The Pacific Ocean is coming along very nicely in its transition from El Nino to La Nina. In the latest news, three models now think we will reach the La Nina threshold in July/August (it was just two models before now). That means there are stronger signs of an active push of moisture from the Pacific Ocean by late winter.
Events in the Indian Ocean are less encouraging. The models were forecasting a "proper" Positive Indian Ocean Dipole (IOD) for winter and spring and there are worrying signs that the ocean is heading rapidly into that situation.
A Positive IOD is like El Nino but in the Indian Ocean. An imbalance of water is developing over there, and if that reaches a critical threshold then we end up with an active push of moisture away from Australia. It would limit the juicy northwest cloud bands that bring our southern farmers rain.
This situation also encourages stronger high pressure that blocks these rain systems from crossing southern Australia - so a Positive IOD can be a double edged sword.
Remember, it all comes down to this: in order to make it rain you need two things to work together - moisture and instability (low pressure).
Let’s go through the most important chart, the Sea Surface Temperature Anomaly (SSTA).
Sea Surface Temperature Anomaly: how much warmer or cooler than average the surface of the water is.
The ocean is the main source of our moisture, and our weather is heavily influenced by the state of the oceans in the tropics.
In the Pacific, the latest El Nino was strong, but had less effect on Australia thanks to two things: a lack of cooling of the waters off Queensland (there was no imbalance of water from west to east - no cool vs warm), and an incredibly warm Tasman Sea that produced its own feed of moisture. Together, they offset the usual effects of El Nino.
Watch the Pacific box change colour over the past month. It was bright red at the start of the year, waning yellow in April, and now we have a steady wave of blue appearing.
When it reaches a point where there is more blue than yellow we’ll cross that threshold into La Nina, and with yellows and reds off Queensland to create an imbalance, we’ll have lots of moisture pushed our way.
Over in the Indian Ocean the box is also warmer than average. That box has increased in temperature anomaly over the past month, from yellow to pale red. That puts us into a Positive IOD.
In April there were lots of warm yellows to the west and south of Indonesia, and in a large area away from Australia’s northwest coast. Now you can see blues appearing just south of Indonesia (rapidly over the past few weeks), plus the cool blues right on Australia’s northwest coast.
If we don’t see that blue spread out further we’ll be in a Positive IOD that has a limited drying effect. If the blue does spread out further then this Positive IOD will actively encourage moisture to be pushed away from Australia.
That covers just the moisture component of the rainfall equation.
The other ingredient is instability from low pressure.
You may have noticed a lot of blocking highs over Australia lately? Well, they are encouraged to be there by the SSTA pattern in the Indian Ocean.
The Positive IOD is likely to continue so the blocking highs may too.
What you need to look for is a ‘split’ in the high pressure. This allows low pressure with energy from the south to meet up with a feed of moisture from the tropics - like we may see next week:
For more on next week’s rain potential, and extended commentary on where our oceans are heading and what it might mean for us, please see my Friday morning video update for Jane’s Weather subscribers:
Putting all of this together, we look at a blend of the weather models, and see the odds for above or below average rain.
These outlooks take into account our sources of moisture, but are limited in knowing what low pressure will do with that. Ultimately, we have a good idea of how much moisture we will have to play with, but a hazy idea of what will turn that into rain - and an even hazier idea of where those weather systems will move.
Overall, it all depends on where the low pressure moves as to who has a good season.
BoM’s seasonal forecast isn’t included in the blended product, so I’ve shown their forecast separately.
The blend of weather models:
BoM’s outlook:
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