Double La Niña, and Tucson Winter Rain

La Niña will probably be with us again, and so the mantra in many places has been that this winter will be a repeat of last winter. But rain is coming to Tucson tomorrow night, and perhaps again on Monday. That expectation has temporarily dampened the chatter here. Eventually we will dry out again for an extended period—that happens every winter regardless of ENSO status—and the La Niña buzz will return. I predict that the total rainfall at the Tucson airport over the next five months, November through March, will be two to four times what it was for the same period last year. That is an easy call, because last year was one of the driest over the last sixty years for that five-month winter period.

In my post a little over a year ago, I plotted Tucson total rainfall, November through March, versus the ocean temperature anomaly used to officially delineate ENSO phases (my plot, as detailed in that earlier post, displays an average of the ocean anomaly over the entire five months). My point about rainfall for this year's current and upcoming five winter months is the same as it was last year. That is, based on what happened in the past, a range of possibilities should be expected: from well below normal to just below normal. But what about the fact that this would be the second La Niña winter in a row?

In the table below are data for selected years from my earlier plot, limited to four (including this one) recent extended periods of cool ocean temperatures following moderate to strong El Niños (red). Two of those winters with cool ocean temperatures came close but did not quite qualify as La Niña (blue), and so the ocean temperature anomalies for those two years are unshaded. Both 12 and 37 years ago at this point we were in the middle of not only a double but a triple La Niña. (Officially, La Niña ended for a few months during the summer of 2000, as it just did during the summer of 2011, but then redeveloped the following winter.) So the two winters of 1974-1975 and 1999-2000, each having come two years after an El Niño, look like good analogues for this winter. The rainfall totals for those two winters, 2.58 inches and 1.22 inches, offer a little more hope for Tucson this year than just a repeat of last winter.

November through March, Ocean Temperature Anomaly and Tucson Rain

37 years ago			|	27 years ago			|	12 years ago			|	Now
Winter	Ocean	Rain	|	Winter	Ocean	Rain	|	Winter	Ocean	Rain	|	Winter	Ocean	Rain
72-73	+1.59	5.77	|	82-83	+2.08	6.81	|	97-98	+2.17	8.38	|	09-10	+1.59	4.94
73-74	-1.84	1.95	|	83-84	-0.49	2.94	|	98-99	-1.22	1.13	|	10-11	-1.25	0.73
74-75	-0.70	2.58	|	84-85	-0.90	6.74	|	99-00	-1.43	1.22	|	11-12	<-0.5?	?
75-76	-1.39	1.83	|	85-86	-0.37	4.51	|	00-01	-0.56	3.94	|	12-13	?	?

The winter of 1984-1985 is a different story. It, along with the winter of 1967-1968, was a La Niña outlier on my earlier plot. Both of those La Niña outliers occurred in the second year after an El Niño. But each of them also followed a winter that did not quite qualify as La Niña. For that reason, it may be appropriate to dismiss those years as completely irrelevant to this year's interactions over the Pacific, since last year was most definitely a La Niña winter. But I'm not so sure as I look upstream at storms over the Pacific these last few days evolving in a way that they tend to do more often in El Niño years. So I will hedge my prediction and say that there is a slight chance that this year could be another stray, errant La Niña winter that brings above normal precipitation to Tucson.