INDIAN OCEAN INFLUENCE AS WE MOVE INTO WINTER - ANOTHER NEGATIVE DIPOLE EVENT? REALLY?

That is the question - do we really have another negative event to look forward/not look forward to during the coming season?


There have been some rumblings of a developing negative IOD developing through the Winter of 2022, which would clearly lead to extensive rainfall and cooler than normal temperatures for large parts of the inland from WA through SA and into the southeast and eastern inland.


But forecasting these things, well ahead of time can be fraught with danger, as we saw last year and in 2020 as well, where there were agencies going too hard too early, and it meant a lot of backlash from people complaining about a bust season, going all out and preparing for widespread rainfall that never arrived.


In this case, we are seeing the needle on the dial turning towards a negative event as we track into July and more so into August. My internal data sets do offer a sharp negative event during the Spring time but I will share that more in the weeks ahead.


At this time, I am leaning to a 60% chance of a negative IOD developing, a 40% chance of a repeat of 2016/2017 where we saw extensive inland flooding for NSW and VIC. The chances are not especially high but they are no especially low either.


With that said, lets review the negative IOD first for those who are not fully across the impacts.


SOME BASIC UNDERSTANDING


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.

NEGATIVE INDIAN OCEAN DIPOLE

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.


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.


Finally the SAM being in a NEGATIVE phase during a NEGATIVE IOD is very productive for rainfall over the nation during Winter but later into Winter and extending into Spring. A NEGATIVE SAM means more frontal weather which is a good thing for those wanting rainfall in the cooler season (not so great for those on the east coast where it is dry with westerly winds).

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


POSITIVE INDIAN OCEAN

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.

CURRENT OUTLOOK FROM THE LOCAL MODELLING

The latest outlook from the BoM model is tilting towards a negative phase developing through June and into August, but the mean suggests it remains neutral, which is the right forecast this far out.

But it is this subtle signal from some of the members which have now starting to surge well beyond -2 units which does not happen very often this far out, last year, they did not go this far negative and we saw how it panned out. Now that is not saying we will have a full blow negative phase just off one run, but it something I will review at the next update and share and compare with you in a fortnight, because if more members join the deeper negative phase, then we may start to see the case for a negative IOD developing in Winter and Spring 2022 gathering pace. We are not there yet and I need to sample more of the data before coming to a conclusion which may not be possible until May.

I think it is important to give the basic mechanics before talking about bigger and more specific impacts in the weeks ahead.



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