The SST anomaly for September 2017 fell for the third consecutive month to -0.43°C in Niño region 3.4, and looks likely to usher in a La Niña event during this coming winter (fig 1). The convoluted way that a ONI event occurs, depends on a three-month running anomaly of -0.5°C or lower (fig 2), so the earliest it could be declared is probably December (combined October-December anomalies), unless the anomaly decides to crash during October!
The coastal waters off the coast of Chile seem to have cooled down since last month. There were SST anomalies 0f +5°C on the chart for the 18th of March last month (fig 1), but the highest that I can see in the same area is +3°C twenty five days later (fig 2). The experts are saying that an El Niño event still remains a distinct possibility towards the end of this year though.
I’m no expert and don’t pretend to understand why the cold Humboldt current seems to be the controlling mechanism for the upwelling of warmer water off the coast of Chile (fig 3).
But this map I found after a little research on the Internet indicate that there are more than just the Humboldt current at play down the coast of South America (fig 4).
I shall of course try to find out and report back, unless one of my readers would like to do it for me.
The statistics that are used to decide whether we are currently in a La Niña or an El Niño event, or if conditions are just neutral, never fail to amuse me. I thought I’d correctly programmed all the rules into my application, but obviously you need to be monitoring more areas of the Pacific than region 3.4 that I do. As far as I make it, conditions over the last 5 months in region 3.4, meant that a La Niña event had started in August (fig 2). Three month averaged SST anomalies have been below -0.5°C for the required five consecutive months, and as far as I could see that was it, bingo. But not so, the event didn’t even happen according to the experts, although they do expect that it could start later this year. In my defence, there have been other short five month La Niña and El Niño events in the past, and why this event doesn’t merit being one of those beats me. Meanwhile, I’ll continue to live in my make-believe ENSO world where it did happen, because I refuse to add any more complexity to that application.
The reason that the La Niña event never really took off, is that some recent warming in the eastern Pacific along the coast of Central America (fig 4), has somehow manage to cut off the supply of colder water that usually tracks from east to west across the equator from the Chilean coast. That’s my very simple explanation, which is almost certainly completely wrong.
The El Niño event is now history since it ended in May (2016) now it’s just a case of waiting to see if and when La Niña arrives. The region 3.4 index has now been negative for the last two months. But because the index is based on a three-month moving average, and because that average needs to be -0.5 or lower (and we’ve only had the August index lower than that) we might have to wait till the New Year to see if August’s -0.54 is the start of a La Niña event – the Oceanic Niño Index [ONI] calculations are more complex than the majority of the other climate indicators are.
The latest El Niño (for region 3.4) which started back in March 2015 continues into what is highly likely to be its final month of May 2016 according to the latest Oceanic Niño Index [ONI] figures that have just been released. I say that because the ONI is a three-month running mean, and the latest value for region 3.4 during May was +0.30°C which is below the 0.5°C threshold and markedly down on the +1.09°C of the previous month. That’s a duration of 15 months, which could have easily have been four months longer if it weren’t for a stalled start in had. Despite this it still makes it the second longest El Niño in the current series back to 1950, and it also had the highest anomaly of +2.3 of any other event. That still might only make it the second warmest behind the 1997-98 event, but as with everything to do with these cycles it may be down to which region and what algorithm you use to decide these things. It’s interesting to see the cold waters spread across the equator from east to west and splitting the anomalously warm ocean.