A dull zero crossing & what it means for Australia

This week the tropical Pacific has seemingly crossed a dull threshold. Not very exciting or significant, but it scares a lot of people around the globe.

For the first time in the past two years, the relative (meaning warming signals have been accounted for) Niño3.4 index has crossed the zero line and enters positive anomalies. This is not historical, happens a lot of times, sometimes randomly, but given this years forecast, many people hold their breath.

We are still not quite in the window of certainty, but more indizien come together. This past week, the Niño3.4 index changed sites from negative to slightly positive: 0.23! Doesn’t sound like much and fair enough, we’re far off official requirements of 0.8 and other key requirements to be met. But hovering to the Niño2 region shows, something is on the way. The tropical Paicifc is warming up, starting in the East (Niño1 & Niño2) and slowly reaching areas along the dateline (Niño3.4).

You can look at the full SST pattern or, in a simplified way, at how the different Niño indices behave relative to each other. A lot of the time, when the eastern indices (Niño 1+2) and Niño 3.4 move in sync, the system is leaning toward an Eastern Pacific El Niño.

Eastern Pacific El Niño

The Eastern Pacific El Niño is the classic version — the one most people mean when they say “El Niño.” The trade winds weaken, upwelling off South America slows, and the eastern Pacific warms. As warming increases, convection follows the heat from unusually warm sea-surface temperatures, bringing large shifts in circulation patterns such as the Walker circulation.

Now, with an increasing likelihood of an El Niño calling out loud, being mega or monster El Niño, where does that come from? Very clearly, from a striking glimpse at the latest seasonal forecast from the ECMWF. You don’t have to study atmospheric or climate science to understand why this looks huge: