The Climate CIRCulator is a monthly newsletter covering climate science and the Northwest written by scientists and communicators.
Weather and climate researchers predicted a strong El Niño in 2014. It failed to manifest. This year, El Niño is back and stronger than ever. What happened?
This is the question taken up in the commentary section of Nature Climate Change under the catchy title, “Playing hide and seek with El Niño.” Peekaboo, however, might be the more apt metaphor. As the paper’s author, NOAA’s Michael McPhaden, points out, El Niño didn’t so much run away and hide as temporarily obscure itself. Here’s what happened.
In spring 2014, researchers observed what appeared to be a developing El Niño that by all accounts looked to be the start of a major El Niño. Then, in summer 2014, it just sort of disappeared, only to reappear in early 2015 as the “monster” El Niño everyone had expected in 2014. Why the surprise? Let’s start with what we know about El Niño.
The tropical Pacific Ocean is in a perpetual slow dance between the ocean and atmosphere. In a typical year, trade winds blowing from east to west across the Pacific lead to warmer water pooling in the western Pacific, around Australia, and cooler water pooling in the eastern Pacific, around South America. This produces a kind of atmospheric conveyer belt, called the Walker circulation.
Partly in response to the winds of the Walker circulation, warm water in the western Pacific creates storms that send a mass of warm air east, up and over the trade winds. The winds in turn drive the ocean currents that keep the cold water in the east and the water warm in the west. This creates storms and, well, you get the idea: Warm water helps drive the atmospheric Walker circulation and its thunderstorms, and the Walker circulation pushes the warm water.
El Niño is the periodic weakening of the trade winds. When this happens, the circulation process is disrupted and that warm water in the west begins to slowly move east. This affects the atmosphere overhead, leading to climate changes the world over as different as droughts and downpours.
In 2014, when researchers noted a relaxation of the trade winds in the western equatorial Pacific, many expected a warming in the waters of the central Pacific. Which is what they got. In fact, the ocean’s response looked very similar to what was observed during the 1997-1998 El Niño, the strongest El Niño on record. Computer model simulations also predicted the likelihood of an El Niño with a high degree of confidence.
Consequently, headlines about a coming “monster” El Niño deluged the popular press. Then the trade winds strengthened, the waters cooled, and nearly everyone thought El Niño was, if not gone, then at least so weak as not to have a noticeable effect on the climate. Then, peekaboo!
The trade winds weakened again, and the already warm water in the central Pacific helped push things along, in McPhaden’s words, “leading El Niño to rise, phoenix-like, from the ashes.”
El Niño predictions were again highly confident. For instance, in July of 2015 NOAA predicted with 90 percent certainty that El Niño would continue through the Northern Hemisphere’s 2015-2016 winter. So why were the initial 2014 projections off the mark?
The proximate reason El Niño failed to manifest in 2014 concerns those warm waters in the central equatorial Pacific, and why they failed to connect with the atmospheric conditions overhead. Normally, this connection is due to the aforementioned atmospheric convection and rainfall, but during the summer and fall of 2014, when those warm waters should have produced rainfall and pushed El Niño along, the atmosphere remained relatively dry. McPhaden offers several educated guesses as to why the atmosphere and ocean failed to connect.
One is that things simply got going too early in the year. Climate conditions favor warm water growth — as measured by sea surface temperature (SST) — later in the year, suggesting that normal climate conditions effectively nipped the nascent El Niño in the bud. Another compelling reason offered by McPhaden concerns the Indian Ocean Dipole (IOD), another periodic climatic pattern often called the “Indian El Niño.” McPhaden speculates warm waters produced by the IOD could have helped “anchor” convection in the eastern Pacific, thereby holding back convection in the central Pacific.
Other educated guesses put forth by McPhaden include larger background climate factors, such as the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation, both of which might have played a factor in El Niño’s initial no show.
What is clear is the behavior of the 2014-2015 El Niño caught researchers by surprise, and McPhaden ends his paper suggesting there’s a lot of fertile research ground in figuring out why predictions were so far off the mark.