.

April 2022

CANSIPS strengthens the cooler signal in the Indian Ocean while other models now agree to weaken the cooler waters and bring the values generally neutral. That is the right call on this current guide. Very little impact to be achieved from the IOD at this time nationally.

May 2022

The IOD is expected to start forming a negative phase if this is correct off the global models. The BoM model says nothing is happening. But the Euro and other international models did pick the last IOD event of 2021 quite well which resulted in significant rainfall developing in the late Spring. They do move towards a warming phase but still within neutral values.

June 2022

The Euro getting close to calling a negative IOD event but other models keep it neutral with some impacts of a wetter start to winter if the Euro is right, where other models say seasonal conditions expected.

BoM Model and all the members.

You can see there is a strong upward trend on some of the members according to the BoM model as we go further out, will keep an eye on that, but with international models not interested in that idea, I am not too concerned. This model performed poorly last year in forecasting the event of 2021 ahead of time

Rainfall Outlook - May through July.

Can see the impacts of those warmer waters over the Indian Ocean in the rainfall projections off the Euro if this is right, but it is a lonnnnnnnnng way out, but early indications would be for seasonal to somewhat above seasonal rainfall chances.

Temperature Outlook - May through July.

The temperatures neither above or below average across this period off the Euro, but it is a long way out as mentioned above. So treat this data with caution. The forecast refines as we get closer.

So we are moving into the planting season for many and the next few months will offer plenty of soil moisture and seasonal temperatures, but will it continue through Autumn into Winter? That remains to be seen.

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