As the clouds gather and we reach for the umbrella, dare we ask ‘that’ question again?

Are we looking at yet another visit from La Niña?

After the East Coast dried out through June and July, the wet signal has returned, along with plentiful amounts of easterly swell. This at a time when we usually see steep southerly swells glancing the coast on their way to Fiji.

Last summer saw a short-lived Niña signal develop throughout the Pacific, though its stunted longevity meant it failed to reach the requisite threshold for the Bureau of Meteorology while just scraping in for NOAA, the American meteorological service.

The surf and weather through autumn played out as you’d expect post La Niña with multiple flooding events and oversized easterly swell impacting the East Coast from March through May, feeding off warmer than normal sea surface temperatures in the Coral and Tasman Seas.

Since then the Pacific has returned to a neutral state while at the same time warmer than normal sea surface temperatures have continued to hug Australian coastlines, with far spread consequences including the South Australian algal blooms and coral bleaching off the Ningaloo coast.

Sydney is also coming in much warmer than normal with SSTs around 21°C, which is 2-3°C above normal for this time of year - see image below left.

A warm tongue of the East Australian Current is bringing warm, blue water to southern locales (Photo Brokensha)

Shifting our eyes back to the equatorial Pacific, and since July we’ve seen a couple of stronger than normal easterly trade-wind bursts, cooling sea surface temperatures slightly in the form of upwelling. This upwelling has promoted the development of a sub-surface cold pool which may feed any further, favourable upwelling winds.

Crucially, the wind forecast for the coming month shows just that, with a significant burst of stronger than normal easterly trade-winds due over the coming fortnight, followed by possible further activity into September.

If this comes off as forecast, we’d see the cool water signal in the Pacific Ocean deepen further while piling up warm water in the western Pacific, pushing us towards the La Niña thresholds. 

However, it’s not just about the sea surface temperature, with a fully fledged Niña requiring the atmosphere to become coupled. If air rising in the west, above the warm pool, begins feeding back to the east this would complete the feedback loop, driving even stronger easterly winds flowing westward towards the rising air in the west.

The last few summers have shown that the intensity of the La Niña signal isn't always reflected in the effects. If there's enough warm water offshore then even a reduced La Niña can bring rain, floods, and lots of easterly swell. 

Above, a typical La Niña lineup: grey, wet, and windy (Neverka).

Since the start of the 2020’s we’ve largely been in La Niña. One of the drivers that explains this persistent La Niña pattern is the Pacific Decadal Oscillation (PDO).

You can think of the PDO as a long-lived El Niño or La Niña-like setup throughout the Pacific Ocean, which has both warm and cool phases. It takes into account the sea surface temperature anomalies (difference from long term average) across the wider Pacific basin, with a horseshoe of warm water wrapping the cooler signal across the central and eastern Pacific Ocean.

A quick look at the basin wide sea surface temperature anomalies shows this pattern clearly, with the PDO index currently showing a reading of negative four, the coolest reading on the reconstructed record dating back to 1854.

With such a low reading, the current setup also increases the chance of a developing La Niña.

The Pacific Decadal Oscillation (PDO) over the past thirty years, showing a record negative dip into July (NOAA)

Lastly, the Indian Ocean Dipole - another regional climate driver - is also hinting at a coming La Niña.

Currently, the difference in water temperature across the Indian Ocean basin - cool off eastern Africa and warmer off Indonesia - makes it a quasi-negative Indian Ocean Dipole (IOD) setup.

A negative IOD and La Niña typically complement each other, which makes it one last point of persuasion. It also makes the weather outlook lean towards wet and the swell more easterly.

We’ll provide updates on any cooling or increased chance of La Niña in the commentary below and future articles.