INDIAN OCEAN AND THE WINTER INFLUENCE. WHEN IS THE INFLUENCE LIKELY TO KICK IN??

Updated: May 15

The Indian Ocean Dipole is forecast to turn negative through the Winter of 2022, and that means that people automatically think wet wet wet. But as I always suggest, the devil is in the detail and not one event is ever the same so while it is safe to bet on a wetter than normal Winter, it now is a question of when and where!


There are plenty of places across the country that would still benefit from a wet Winter, namely many areas in SA which have been missing out on the heavier rainfall. But on the flip side, there are many other areas that do not want a wet Winter with the risk of flooding remaining elevated. That heading into a wet Spring is not great news and could set the scene for a similar flood profile in the southeast and east as we saw in 2016. Statistically we are due for another substantial flood through the southeast and east.


The Indian Ocean Dipole is forecast to remain a feature into Spring which I have touched on yesterday, this post is talking about the start of the event and when we should start to see influence across Australia on the current guide.



JUNE 2022

There will be minimal impact from the start of Winter, even though we have elevated SSTs in the Indian Ocean (each colour of red is 0.5C above normal, the deeper the colour the warmer you get) the energy and moisture from the Ocean will be influencing the nation's weather will be not felt really until the end of the month I suspect. So, the Indian Ocean Negative Phase developing and deepening as we move forward through June.

Rainfall outlook for June reflects that outlook above, with the lingering La Nina having a larger influence I suspect over the east with more easterly winds. Could see some enhanced rainfall via the influence from the Indian Ocean through the northwest tropics and over the southwest of WA but here too, not overly strong influence.

Sea Surface Temperatures for June is forecast to remain very much average through the IOD region which is northwest of Australia and south of Indonesia. Many of the models have the SSTs about 1-2C above normal through June into July but the warmer waters are building through June. There is sometimes a lag of 2-4 weeks between the observed SSTs to then playing out in the atmosphere. (Also note the La Nina signal in the Pacific, still having influence over the east).

Rainfall Outlook from the model, lots of green on the board suggesting an overall 60-70% chance of above average rainfall through large areas of the nation grading to 90% through NSW and parts of SA into WA and grading down to 50% through southwest areas of the nation and along the east coast. Other models are much more aggressive than this and others are somewhat more subdued, but I think this is the right fit for the current guidance.

Temperatures through June into July should be relatively seasonal for many areas and that cooler bias is something that builds as we go through Winter which you can see below. Above average temperatures over the north of the nation will continue in response to the elevated SSTs through the Indian Ocean extending east through the tropical waters north of Australia and meeting the impacts of the La Nina with the lingering warmer waters in the Coral Sea. This will lead to elevated moisture levels through Australia.

JULY

This is where we should see the increasing impacts of the negative IOD on Australia with a modest influence starting to unfold via the jet stream ahead of cold fronts being the main feature of this period. So elevated rainfall running through northwest, central, southern and southeast Australia likely. Seasonal rainfall likely over the southwest but no impacts are forecast at this time over the northeast of the country, but the impacts of the elevated SSTs over in the Coral Sea would impact rainfall and temperatures here.

The rainfall outlook for July shows the increasing impact of higher moisture streaming in via the jet stream from northwest and west of the nation over the Indian Ocean and feeding into cold fronts. Strong rainfall signals for the southwest, along the north of the nation and over in the southeast and eastern inland west and north of the divide through the food bowl, will be the areas that are feeling the biggest impacts. Some low risks of flooding for now over the latter part of the month over the southeast inland. Higher moisture values over eastern Australia is in response to the higher SSTs in the Coral Sea. So that is why there is a green shading for much of inland QLD which should be dry at this time.

The SSTS are reflecting the persistent warm waters around Australia which is connected to the IOD but also thanks to the impacts of the ENSO lingering in La Nina. This could pose some threat to increasing rainfall, humidity and temperatures over northern Australia. But you could see below average temperatures through the daytime over southern areas of the nation thanks to cloud and rainfall.

Noting the very strong signals for rainfall across the country via the IOD and the ENSO region being in cool neutral leading to warm waters stacking up across the country. Some strong signals for widespread rainfall emerging from the Indian Ocean influence from July with the heaviest of the rainfall being felt over the southeast and eastern inland extending northwest through SA and WA.

Temperatures starting to reflect the deepening IOD over the northwest of the nation with more cloud, rainfall and this leading to temperatures being below average. Over northern areas of the nation temperatures are above average reflecting those very warm waters. Higher humidity keeping the daytimes up over the north and northeast, but overnights will also likely be above average across much of the nation reducing the frost risk.

AUGUST

The increasing influence of the IOD is apparent from August and as we roll into Spring with a heavier impact expected from this time. Temperatures could be up to 2.5C above normal in the Indian Ocean which will lead to excessive moisture being transported across the nation from northwest to southeast. This is where we see temperatures below average in the areas expecting above average rainfall in southern parts of the nation but temperatures likely above average over northern areas of the nation reflecting the higher moisture/humidity values. The influence from the IOD will be broad from this point on and more information on the impacts into Spring will be available in June 2022 so stay tuned to that.

Rainfall increases further across the nation in response to the influence of the IOD. The rainfall could lead to flooding over some Outback areas, over the southwest of WA thanks to the location of the long wave tapping into moisture and bringing persistent rainfall but over the food bowl in the southeast, this is where we could see significant flooding issues, possible crop damage and loss and disease risks increasing as we see more humidity than normal. That is also the case over northern Australia as those elevated SSTs strengthen further leading to early onset rainfall and build up conditions.

The SSTs across the Indian Ocean strengthen further as we head into August and then into September with a deep moisture profile emanating from this region. Noting that the SSTs across the north and west are very high and lingering elevated SSTs in the Coral Sea could also see elevated moisture be present over much of the country.

Noting the excessive rainfall spread over the north and northwest thanks to the Indian Ocean. The peak of the IOD is expected during October but we should see an uptake of rainfall throughout the nation. Some areas seeing the chance of rainfall being above average at 95% chance this far out! That is remarkable and when you are dealing with a conservative model such as Euro, you pay attention but matching that against other data sets and climate agencies, the signals are very strong. Most of the nation should see above average rainfall through August and then into Spring.

The temperatures reflect the increasing influence of the IOD with well below average temperatures over the southeast and southern interior running via the jet stream. The warmer than normal weather bias continues over the north reflecting the very humid and warm air responding to the elevated SSTs. Nighttime temperatures should remain above average for many areas.

THE 9 MONTH OUTLOOK

Very dramatic look at the negative phase through Spring 2022. We could see the IOD as strong as what we saw through 2016 which brought severe inland flooding to NSW and caused significant crop damage and structural and infrastructure damage so something to watch. Certainly, get in and continue your life as normal, but note that there is the threat of severe weather leading to flooding through Winter but more likely as we move into Spring 2022. But understand, that the risk of flooding extends northwest and west through VIC, SA and into WA. The northern tropics also feeling a prolonged period of higher humidity with an interrupted dry season and above average rainfall through most months this year.

Indian Ocean Dipole - Influences when both NEGATIVE and POSITVE.

This is the overall impact and timing guide for the Indian Ocean Dipole influence whether it be in Negative, Neutral or Positive phase.

REVIEW

A negative IOD phase - Average impacts across the nation.

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 without 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, 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.


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 buildup temperatures over the northwest of Australia through the heat engine, not the buildup 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.

More coming up on the influence moving into Spring 2022 will be available in a month.