Elusive Effects of El Niño

Seems only yesterday, but it's been three years since everyone was talking about expectations for the then upcoming 2015-2016 winter. That summer of 2015 the Oceanic Niño Index (ONI, defined and calculated by the Climate Prediction Center [CPC]) had already run past plus one, and was expected to (and did) exceed plus two during the winter of 2015-2016. This year the latest Diagnostic Discussion by the CPC places the odds for El Niño at 60% this fall, and 70% during the upcoming winter. The CPC's consensus of forecasts takes the ONI to around 0.6 by this October and to about 1.0 during the winter.

This time three years ago I thought that the chatter about the upcoming effects of El Niño was excessive. This summer on the contrary it seems that excessive reticence has replaced overconfidence. Three years ago in an initial rant about the chatter, I posted a scatterplot of the previous 65 years of precipitation at Tucson airport, totaled for September combined with October, versus October ONI. Here are the final numbers for the last three years—an additional three points that can now be added to that scatterplot for fall.^*

Year	October ONI	September plus October Rain, inches
2015	+2.4	4.65
2016	-0.7	1.70
2017	-0.7	0.03

I won't rehash my old rants about fall; they can be read from three years ago here, and two years ago here. The point about expectations for this fall is that to the extent that an increased level of concern about the kind of thing that can happen in the fall because El Niño was justified three years ago, that level of concern is equally justified this year.

There will be a separate post for winter, including a fresh scatterplot^* for that season, as well as wonderings about why the scatter for that season is what it is. But that post can wait for a month or two.

* The plots three years ago were made with the previous version (v4) of the CPC's ONI's SST anomalies. The current version (v5) mostly differs by no more than 0.1 ONI (degree C), but in some cases the difference is 0.2 or even 0.3. Notably: October 1983 is now -0.8 (previous -0.6); and January 2010, which previously at +1.3 was right in the middle of the "moderate" El Niño category, now at +1.5 edges into the strong category.