Rapid equatorial Pacific SST's amidst a negative PDO

 Rapid intensification of equatorial SST's, amidst a negative PDO.

The Pacific Decadal Oscillation (PDO) is currently negative. As of March 2026, the PDO index was recorded at -1.25°C, maintaining a persistent negative (or "cool") phase that has largely dominated the North Pacific basin over the last several years.  

Even though a negative PDO usually runs counter to El Niño, its interaction with the rapidly developing 2026 El Niño is creating a fascinating and highly unusual climate scenario.

How a Negative PDO Changes the El Niño Dynamics.

The PDO is essentially a long-term, decadal cousin of ENSO that is centered in the North Pacific (poleward of 20°N). When it is in a negative phase, it features a massive horseshoe-shaped footprint of cooler-than-average waters along the west coast of North America and into the subtropics, wrapping around a warmer core in the central North Pacific.  

Typically, a negative PDO acts as a "brake" on El Niño development by reinforcing stronger trade winds and keeping the eastern Pacific cool. However, the 2026 El Niño is behaving quite differently due to its overwhelming thermodynamic strength:

Tug-of-War with the Trade Winds:

Normally, a negative PDO tries to push back against El Niño by strengthening the easterly trade winds. Right now, however, the atmosphere over the equatorial Pacific has completely decoupled—the trade winds have already collapsed or reversed into westerly wind bursts, rendering the PDO's usual dampening effect ineffective.

The Subsurface "Fuel Tank": The sheer volume of subsurface heat content currently sitting in the equatorial Pacific is nearly twice what was observed at this exact same point during the development of the major 2023 El Niño. This massive reservoir of warm water is overpowering the mid-latitude cooling signals of the PDO.

Suppressed Extratropical Warmth: While the equatorial regions (like Niño 1+2 and 3.4) are experiencing extreme, borderline "Super El Niño" warming, the negative PDO is keeping the waters immediately off the coast of North America much cooler than they would be during a classic, fully constructive positive PDO/El Niño pairing.

What This Means for the 2026 Peak and Beyond.

Because the equatorial forcing is so incredibly dominant, the negative PDO will not prevent this El Niño from peaking as a strong, potentially historic event later this winter.

Instead, the main impact of this clash will likely be seen in the global weather teleconnections (the atmospheric bridge that alters weather patterns worldwide). The combination of a strong tropical El Niño and a negative mid-latitude PDO can distort the jet stream in unusual ways. For instance, it can warp the typical winter storm tracks over North America and Europe, making downstream weather impacts—such as winter rainfall patterns—harder to predict using historical El Niño analog years alone.

Written 27th June 2026

David I Birch