La NiƱa continues as the Northern Hemisphere heads into winter, and forecasters are confident that it will hang around through the rest of the winter. This La NiƱa, the second in two years, will likely transition to ENSO-neutral sometime in the spring. Thereās about a 60% chance the late spring and summer will feature neutral conditions.
Magnifying glass
Time for a closer look at the tropical Pacific, the home of the El NiƱo-Southern Oscillation climate pattern!
La NiƱaās hallmark cooler-than-average ocean surface is readily apparent across much of the tropical Pacific. Over the past few weeks, cool anomalies have increased in the eastern Pacific, and in general, we see a well-established La NiƱa pattern. Also characteristic of La NiƱa is the warmer-than-average surface temperatures in the far western Pacific.
The wavy features in the animation above are called tropical instability waves. The equatorial eastern Pacific is usually cooler than the water just to the north and south and cooler than the water in the far western Pacific. (Scientists refer to this region as the ācold tongue,ā unfortunately.) This cooler surface right along the equator is due to cold water upwelling from the deep ocean. Any time there is a temperature difference, or gradient, between cold and warm, nature wants to smooth it out, and tropical instability waves help to mix the cooler equatorial surface with the warmer off-equatorial water to the north and south.
During La NiƱa, the cold tongue is even colder (shudder), so we often see prominent tropical instability wave action. These waves move relatively quickly, so they usually donāt show up in monthly or seasonal sea surface temperature maps.
Binoculars
The sea surface temperature in the NiƱo-3.4 region of the tropical Pacific, our primary ENSO monitoring index, was about 0.9 Ā°C cooler than the long-term (1991ā2020) average. This is according to ERSSTv5, our most reliable dataset, and comfortably within the La NiƱa category of more than 0.5 Ā°C cooler than average. Forecasters are confident that La NiƱa will continue for the next few months, partly based on computer model predictions. There is also a large pool of cooler-than-average water under the surface, adding confidence that La NiƱaās demise is not imminent.
Forecasters think the most likely scenario is for La NiƱa to transition to neutral by the late spring. Neutral is the most common state for ENSO in the spring, and most computer models predict the NiƱo-3.4 Index to be in the neutral range (between -0.5 and 0.5 Ā°C) by the AprilāJune season.
We spend so much time and energy on El NiƱo and La NiƱa because they have a substantial impact on winter weather and climate around the globe. (Also, because theyāre interesting!) La NiƱa shifts the Pacific jet stream, the fast-moving river of air that steers storms across North America. Specifically, during La NiƱa, the jet stream retracts to the west, and an area of high pressure tends to form in the North Pacific. These changes lead to a reduced number of storms coming onshore along the southwestern U.S. coast and hence drier conditions. On the other hand, the storm track tends to get deflected to the north, leading to wetter conditions in the Pacific Northwest. For lots more detail about how this works, check out Tomās post on La NiƱa and the jet stream.
The outlook for this winterās climate in the U.S. was detailed by Mike Halpert in his recent post. Mike discusses the higher chance of dry conditions throughout the southern tier, a hallmark of La NiƱa. Also, for something you could probably see with your naked eyes, check out Tomās discussion of the December 2021 outlook.
Telescope
Since La NiƱa is linked to drier conditions in the southern tier of the U.S., it can exacerbate drought. The current drought outlook does not expect relief for the severe drought in the Southwest this winter and is anticipating the expansion of drought eastward into Texas and Oklahoma.
Can we make any predictions about the state of ENSO for the winter of 2022ā23? Not at the moment. Most of our computer models do not extend out that far, and the evolution from past La NiƱa events does not give us a clear picture. Overall, of the 12 first-year La NiƱas in our historical record, four lasted only one year. Eight turned into double-dip La NiƱa, just like our current event. Of those, two recurred for a third year (triple-dip?). Nat dug into the science behind La NiƱaās tendency to recur in consecutive winters in this post.
One interesting thing to note about La NiƱa 2020ā22 is that, unlike the other 11, it did not follow a full-blown El NiƱo. The tropical Pacific was warmer than average in 2019ā2020 but did not meet El NiƱo thresholds.
Official Friend-of-the-Blog Mike McPhaden had this to say about recurring La NiƱa in general and this year in specific: “The likelihood of a double-dip La NiƱa increases for stronger preceding El NiƱos because the subsurface heat content in the equatorial band is usually more depleted following strong El NiƱos, providing extra inertia to sustain subsequent surface cooling over multi-year periods.Ā What then accounts for the current 2-year cold event, which was preceded by only very weak El NiƱo-like warming, is an interesting research topic.”
And so, as much as weād like to get an idea of the next winter, itās still too soon. The minute forecasters have a clearer picture, though, weāll pass it on to you!
This post first appeared on the climate.gov ENSO blog and was written by Emily Becker.