TLDR:

• El Nino peaked late last year.
• We’re currently in a neutral ENSO state, trending towards La Nina.
• A tongue of cool water has appeared in the eastern Pacific Ocean, a key signature of a developing La Niña.
• Sea surface temperature observations in the critical Niño 3.4 monitoring region have dipped below 0°C, with further cooling expected over the coming months as stronger than normal trade-winds persist.
• La Niña is likely by spring, peaking later in the year.

Climate monitoring is a great lesson in patience.

The major drivers play out on weekly to seasonal timescales, sometimes interacting with each other, and it’s only afterwards that we start to see the downstream effects in the ocean and atmosphere.

This brings us to an update on the current transition in sea surface temperatures across the equatorial Pacific Ocean.

It’d be easy to forget that we recently had an El Niño. Arguably, it was overshadowed by the preceding La Nina ‘three-peat’ and it was also weakened by the weight of warm ocean water surrounding eastern Australia, yet it peaked late last year and has been weakening ever since. We’re now in a neutral phase.

Since El Nino’s peak, we’ve seen a steady release of surface heat across the Pacific Ocean. The catalyst for this was a relaxation of the westerly winds that drive El Niño - and in turn pile up warmer water (and also elevate sea levels) in the eastern Pacific.

Once these winds backed off, the warm, elevated water started to flow back to the west where sea levels were relatively lower. It’s a simple ‘conservation of energy’ situation.

This, in addition to recent bursts of strong easterly trade-winds, caused a pool of cold water to upwell in the eastern Pacific Ocean. This cold pool originated in the western Pacific Ocean late last year, with it making its way east, hidden below the surface during the last six months.

With the strong easterly trades as a driver, a tongue of cold water is now visible across the eastern Pacific Ocean. Not only is this tongue of cold water a downstream effect, as referenced earlier, it’s also the key signature of a developing La Niña event.

If it happens, it’ll be the fourth in five years.

Sea surface temperature anomaly chart for May 28th. Note the upwelling signal in the east. (NOAA)

The above explanation will satisfy most people, however the curious surfer might ask, ‘Well, what’s driving the stronger than normal easterly trade-winds?’

The answer to that - and it also includes the related westerly wind bursts - is the Madden Julian Oscillation, or MJO.

The MJO is the wave of tropical activity that circumnavigates the globe. When the MJO is in an active phase, it produces strong westerly winds within its vicinity (aiding cyclone formation), while at the same time, drawing in strong easterly trade-winds to the east.

This has been the case over the last week or so in the Arabian Sea and Bay of Bengal, thanks to an active MJO stalling in the region..

Surfers currently in the Maldives and Sri Lanka would be well aware of this stalling MJO signal with strong, blustery westerly winds ruining the surf across some breaks. It also spawned Severe Tropical Cyclone Remal in the Bay of Bengal over the weekend.

It’s this instability that’s driving the strong easterly winds over in the Pacific basin, promoting upwelling in the eastern regions.

The teleconnections at play here, spanning tens of thousands of kilometres between the Indian and Pacific Oceans, are astounding in scope. The world shrinks under their influence.

The yin and yang of the MJO (May 19-27). Stronger than normal west winds (red) depict the location of the active MJO, with stronger than normal east winds (purple) feeding in from the Pacific Ocean (NOAA)

It’s also these westerly winds that have likely killed off the positive Indian Ocean Dipole event for the coming season, preventing strong, upwelling south-east winds from developing. Surfers in Indonesia will be thankful for that.

Thanks to this active MJO signal, and the stronger than normal easterly trade-winds feeding into it, sea surface temperature observations in the all important Niño 3.4 monitoring region* have just dipped below the 0°C mark. They’re expected to keep cooling over the coming fortnight with another episode of strong easterly winds on the cards.

If it happens, this should push us further towards La Niña, with a peak due later in the year.

With the slow transition between El Niño to neutral over our autumn, it hasn’t influenced the East Coast’s swell regime as favourably as we’d have hoped, with relentless south swells but no meaningful easterly energy.

As the Pacific Ocean continues to transition through winter, high pressure looks to remain dominant across most of the country, with no clear swell signal in favour of more east energy visible for the East Coast, rather a continuation of southerly swells.

Over to the west, the Indian Ocean looks to provide plenty of mid-latitude storms and this points to westerly swells for South Australia and Victoria, though with favourable winds.

We’ll continue to monitor the cooling in the eastern Pacific Ocean and evolution of the highly likely La Niña over the coming months.

//CRAIG BROKENSHA

*The Nino 3.4 region is an area in the central-eastern Pacific Ocean which best represents the state of ENSO (El Niño/La Niña or neutral). When the average of sea surface temperatures in the region are above a certain threshold, El Niño is declared, and when below a certain threshold, La Niña is declared.