El Niño Winter, 2023-2024

From Dr. Daniel Swain, aka Weather West. While this is quite long, as he says toward the end of this article, it is an important conversation, and I would urge you to read the entire thing: “Some additional thoughts regarding the still strengthening El Niño and its implications for western U.S. hydroclimate during winter 2023-24.

International dynamical models ensemble is now unanimous in predicting strong to very strong #ElNiño event that will peak sometime this winter. This is a remarkably strong signal, so I’m reasonably confident that the oceanic signature of a strong EN will actually occur.

Additionally, there’s good consensus that this #ElNiño event will be centered in the eastern Pacific basin, versus the central Pacific (i.e., this is not “#ElNiño Modoki”). High confidence in strong, east-based event signals increased likelihood of WUS hydroclimate impacts.

Canonically, that would mean increased odds of a drier than usual winter across the PacNW & Hawaii and of a wetter than usual winter across much of California, Arizona, and New Mexico. But although El Niño’s never the only game in town, that’s especially true this year.

In addition the likely strong El Niño event clearly apparent in the eastern tropical Pacific, nearly the entirety of the global oceans have been (and are projected to remain) far warmer than average (and record warm in some places). This complicates things considerably.

The co-occurrence of numerous pockets of exceptionally/record warm ocean water outside of the tropics with a strong east-based #ElNiño event has essentially not been observed before. The oceans presently have, and are expected to retain, a configuration without modern analogue.

Because ENSO teleconnections are driven by anomalous ocean temps gradients as well as absolute ocean warmth, this has potential to throw wrench into things. How? Depends: different mechanisms could either interfere with or reinforce “classic” ENSO teleconnections.

Extraordinary warmth in mid-latitude oceans will reduce anomalous latitudinal ocean temperature gradient, for example, perhaps weakening subtropical jet response. But those same warm waters will add additional moisture atmosphere, potentially enhancing those storms that do form.

All of that said: here’s a probabilistic take from the large ECMWF seasonal ensemble. Here, I’m focusing on potential for conditions to fall into in the wettest (or driest) third of Jan-Mar periods on record, rather than just looking at averages.

All of that said: here’s a probabilistic take from the large ECMWF seasonal ensemble. Here, I’m focusing on potential for conditions to fall into in the wettest (or driest) third of Jan-Mar periods on record, rather than just looking at averages.

There’s a strong signal toward emerging/worsening drought (precip in the lowest 33% of years) in the Pacific Northwest and SW British Columbia, as well as the entire Hawaiian island chain, this winter.

Meanwhile, there’s a modest signal toward a second consecutive wet winter in the Pacific Southwest, especially coastal/central & southern CA. Notably, however, this is less certain than the drought signal in the PacNW & Hawaii.

These probabilistic outlooks represent tilts in the odds, not certainties. A wide range of outcomes remains possible. But given potential for dry winter in PacNW to worsen existing drought, & for wet winter in CA to follow wet antecedents, this is an important conversation.

Finally: Despite caveats, there is meaningful physical & statistical information embedded in this. So just as I’d advise folks to be wary of social media folks “overhyping” El Niño, I’d advise same regarding folks saying ENSO tells us nothing about SW US hydroclimate!