El Niño and Tucson Winters

The scatterplot below is an update of a similar figure presented in my post three years ago.

The Oceanic Niño Index (ONI) is defined and calculated by the Climate Prediction Center [CPC] as a climate-adjusted three-month average of the sea surface temperature anomaly for the 3.4 region of the tropical Pacific. Thus the January ONIs are already averages over the three months of December, January and February. Historical calculations now use version 5 of the CPC's reconstructed sea surface temperature dataset. The figure that I presented three years ago was from version 4. In some cases the plotted points on this figure have shifted a few tenths of a degree left or right from where they were three years ago. But the overall scatter remains the same. This updated figure includes the last three winters. The last two winters would be labeled 17 and 18, but those two points are packed (unremarkably) among similar years having negative ONI with below average precipitation. (For all 69 years, the mean of the five-month total precipitation is about 3.7 inches; the median is about 3.3 inches; the upper one-third starts at 4.1 inches.) The winter of 2015-2016 (labeled 16) is remarkable because though it is the rightmost point, it is also below average.

Being a three-month running average, the September ONI that will become available at the CPC site tomorrow will lag the latest developments. According to the weekly updates from the CPC the 3.4 anomaly has climbed rapidly in October so that for the latest week it stands at plus 1.1 degrees. The models had correctly predicted the rapid warming in October. If their prediction of a leveling off for the rest of the winter holds, then the January 2019 ONI would be close to plus one, which is the dividing line between a weak and a moderate El Niño. Accordingly the CPC's outlook for the upcoming winter (three months, December through February) issued two weeks ago expects an El Niño pattern across the country. Their outlook puts Tucson's odds into wetter-than-average territory, consistent with the plot above, where of the 24 winters in El Niño territory in January, 15 had above average precipitation, 14 of the 24 were above 4.1 inches. For the 13 weak El Niño winters alone, the two decades of the 1950's and the 2000's are tilted toward below average; the decades of the 1970's through the 1990's, along with so far the decade of the 2010's are tilted toward above average. The fact that the El Niño winter three years ago was below average is unlikely to have ushered in a new era of below average El Niño winters, not anymore than the winters of 1984-1985 or 1967-1968 ushered in new eras of above average La Niña winters. This coming winter's precipitation in Tucson might not be above average, but it more likely will be.