This article originally appeared on Climate.gov and was published by Emily Becker
Our second-year La NiƱa has materialized, as indicated by the ocean and atmosphere in the tropical Pacific. Thereās anĀ 87% chance of La NiƱa this winter, the season when North American weather and climate are most affected by ENSO (El NiƱo/Southern Oscillation, the entire El NiƱo/La NiƱa system).
The winning numbers
The first step on our āIs it La NiƱa?ā decision tree asks āis the monthly NiƱo-3.4 Index equal to or less than -0.5Ā°C?ā TheĀ NiƱo-3.4 Index, our primary metric for ENSO, is the anomaly in sea surface temperature in the central equatorial Pacific (anomaly = the difference from the long-term average; āaverageā isĀ 1991ā2020Ā nowadays).
The September NiƱo-3.4 Index was -0.5Ā°C, according toĀ ERSSTv5, our primary dataset. So itās on to the next box on the decision tree! Do we think that anomaly willĀ remainĀ cooler than the La NiƱa threshold? Again, yes! Nearly allĀ computer models are currently predictingĀ that the cooler-than-average conditions will remain through the winter. Another source of confidence for the NiƱo-3.4 Index to remain cooler than the La NiƱa threshold is the large amount of cooler-than-average water beneath the surface of the equatorial Pacific. This subsurface water provides a source for the surface, and it has been intensifying in recent weeks.
The final requirement to get to La NiƱa conditions is that the atmosphere is showing signs of responding to that cooler-than-average surface water. Over the past month, the low-level winds near the equator, which usually blow from east to west (the trade winds), were stronger than average, as were the west-to-east winds high up in the atmosphere.
These signs, along with more rain than average over Indonesia and less in the central Pacific, tell us that theĀ Walker circulationĀ is juiced up. A stronger Walker circulation is the expected atmospheric response to La NiƱa. The Walker circulation is driven by the difference between the very warm ocean surface in the far western Pacific and the relatively cool eastern Pacific; during La NiƱa, this difference is enhanced, leading to a stronger Walker circulation.
Follow the blue brick road, and we get to La NiƱa!
Buy one, get one free
La NiƱa has probably earned silver-elite, frequent-flier status here, as this is the fourth time weāve written a āLa NiƱa is hereā post on the ENSO Blog! (Earlier posts areĀ September 2020,Ā November 2017, andĀ November 2016.) Thatās four of the seven-and-a-half years weāve been standing on this digital street corner yelling about ENSO.
Overall, La NiƱa is not more common than El NiƱoāin the historical recordĀ dating back to 1950, there have been 25 El NiƱos and 24 La NiƱas (counting this year). However, La NiƱa often occurs in consecutive winters, while El NiƱo rarely does. The reason for this is difficult to explain in brief, but researchers think itās partly due to differences in atmosphere-ocean coupling between La NiƱa and El NiƱo.
As you can see in the map below, the center of the ocean surface temperature anomaly tends to be a bit weaker and a bit farther west during La NiƱa than El NiƱo. The difference in strength and location means the trade winds response is not exactly opposite in La NiƱa and El NiƱoāunsurprisingly, the response tends to be weaker and centered farther west during La NiƱa than El NiƱo.
El NiƱo events endĀ when the equatorial temperature anomalies are dispersed away from the equator, and this process tends to be less robust in La NiƱa. The less-clean end to La NiƱa means that itās not hard for the system to tip back into La NiƱa the next year. For more on how this works, check outĀ Natās excellent postĀ on double-dip La NiƱa.
Holiday sale
Weāve already seen one likely effect of La NiƱa this yearāaĀ more active Atlantic hurricaneĀ season, withĀ nearly twice as many storms as averageĀ so far this year. But the most substantial La NiƱa effect on North American rain, snow, and temperature happens during winter. In summary, La NiƱa winters tend to be drier and warmer across the southern third of the U.S., and cooler in the northern U.S. and Canada. The Pacific Northwest, the Tennessee/Ohio Valleys, and parts of the Midwest tend to see more rain and snow than average.
For more information, check out these posts on howĀ La NiƱa affects the Jet Stream,Ā drought during second-year La NiƱa,Ā snow during La NiƱa winters, andĀ La NiƱaās role in South American climate. NOAAās Climate Prediction Centerās winter weather outlook will be released soon, and you can find the current seasonal forecastsĀ here.
Sorry for being so selfish, but unless this bitch is like 2010-2011 for NorCal, we don’t want to hear it. We need precipitation!!