La Niña December update: visual aids


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[November 2021 sea surface temperature departure. Image from Data Snapshots on NOAA Climate.]

[From NOAA Climate written by Emily Becker] La Niña continues as the northern hemisphere moves towards winter, and forecasters are confident that he’ll hang around for the rest of the winter. This La Niña, the second in two years, will likely transition to ENSO neutrality during the spring. There is about a 60% chance that late spring and summer will be neutral conditions.

Magnifying glass

It’s time to take a closer look at the tropical Pacific, the home of the El Niño-Southern Oscillation climate model!

[Sea surface temperature difference from the long-term average from mid-October 2021 to early December. Graphic by Climate.gov, based on data from NOAA’s Environmental Visualization Lab. Description of historical baseline period here.]

La Niña’s characteristic ocean surface, cooler than average, is evident across much of the tropical Pacific. In recent weeks, cold anomalies have increased in the eastern Pacific, and in general we see a well-established La Niña pattern. The warmer-than-average surface temperatures in the far western Pacific are also characteristic of La Niña.

The wavy features of the above animation are called waves of tropical instability. The eastern equatorial Pacific is generally cooler than the water just north and south, and cooler than the water in the far western Pacific. (Scientists unfortunately call this region the “cold tongue.”) This cooler surface along the equator is due to the upwelling of cold water from the deep ocean. Whenever there is a temperature difference, or slope, between cold and hot, nature wants to soften it, and tropical instability waves help mix the cooler equatorial surface with warmer waters outside the equatorial north and south.

During La Niña, the cold tongue is even colder (chill), so we often see the wave action of significant tropical instability. These waves move relatively quickly, so they typically do not appear on monthly or seasonal sea surface temperature maps.

Binoculars

The sea surface temperature in the Niño-3.4 region of the tropical Pacific, our primary ENSO surveillance index, was approximately 0.9 ° C cooler than the long-term average (1991-2020). It depends ERSSTv5, our most reliable dataset, and comfortably in the La Niña category of over 0.5 ° C cooler than average. Forecasters are confident that La Niña will continue over the next few months, in part based on computer model predictions. There’s also a large pool of cooler-than-average water below the surface, which builds confidence that La Niña’s demise is not imminent.

[The official CPC/IRI ENSO probability forecast. The bars show the seasonal chances for each possible ENSO state—El Niño (red), La Niña (blue), and neutral (gray) from late fall 2021 through summer 2022. The forecast is based on a consensus of CPC and IRI forecasters, and it is updated during the first half of the month, in association with the official CPC/IRI ENSO Diagnostic Discussion. It is based on observational and predictive information from early in the month and from the previous month. Image from IRI.]

Forecasters believe the most likely scenario is for La Niña to go neutral by the end of spring. Neutral is the most common state for ENSO in the spring, and most computer models predict the Niño-3.4 to be in the neutral range (between -0.5 and 0.5 ° C) d ‘here the April-June season.

We devote so much time and energy to El Niño and La Niña because they have a impact on winter weather and climate around the globe. (Also, because they’re interesting!) La Niña moves the Pacific Jet, the fast-moving river of air that directs storms across North America. Specifically, during La Niña, the jet stream retracts westward and an area of ​​high pressure tends to form in the North Pacific. These changes are leading to a reduction in the number of storms arriving on the coasts along the southwest coast of the United States and, as a result, drier conditions. On the other hand, the storm’s track tends to be deviated to the north, resulting in wetter conditions in the Pacific Northwest. For more details on how it works, check out this post on La Niña and the jet stream.

[Average location of the jet stream and typical temperature and precipitation impacts during La Niña winter over North America. Map by Fiona Martin for NOAA Climate.]

The climate outlook for this winter in the United States has been detailed in this recent post. Mike Halpert discusses the higher likelihood of dry conditions throughout the southern level, a hallmark of La Niña. Also, for something you could probably see with the naked eye, check out the discussion on the Outlook December 2021.

Telescope

Since La Niña is linked to drier conditions in the southern part of the United States, it can make the drought worse. The stream drought outlook does not expect relief for the severe drought in the southwest this winter and anticipates the drought expanding eastward into Texas and Oklahoma.

Can we make predictions about the state of ENSO for the winter of 2022-2023? Not for the moment. Most of our computer models do not extend that far, and the evolution of past La Niña events does not give us a clear picture. Overall, of the 12 La Niña freshmen in our historical record, four only lasted a year. Eight turned into a La Niña Double Dip just like our current event. Among these, two have reoffended for a third year (triple dip?). Nat dug into the science behind La Niña’s tendency to reproduce in consecutive winters in this post.

[This graph shows 3-month-average sea surface temperature anomalies in the Niño 3.4 region of the tropical Pacific (the Oceanic Niño Index) for 2019–21 (blue line) and all other years (gray lines) based on the twelve first-year La Niña winters since 1950. The average of all twelve is shown by the black line. Four La Niñas lasted only one year (light gray lines), six lasted two years (medium gray), and two recurred for three years in a row (dark gray).  Climate.gov graph inspired by original from Mike McPhaden.]

One interesting thing to note about La Niña 2020-22 is that, unlike the 11 others, it did not follow a full-fledged El Niño. The tropical Pacific was warmer than average in 2019-2020 but did not reach El Niño thresholds.

Mike McPhaden had this to say about recurring La Niña in general and this year in particular: providing additional inertia to support subsequent surface cooling over periods of several years. What then explains the current 2-year cold event, which was preceded only by a very weak El Niño-type warming, is an interesting research subject. “

And so, while we’d like to get a feel for next winter, it’s still too early. Minute forecasters have a clearer picture, however, we’ll get it to you!

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