January El Niño update: It’s got a lot going on

Happy New Year! December was an action-packed month, El Niño-wise. We’ll take a look at what happened in the tropical Pacific and around the world.

Out with the old

El Niño put up some pretty impressive numbers in December. The Niño3.4 index, which compares ocean surface temperatures in the east-central Pacific to the long-term average, broke the record in December, coming in at 2.38°C above average, surpassing December 1997’s 2.24°C. (This is using the ERSSTv4, the most historically consistent sea surface data we have.)

El Niño is ultimately measured on seasonal timescales, though, so the average of the sea surface temperature anomaly (departure from the long-term average) over three months is what we really pay attention to. In October–December 2015, the Oceanic Niño Index was 2.25°C, tied with the same period in 1997.

Ranking of October-December average sea surface temperature departures from the mean for all El Niño episodes since 1950. This measurement, the Oceanic Niño Index, uses ERSSTv4 data. Figure by climate.gov, data from CPC.

The atmosphere made a good showing in December, too. Until recently, the atmospheric part of this El Niño was lagging behind the other very strong El Niño years, 1982-83 and 1997-98. However, satellites saw much more cloudiness than average in the central Pacific during the past month, even more than in December 1982 or December 1997).

Blocking of outgoing longwave radiation (OLR) during December 2015. Blue shading shows areas where more clouds than average were present, indicated by greater-than-average absorption of surface OLR (less OLR exiting the top of the atmosphere). Climate.gov figure from CPC data.

The Equatorial Southern Oscillation Index, which measures pressure anomalies on either side of the  tropical Pacific, was also at record levels for the month. After a fairly moderate response in the low-level winds during December, this month has begun with a strong westerly wind burst. Whether this burst was solely related to El Niño, however, is something we’ll talk about a bit later in this post.

Recent average winds near the surface of the tropical Pacific: departure from the 30-year average. Purple shading area indicates where the normally easterly (blowing from east to west) winds are weaker than normal. Climate.gov image from CPC data.

First, a quick spin around the world

So El Niño is certainly cranking away in the equatorial Pacific. Is it influencing the weather around the world in the ways we’d expect? The main season for El Niño impacts in the U.S. (January–March) is just beginning in the U.S., but it’s winding down in other areas of the world.

Australia’s typical El Niño impact is dry conditions over most of the continent from about July through December, but through this period there hasn’t been a very clear deficit except in portions of eastern Australia. It’s possible that a record warm Indian Ocean had a strong effect on the climate in Australia this year, a reminder that the climate system has a lot of moving parts, and impacts from El Niño are expected, but not guaranteed.

That said, in other areas of the world, El Niño impacts were clearer. Much more rain than normal fell in eastern Africa, as their “short rains” rainy season (October–December) was enhanced by El Niño, while southern Africa has had continued dry conditions. Uruguay, southern Brazil, and Paraguay also experienced a lot of rain, and northern South America has been dry, as often happens in September–December during El Niño.

Typical rainfall patterns during El Niño events. Such teleconnections are likely during El Niño events, but not certain. Map by IRI.

You said there was a lot going on. I want more!

El Niño isn’t enough for you? In addition to Michelle’s post about all the different climate patterns that contributed to the record warmth across eastern North America in late 2015, the Madden-Julian Oscillation (MJO) was also active during December and early January. The MJO is a collection of storms and circulation changes that slowly moves eastward around the globe at the equator.

Similar to El Niño, the area of storminess perturbs global atmospheric circulation through tremendous heating on the equator. But unlike El Niño, the MJO passes through an area in a matter of weeks.  In December, this large area of storms and convection was moving across from Indonesia through the central Pacific, where the westerly wind burst that I mentioned earlier occurred.

A westerly wind burst during January 5-9, 2016. Purple shading area indicates where the normally easterly (blowing from east to west) winds are weaker than normal or have reversed. Climate.gov image from CPC data.

Remember that El Niño brings weaker easterly trade winds. Sometimes the winds reverse course altogether, and blow from the west. Weaker trade winds help intensify surface warming. Here’s what previous guest blogger Adam Sobel had to say about how the MJO and El Niño could be interacting:

Whenever the MJO acts up during an El Niño event – like now - there is a tendency to say that the MJO is making the El Niño stronger, or even causing it to exist in the first place. A recent article about the MJO in Science started off by claiming that a clutch of MJO events last March caused the onset of the current strong [El Niño] event.

There’s a clear logic to this: westerly surface winds on the equator are a critical part of El Niño, and the active phase of the MJO brings those. And whereas the MJO’s clouds, rain, and surface westerlies normally tend to fizzle when the active phase reaches the Central Pacific, during El Niño, the warmer waters near and east of the dateline allow the MJO to hold itself together longer. This allows the MJO’s westerlies to reinforce surface warming.

But the MJO is, like its name says, an oscillation. The westerly anomalies are followed quickly by easterly ones, so that the average effect might be very little over the longer time scale of ENSO. And we know there would be central and eastern Pacific westerlies during El Niño even without the MJO. Westerlies have been present for the past several months, but for much of that time there has been no sign of the eastward propagation that is the MJO’s trademark.

Clearly, the question of how the MJO and El Niño act to reinforce or weaken each other is still up for debate. 

In with the new

Regardless of the cause, there was a very strong westerly wind burst near the Date line early this month. Often, but not always, these wind bursts kick off downwelling Kelvin waves, sending an area of warmer subsurface waters eastward. We’ll have to wait to see if this develops to know what impact it could have on slowing the decay of El Niño through the spring.

Speaking of the decay of El Niño… since this past summer, computer models have been predicting that December would be the peak in one-month sea surface temperature anomalies in the Niño3.4 region. Forecasters expect the sea surface temperature anomalies in the tropical Pacific to decrease gradually over the next several months, and we’re favoring a transition to neutral conditions by the late spring or early summer. Stay tuned over the next few months to see what impact this very strong El Niño has on the U.S. and the rest of the world.

* Follow Dr. Sobel on twitter: @profadamsobel, and check out his book about Hurricane Sandy, Storm Surge.