The BOM is on alert for a third consecutive La Niña season. What does that mean? And what causes it?

The Bureau of Meteorology is on alert for another La Niña season.

There’s a 70 percent chance Australia’s east coast will struggle with the climate driver for a third straight year.

This is very unusual and other countries have already declared La Niña. Let’s update what it could mean for this spring and summer.

1. What is a La Nina?

La Niña is one of the most important climate drivers for eastern Australia. It brings with it a greater chance of precipitation.

Basically it is a phenomenon determined by changes in winds and water temperatures in the tropical waters of the Pacific Ocean.

Along with the complementary El Niño, the phenomenon is known as the El Niño Southern Oscillation, or ENSO.

So for Australia, El Niño typically results in drier and hotter conditions, while La Niña usually results in cooler and wetter conditions.

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You can find out about the latest update here: Australia remains on La Niña alert amid near-record-breaking Indian Ocean dipole event

2. Why should we care?

La Niña brings with it an increased likelihood of above-average rainfall, and the east coast has already endured two La Niña seasons in the last two years.

Floods devastated communities in the eastern states a few months ago.

Now we could be looking at a third season in a row. According to the Bureau, this has only happened three times since 1900.

And two of those triple-dip years coincided with major flooding events.

For more information on previous triple La Niña seasons, click here: Australia’s east coast faces third consecutive La Niña season, experts predict

3. What actually causes a La Niña?

Basically, the process is triggered by trade winds. These are the winds that blow from east to west around the equator.

Under normal conditions, the Walker Circulation drives the trade winds from the east to the west of the Pacific.(Supplied: Bureau of Meteorology)

When these winds blow stronger, they can change ocean currents.

This eventually leads to warmer sea surface temperatures in the western Pacific north of Australia.

The warm water means that the conditions for precipitation become more favorable due to the higher humidity in the atmosphere.

When certain thresholds are reached, the weather authorities declare a La Niña.

Map of the Pacific Ocean with a clockwise circle of arrows indicating strong airflow and increased convection over the west
When there is a La Nina, the Walker circulation strengthens, bringing wet and warm conditions to Australia.(Supplied: Bureau of Meteorology)

So La Niña does not directly cause rain, but only prepares the conditions for it.

More details can be found here: What is La Niña and what does it mean for Australian weather?

4. Why have other countries declared La Niña but not Australia?

Authorities in the United States, United Kingdom, Japan and India say La Niña has been underway for some time.

The Bureau of Meteorology (BOM) doesn’t do this because they have a stricter process for declaring a La Niña. Climate change is also making things more difficult, which you can read more about here.

A semicircular scale with an arrow pointing to
BOM’s ENSO outlook indicates a 70 percent chance of La Niña forming in the coming months.(Supplied: Bureau of Meteorology)

That doesn’t mean the BOM won’t be declaring La Niña this year. The bureau’s next climate driver update is in about two weeks, on September 13.

In a climate outlook last week, the bureau said: “September to November rainfall levels are likely to be above mean for most of Australia’s eastern half, but below mean for western Tasmania and south-western Western Australia.”

Review of the forecast: BOM’s latest outlook signals muggy spring weather for eastern Australia

5. Is it just La Niña that could cause higher rainfall?

No, other factors play a role. Another big driver is the dipole in the Indian Ocean.

A negative Indian Ocean dipole event brings more rain to Australia. It is defined by temperature differences in the east and west of the Indian Ocean.

During a negative event that we are currently witnessing, winds are becoming more westerly eventually bringing more precipitation to the Top End and South Australia.

Check out more here: What is a ‘negative dipole in the Indian Ocean’ and why does it mean more rain for eastern Australia?

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