The winter and spring rain outlook may hinge on what happens in the Indian Ocean

25 June 2024
An article by  Jane Bunn - Jane's Weather

In this seasonal update we look at the slow transition of the Pacific Ocean from El Nino to (potentially) La Nina, and try to decipher the very unclear Indian Ocean. 

One ocean may encourage lots of rain, while the other tries to dry it up. But the ‘dry up’ may not succeed thanks to a lack of imbalance. This ‘lack of imbalance’ may be the big player in what we see over the month’s ahead. 

OUR CURRENT MOISTURE

SEA SURFACE TEMPERATURE ANOMALY

It’s always good to begin with the Sea Surface Temperature Anomaly (SSTA) chart, as that sets the scene for everything that is about to unfold. 

This shows us how much warmer or cooler than average the top of the ocean is.

Sea Surface Temperature Anomaly: how much warmer or cooler than average the surface of the water is. The boxed areas highlight the regions that drive our weather patterns.

In the Pacific, we are in transition. The box was all yellow during the last El Nino, and now it is slowly heading towards blue. Very slowly, as it hasn’t changed much over the past month. But, there is plenty of cold water just under the surface ready to transform that box properly into blue. 

Closer to Australia we have warm yellows and reds off Queensland and Indonesia, meaning there is a proper imbalance setting up. 

If more blue appears in that box, and the water off Queensland remains yellow -> moisture should be actively pushed towards Australia. The imbalance from one side to the other is what kicks this off. 

In the Indian, the box is firmly yellow. This is known as a Positive IOD (the El Nino of the Indian Ocean). 

However, there is no imbalance of blue water off Western Australia and western Indonesia. This lack of imbalance may be very important for our rain outlook.

If the box remains yellow, and blue develops off WA -> moisture should be pushed away from Australia. If blue doesn’t develop off WA, then the outlook is unclear. We are most likely not going to see an imbalance. 

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THE FORECASTS

PACIFIC OCEAN

The forecast for the Pacific Ocean across all the different models that provide guidance is not clear cut, but they are generally heading in a similar direction:

  • 3/7 models have us crossing the threshold (the SSTA box turning blue enough) in July to late August (the bottom projections in this graph that take us into the green area)
  • 2/7 models have the box turning slightly more blue, but not going all the way (the projections in this graph that go close to the green area)
  • 2/7 models have the ocean remaining as it is, with no big influence from the Pacific Ocean (the projections in this graph that remain in the middle of the white space)

-> Overall, we’re not heading into another El Nino. We may go into La Nina or remain in neutral. Most guidance says if not La Nina then close to it. 

-> The warm water off Queensland helps ensure that we have a source of moisture anyway, even if the ocean doesn’t completely commit. We also have the warm Tasman Sea that produced a lot of last summer’s rain too. But we only have these feeds of moisture when the weather pattern allows it. 

Forecast guidance from a range of seasonal weather models about the temperature of the box in the Pacific Ocean. The brown area indicates warm yellows in the box, while the green area indicates cool blues in the box.

 

INDIAN OCEAN

The Indian Ocean is really struggling to tell us what is most likely to occur:

  • 1/5 models like a negative IOD (the La Nina of the Indian Ocean) from August (the projection in this graph that moves into the green area). This model is very different from the rest, and would indicate that the Indian Ocean box would be predominantly blue in just a few weeks time
  • 2/5 models like a positive IOD in July and August, then heading into neutral (the projections that cross into the brown area then back into the white)
  • 2/5 models come close to a positive IOD in July and August, then head into neutral (the projections that remain in the white area)

-> The Indian Ocean is far from clear cut, but there is another spanner in the works too: this doesn’t take into account all the warm water off WA that provides a source of moisture when the weather pattern allows it. 

Forecast guidance from a range of seasonal weather models about the temperature of the box in the Indian Ocean. The brown area indicates warm yellows in the box, while the green area indicates cool blues in the box.

 

COMPARISONS TO PREVIOUS YEARS

We can compare these projections with what has occurred in the past.

The past 16 La Nina’s have produced a wetter than average winter and spring for a vast majority of the nation:

Winter and spring rainfall deciles over the past 16 La Nina years show that much of the country is in decile 7 or 8 (and some as high as 9). No part is drier than average. 

 

The past 16 La Nina’s have produced a wetter than average summer over the northeastern half of the nation:

Summer rainfall deciles over the past 16 La Nina years show that much of the northeastern half of the country is in decile 7 or 8. The rest is 50/50. No part is drier than average. 

 

On the flip side, the past 8 Positive IOD years tell a very different story. They produced a drier than average winter and spring across a vast majority of the nation:

Winter and spring rainfall deciles over the past 8 Positive IOD years show that much of the country is in decile 2 to 4. No part is wetter than average.

 

There is no summer map, as the IOD isn’t active in summer.

So, this may be the battle between east and west. East encouraging rainfall, west trying to dry it out. 

But… it’s not a ‘normal’ drying from the west, not a ‘normal’ positive IOD, as there we still have lots of moisture to play with thanks to the warm water off WA.

We just need the blocking high pressure systems to lose their dominance over our weather pattern so that those in southeast Australia can access that moisture. Western Australia has seen plenty of it. 

Unfortunately, positive IOD’s encourage high pressure over the southeast, so it may be a case of lots of moisture to play with, but no low pressure in the right area to turn it into rain. Or, as we head into spring and summer, the blocking high’s trap a trough or slow moving low, with a feed of Tasman Sea moisture, just like we saw last year. 

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