Written off last month as all but a goner, La Niña — El Niño's cool opposite — now seems to be stirring again in the Pacific, albeit weakly.
The two images in this animation show the evolution of sea surface temperatures in the equatorial Pacific Ocean. The first image shows a spear of abnormally warm water along the equator on Oct. 12, 2015 indicative of El Niño conditions. The second image, from exactly one year later, shows the dramatic cooling of a possible La Niña. (Source: earth.nullschool.net) A month ago, forecasters were writing the obituary for La Niña. Still in gestation at the time, it looked like it was going to be stillborn. Not any longer. The latest forecast from the Climate Prediction Center pegs the odds of La Niña coming to term at 70 percent during fall in the Northern Hemisphere.
Source: Climate.gov When a La Niña does develop, it has impacts on temperature and precipitation around the world. For a summary of what typically happens, click on the graphic at right. And for more detail on typical U.S. impacts, check out this explainer on Climate.gov. But even if this baby does come to term, in all likelihood it will have a low Apgar score (sorry! my wife is a nurse-midwife...). Or as the Climate Prediction Center puts it:
. . . the forecaster consensus now favors the formation of a weak La Niña in the near term, becoming less confident that La Niña will persist through the winter.
In fact, the latest forecast says there is just a 55 percent chance that La Niña will persist into the winter, assuming it fully forms at all. So the bottom line for now is that it's unclear how much of an impact this La Niña might have. That said, 70 percent is pretty good odds. What happened over the past month to change the forecast? In short, the ocean started talking to the atmosphere. That takes a bit of explanation... When a La Niña develops, surface waters in the central and eastern tropical Pacific cool to below normal. That, in turn, causes air above the cool waters to sink, suppressing storminess. Meanwhile, waters on the western side of the Pacific, near Indonesia, warm up. This enhances updrafts of air, causing increased thunderstorminess. It all looks like this:
An atmospheric loop known as the Walker Circulation is illustrated here, overlaid on a map of average sea surface temperature anomalies. Cool waters prevail in the central and eastern Pacific Ocean, with sinking air that suppresses storminess above it. Meanwhile the western Pacific is warm, enhancing the rising branch of the circulation near Indonesia and leading to greater storminess. (Source: NOAA Climate.gov drawing by Fiona Martin. ) As the diagram above shows, the atmospheric circulation pattern includes strong winds blowing from east to west across the equatorial Pacific. These trade winds are crucial, as explained by Emily Becker at the awesome ENSO Blog:
This atmospheric circulation works to enhance the cooler ocean surface, both by blowing across the surface just like you’d cool the surface of your coffee, and also by causing cold water from the deep ocean to rise up. Have you ever blown on the surface of a cup of coffee that has cream in it, but hasn’t been stirred up? You’ll see the cream rise up to the surface, brought there by the circulation you’ve created.
But here's the thing: Back in early September, even as cool water was spreading along the equator in the eastern and central Pacific, the Walker Circulation action just wouldn't rev up. But starting late in the month, that changed. The east-to-west trade winds began cranking up, less rain started falling in the central Pacific, while storminess became enhanced over by Indonesia. And that's why forecasters are more bullish on La Niña. At least for now. How unusual is it for La Niña's prospects to grow, fade, and then rise again? Anthony Barnston, chief forecaster at Columbia University's International Research Institute for Climate and Society, answered that question over at the ENSO blog:
The path from strong El Niño to a La Niña of some given strength often does not occur in a straight line, but rather with some bumps in the road caused by disruptions in the formation of good ocean-atmosphere coupling, by intra-seasonal activity like the Madden Julian Oscillation, or others. So it is not unusual, particularly if we look at 1-month data.
We do know one thing for sure: Weather happens!