Guest Post by Bob Tisdale
First: Congratulations to the teeny-tiny 2014/15 El Niño. It’s now official. NOAA included it as an “official warm event” on their Oceanic NINO Index this week.
NOAA’s CFS.v2 model (dated April 9, 2015, source here) is now forecasting a strong El Niño for this upcoming ENSO season, or as blogger Alec, aka Daffy Duck called it, “a supercalifragilistic el nino” when he brought the forecast to my attention here. See Figure 1. That forecast is noticeably higher than the one that shows up on page 25 of the NOAA weekly ENSO update, which was dated April 4th.
If you live in California and are hoping for drought relief, I wouldn’t get my hopes up just yet. It’s still (boreal) springtime, and ENSO forecasts have a hurdle called the “spring prediction barrier”. Basically, El Nino forecasts have a bad history during and before boreal spring. [See of Torrence and Webster (1997) The annual cycle of persistence in the El Niño/Southern Oscillation for more info on the spring prediction barrier.] The spring prediction barrier is especially true for El Niño development predictions…not so much for El Niño decay or La Niña development and decay. [See Duan and Wei (2013) The ‘spring predictability barrier’ for ENSO predictions and its possible mechanism: results from a fully coupled model.]
There are a number of aspects that may help in the development of a stronger El Niño this year than last. (1) This year’s downwelling Kelvin wave is a little later in the year, so it will be impacting equatorial sea surface temperatures later in the normal ENSO development “season”. Keep in mind that El Niño events are normally tied to the seasonal cycle. [See the post Why Do El Niño and La Niña Events Peak in Boreal Winter?] (2) The eastern tropical and equatorial Pacific (surface and subsurface) are warmer this year than last. [See the March 2015 ENSO Update – Will the 2014/15 El Niño Become the 2014/15/16 El Niño?] (3) The downwelling Kelvin wave making its way across the equatorial Pacific is a little weaker than the one last year, but it’s still a reasonably strong downwelling Kelvin wave.
But as we saw last year, the development of an El Niño can be suppressed by a lack of westerly wind bursts over the course of the year. Strong westerly wind bursts early in the year only initiate the process. They are needed throughout the rest of the year to strengthen the El Niño. Also, the upwelling (cool) Kelvin wave that should come next can offset the strength and impacts of the existing downwelling (warm) Kelvin wave. So part of the development will depend on the strength and timing of that upwelling (cool) Kelvin wave, too.
A COUPLE OF NOTES ABOUT BACK-TO-BACK AND MULTIYEAR EL NIÑOS
For this discussion, we’ll refer to NOAA’s Oceanic NINO Index again. Of the 20 El Niño events since 1950 that are shown on the ONI index (in red), there are 2 pairs of back-to-back El Niños (the 1957/58 & 1958/59 El Niños, and the 1976/77 & 1977/78 El Niños) and two multiyear El Niños (the 1968/68/69 El Niño, and the 1986/87/88 El Niño). El Niño events normally evolve in one year and decay in the next, so I’m defining multiyear El Niños as those that developed in year one, extended through the second year, and decayed in the third. Bottom line: multiyear and back-to-back El Niños can and do happen. In other words, just in case some alarmists decide to get goofier than normal and try to make a fuss about a multiyear El Niño or back-to-back El Niños, there would be nothing unusual about either type coming to pass this year.
And for those wondering, there is no set rule about the strengths of the first and second years of multiyear and back-to-back El Niños. See Figures 2 and 3. For the multiyear events, the second season of the 1968/68/69 El Niño was slightly weaker than the first year, but during the 1986/87/88 El Niño, the second season was definitely stronger than the first. Looking at the back-to-back El Niños, the 1976/77 & 1977/78 El Niños were comparable in strength, while the 1957/58 El Niño was stronger than the 1958/59 El Niño. In fact, the 1958/59 El Niño was so weak it doesn’t appear on the Old Oceanic NINO Index, which uses a fixed set of base years for the climatology. [See the post Comments on NOAA’s Recent Changes to the Oceanic NINO Index (ONI).]
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As I noted a number of times last year, we’ll just have to watch and see what Mother Nature has in store for us.
[Thanks to blogger Alec, aka Daffy Duck for the heads-up.]