After nearly three years of El Niño conditions La Niña has arrived.
As equatorial sea surface temperatures (SSTs) across the east central and eastern Pacific Ocean continue to cool, what will be the affect La Niña will have on global temperatures this year?
September 2020: La Niña is here
La Niña conditions were present in August, and there’s a 75% chance they’ll hang around through the winter. NOAA has issued a La Niña Advisory. Just how did we arrive at this conclusion, and what does a La Niña winter portend? Read on to find out!
The answer to the first question, “Is the monthly Niño3.4 sea surface temperature anomaly equal to or less than -0.5°C?” is an easy “yes.” August’s value was -0.6°C according to our most consistent sea surface temperature dataset, the ERSSTv5 (though that is not the only SST dataset we monitor). For a quick refresher, the Niño3.4 sea surface temperature anomaly is the difference from the long-term average temperature of the surface of the Pacific Ocean in the Niño3.4 region. In this case, the long-term average is 1986-2015.
The second step is “Do you think it will stay more than half a degree cooler than average for the next several months?” and again, the answer is “yes.” Most of the dynamical computer models predict that the sea surface temperature will remain below the La Niña threshold of -0.5°C through the winter.
Now, on to the critical third step: “Is the atmosphere showing signs of a response to the cooler-than-average sea surface?” Another “yes!” La Niña intensifies the contrast between the warm far western Pacific and much cooler eastern Pacific, and so La Niña’s atmospheric response is a strengthening of the Walker circulation. This large-scale circulation pattern is characterized by air rising over the very warm waters of the far western Pacific and Indonesia, traveling eastward high in the atmosphere, sinking over the eastern Pacific, and traveling back westward near the surface. (Creating the trade winds—more on those in last month’s post.)