As our regular readers will be very aware, La NiƱa has been rolling along in the tropical Pacific for many months, and our third La NiƱa winter in a row is underway. La NiƱa is the cool phase of the El NiƱo-Southern Oscillation (āENSOā for short) climate pattern. The current forecast is for La NiƱa to continue into the winter, with 50-50 chances for La NiƱa and neutral in the JanuaryāMarch average.
Conditions in the tropical Pacific ocean-atmosphere system have been consistent with La NiƱa for long enough that, in November, one of our frequent readers commentedāaccuratelyāthat they might as well have just re-read the October post! However, while La NiƱa seems stuck in a rut right now, the last thing we want is to give the impression that ENSO is boring! So this month, Iāll do a quick recap of current conditions before looking into a question Iāve been curious about for a while now: how does ENSO affect daily temperatures during winter?
Current events
The sea surface temperature in the tropical Pacific is still well within La NiƱa territory, at about 0.9 Ā°C cooler than the long-term average in November, according to ERSSTv5, our favorite sea surface temperature dataset. This anomalyāthe difference from the long-term averageāhas been gradually warming up over the past few months, since reaching -1.1 Ā°C in September.
The atmosphere also continues to reflect La NiƱaās amped-up Walker circulation, with more rain than average over Indonesia, drier-than-average conditions over the central Pacific, and stronger winds, both near-surface and high in the atmosphere, all observed during November. The near-surface windsāthe east-to-west trade windsāfluctuated a bit, slowing a little in mid-November. Since these winds cause upwelling, their slow-down may have contributed to the slight weakening in the sea surface temperature anomaly.
Whatās next?
Forecasters are very confident that La NiƱa will continue in the short term, followed by a transition to neutral conditions. The exact timing of the transition is not clear, with equal chances of both La NiƱa and neutral for the JanuaryāMarch average. Confidence that La NiƱa will have exited by the FebruaryāApril period, however, is fairly high, with a 71% chance of neutral. This forecast indicates that we can expect La NiƱa to influence our winter climate conditions this year. Check out the September post for a round-up of La NiƱaās typical effects on winter temperature, rain and snow, and other weather and climate.
Speaking of La NiƱa impactsā¦
When we talk about the expected La NiƱa influence on seasonal temperature, we almost always talk about how the seasonal average is shifted during ENSO events. For example, during La NiƱa, winter in the northern U.S. tends to be cooler than average. The seasonal average temperature is very important, with implications for the amount of energy used for heating, and so on. Also, looking at seasonal averages means that we are filtering out short-term fluctuations and can be more confident that the shifts in climate are truly linked to ENSO.
However, you and I generally experience weather from a day-to-day perspective. Weāll notice big shifts in temperature from one day to the next or unusually warm or cold days. To get an idea of how ENSO affects daily temperatures, Iāll start by looking at the range of daily average temperatures within each winter. How much does the temperature normally vary from day to day? Then we can ask, for example, do La NiƱa winters have a wider range of daily temperatures in some locations than average? Narrower? No change? This is just a starting place, but we have to start somewhere!
The footnote has details about the calculations and data I used for the maps I show below. Also, Iām far from the first person to look at ENSOās impact on daily characteristics of temperature, of course, and the footnote includes some information on earlier studies.
Free range
The standard deviation is a common statistic to understand the spread of a set of numbers. For example, if most of your daily winter temperatures occur in the range of 40ā50 Ā°F, the standard deviation will be smaller than if most of your daily temperatures are in the range of 35ā55 Ā°F, even though the average temperature may be the same at both locations. This statistic is widely employed in weather and climate science, and itās what I use here.
First, letās see what the typical range of daily winter temperature is across North America. The central regions of the continent have the greatest variability on average, while the Pacific coast has a relatively narrow range of daily winter temperatures, modulated by the nearby ocean. The very moist climate of the tropics features the least daily variability.
La NiƱa winters tend to feature a wider-than-average range of daily temperatures across most of the continent, with the exception of the Canadian provinces around the Hudson Bay, southern Mexico, and the Caribbean. The geographic extent of this pattern surprised me, as I would have expected it to be more localized, like the seasonal average patterns. The greatest increase in daily variability during La NiƱa is along the Gulf Coast.
El NiƱoās effect on the range of daily temperatures in winter is generally opposite to La NiƱaās, with mostly decreased variability when compared to average, especially across the western half of North America.
Overall, I think this is an interesting result: La NiƱa makes daily temperatures in winter more variable in most places, and El NiƱo makes them less variable. But it presents an incomplete picture, of course. These maps show the average changes during El NiƱo and La NiƱaāto understand how consistent this effect is, we could illustrate each winter separately, like in Natās post last month that shows every winterās precipitation and temperature patterns. What does a wider range of daily average temperature really mean? Why are we seeing these patterns? Would we expect to see the same patterns in the daily high or daily low temperature? These and other questions will have to wait for future postsā¦
Footnote
Details on the analysis:
- ENSO criteria: Any year with the DecemberāFebruary Oceanic NiƱo Index (ONI)greater than 0.5 Ā°C was included in the āEl NiƱoā sample, and any year with ONI less than -0.5 Ā°C was included in La NiƱa. Not including ONI equal to 0.5 Ā°C or -0.5 Ā°C means I excluded the very weakest events, as well as borderline not-quite-ENSO years.
- The El NiƱo maps show the standard deviation averaged over the resulting 20 El NiƱo winters divided by the overall average winter standard deviation. The La NiƱa maps show the average during the 18 La NiƱa winters divided by the overall winter average.
- Daily temperature data: I used Berkeley Earth daily average temperature dataset. Itās also available here.
- Years included: 1950ā2019. Berkeley Earth is available through near-present, but the data I downloaded ended in 2019. Iāll update with 2020ā2022, but donāt expect the overall results to change.
- Programming language: I used Python. Jupyter notebook available upon request.
- Earlier studies: The most cited paper on this topic is Smith and Sardeshmukh 2000. My results mostly reproduced theirs, with the expected differences because we used different temperature data, they looked at 1959 to 1998, and we even had different criteria for defining ENSO events. Also, thereās a recent study, Yang et al. 2022 (Nat is a co-author) that my results agree with. Iām going to discuss that paper a little more next monthā¦ stay tuned!
This post first appeared on the climate.gov ENSO blog and was written by Emily Becker.