The 'shark' will soon gobble up La Niña's cool surface waters. What might this mean for the climate later this year?
A cross section of the equatorial Pacific Ocean showing how water temperature departed from average during the five-day period centered on 3 April 2018. The vertical axis shows depth below the surface. The horizontal axis is longitude, from the western to eastern tropical Pacific. (Source: ENSO Blog/Climate.gov from CPC data) It's not every day that you see an animated graphic like the one above hosted on the website of an ordinarily staid U.S. government agency. And yes, that is indeed an illustration comparing a complex Earth system phenomenon to, well, a shark. The comparison comes from the fabulous folks at the ENSO Blog, published under the aegis of the National Oceanic and Atmospheric Administration (but technically not an official communication from NOAA). The ENSO bloggers focus with unusual clarity — and good humor — on El Niño, La Niña, and their impacts. So, back to the shark. It's actually a gargantuan blob of warm water as wide as the Pacific. And it is rising relentlessly from the depths along the Equator. Here's why you should care: That shark may actually influence the weather you will experience a few months down the road, not to mention just how much warmer than average our planet will be for the remainder of the year, and moving into 2019 as well. The graphic represents a cross section of the Pacific Ocean along the equator. It shows how water temperatures departed from average in late March and early April, from the surface down to 300 meters (nearly a thousand feet), and from one side of the Pacific to the other. Those blue colors you can see near the surface, and particularly concentrated in the eastern Pacific, are a hallmark of La Niña. In fact, we've officially been in the midst of a cooling La Niña episode since November of last year. This climatic phenomenon is characterized by a number of things, most especially that cooling of the surface waters along the equator in a large portion of the Pacific Ocean. This, in turn, is associated with influences on rainfall patterns and, much farther afield, with a shift in the mid-latitude jet stream across North America. As described by a post at the ENSO blog back in September:
During La Niña, the Pacific jet stream often meanders high into the North Pacific and and is less reliable across the southern tier of the United States. Southern and interior Alaska and the Pacific Northwest tend to be cooler and wetter than average, and the southern tier of U.S. states—from California to the Carolinas—tends to be warmer and drier than average. Farther north, the Ohio and Upper Mississippi River Valleys may be wetter than usual. During El Niño, these deviations from the average are approximately (but not exactly) reversed.
The climate system is a very complex beast, so things don't always turn out exactly as the averages suggest. This winter was no exception. To offer one conspicuous example, interior Alaska has been much warmer than average lately. Be that as it may, La Niña unquestionably is one of the major climatic influences on the weather we experience. And now, it is fading. Neutral conditions are forecast to arrive within the next couple of months. If the forecast turns out to be accurate, those conditions will be heralded by temperatures in the central Pacific Ocean returning to near the long-term average. Cue the gargantuan shark. Last summer, the subsurface waters of the Pacific along the equator were running cooler-than-average down to about 300 meters. Thank you, La Niña. But in November, they began to warm up. And by last March, those waters were, on average, the warmest they'd been since February 2016. That was during the last warming El Niño episode — which was a real humdinger. And this warmer-than-average blob — which just happens to be sort-of-kind-of in the shape of a shark — has been slowly moving to the east under the surface. Just so you know, scientists call this a "downwelling Kelvin wave." (I don't know about you, but I think "shark" seems a lot more compelling...) In the next few months, forecasters predict that the shark will surfaces, and when it does, "it’ll swallow the remaining cooler surface waters," writes NOAA's Emily Becker at the ENSO Blog. And that will mean: La Niña, RIP. This could help nudge up global temperatures, which in recent months have actually been running just off the record warming pace of the past two years.
Forecasting what will happen next is difficult at this time of year. Keep that caveat in mind when I run this forecast by you: There is a 45 percent chance of a warming El Niño episode taking hold during October through December. That's compared to a 35 percent of it happening randomly, according to Becker. Like La Niña, El Niño can have a really big impact on weather in far-flung parts of the world. It also tends to boost global average temperatures, which are already rising over the long run thanks to our emissions of carbon dioxide and other greenhouse gases. As always, stay tuned!