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.

Fall Festival of Rain

Tucsonans don't complain when it rains. Rain makes the actual festivals more fun, more reason to celebrate while packed under the tents. Tucson so far this fall has escaped any serious flooding such as has affected points south, west and north of here.

In a post two months ago, I noted the Climate Prediction Center's odds for El Niño during the upcoming fall and winter, and revisited my rant from three years ago regarding chatter about fall rain in Tucson related to El Niño. Since this fall's rains have been so good, I decided to present a fresh scatterplot for fall. For more explanation about the reasoning behind the scatterplot and for additional sources, follow the links in the previous post.

The Oceanic Niño Index (ONI) is defined and calculated by the Climate Prediction Center [CPC], and won't be available for October until the end of November. For now I'm forecasting 2018's October ONI will be approximated by the latest (mid-October) weekly Niño 3.4 region SST departure. As for this year's fall (September plus October) rain, so far for the period from 1 September through the morning of 16 October the airport has had 3.44 inches; that running total will probably increase by the end of the month. (I've had 4.76 inches for the same period, including 0.84 overnight last night.)

Considering just the last six years (2013-2018), there is a trend toward higher precipitation totals when moving from colder to warmer ONI. The decade of the 2010's has joined the decade of the 1970's in favoring above normal fall precipitation when the ONI is in positive territory. The exceptional years (fall precip well below normal but positive ONI) for those two decades were 2012 and 1979. Fall of 1976 was close to normal.

Considering the wet years appearing in the negative territory of October ONI, with the exception of the decade of the 1950's, in each decade there is one unusually wet fall. These wet falls have followed El Niño fall/winters by one (1964, 1970 and 1983) two (1996 and 2011) or three (2000) years. Certainly it's possible that the climate system sometimes stores somewhere a lingering effect of a previous El Niño, maybe even for a few years, and that that lingering effect influences Tucson precipitation in a specific La Niña fall. But most often any lingering effect does not appear to have an influence. As a specific example, most recently the fall of 2016 does not appear to have been influenced by a lingering effect of the previous fall/winter's strong El Niño. It is as much nonsense to call the fall of 1983 an El Niño year as it would be to apply the same label to the fall of 2016.