It’s official! La Niña is here!
And the National Weather Service’s Climate Prediction Center places the odds of La Niña conditions persisting through the upcoming December through February winter season at greater than 75%.
La Niña involve a periodic cooling of ocean temps in the Equatorial Pacific accompanied by stronger than normal tropical easterly winds. The increase in easterly winds in there pushes surface water away from South America which results in increased upwelling of cooler, deeper ocean water there. That drops ocean surface temps there.
A La Niña is officially declared when, over a three month period, ocean temps along the equator in the Pacific average 0.5-degrees C or more below normal.
Changing ocean temps change the air temps above. And since the atmosphere of our planet is one vast interconnected natural system, the impacts of temp changes in one region ripple across the planet and produce weather changes elsewhere.
Though the weather impacts of no two La Niña are precisely the same across the U.S. and the planet, there are some general characteristics of weather which have been noted in a number of La Niñas over the years. Among them has been a tendency for a reduction of vertical wind shear over the tropical Atlantic during all or part of hurricane seasons—a factor which can enhance tropical development there. With months to run in the 2020 Atlantic hurricane season, that could have real consequences. In fact, it’s been the anticipated of La Nina conditions developing which has backstopped the more active than normal forecasts for the 2020 Atlantic season.
Other cold season weather features during La Niña are laid out in the National Weather Service graphic posted here.
Here in Chicago, La Niña have often boosted precipitation in the climatological winter season–the period from December through February. And, an increased volatility in winter temps has been noted–with swings from milder Pacific air to periodic arctic surges. Interestingly, an in-house study we conducted several years ago indicated that by the time the books on many La Nina seasons here in Chicago have been closed, the overall Dec through Feb temps have come in near to modestly above normal despite period of intense cold.
Just how this winter may work out remains to be seen–but we’ll be monitoring developments closely and with great interest!
For more on LaNina, here’s a great post (though a bit technical) from NOAA which you may find interesting.