Starting almost 14 years ago I have been periodically posting here a figure similar to the one below. Last year at this time we had just come out of three straight La Niña winters. Now it's time for the scatterplot to incorporate this past winter, which was dominated by a strong El Niño.
Last year's update was the first in a few years, so in that post I reviewed my reasoning about what is plotted in the figure. Please see last year's update for a more complete discussion. The reason I use a 5-month winter for Tucson is that even in a wet winter an entire month can be well below average, and conversely for a dry winter. It's better in El Niño winters to take note of 4 out of 5 wet months, instead of focusing on mid-winter and so sometimes discounting a precipitation total that results from combining only 2 wet months with 1 dry. As it turns out for this past 5-month winter it was November that was the dry month. A wet March made up for November. So for this past season in particular the net impact of broadening the definition of winter from 3 to 5 months was a wash.
On the horizontal axis, the ONI is a number calculated monthly as an objective way to summarize the status of ocean temperatures in a critical part of the tropical Pacific. Since the calculation is a running mean, the most recent value lags by a month. There is also a persistence requirement, as explained by the Climate Prediction Center in their weekly updates. Generally a value greater than 0.5 means El Niño conditions are present, while less than -0.5 is La Niña. This past winter the ONI edged up to 2.0 in December before dropping back to 1.8 in January. See my last update a year ago for more discussion about what I have plotted here. Though the ONI for this month (April) will probably still be in El Niño territory, model outlooks have it falling rapidly to below -1.0 by this October.
The TUS 5-month total precipitation for this past winter was 6.16 inches, which makes it the wettest winter in 26 years, and ranks it 11th wettest of the last 75 years (i.e., of the 5-month winters 1949-1950 through 2023-2024). Last fall, even though an El Niño of moderate (ONI > 1.0) or strong (ONI >1.5) intensity was expected for the 2023-2024 winter, for the West a minority contrary forecast had a near normal outlook for winter precipitation, apparently based on giving maximum weight to recent trends (subsequent to the 1997-1998 winter). Now that the winter of 2023-2024 is a fact, the underperforming winter of 2015-2016 looks even more like an outlier, not part of a new trend.
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