July 2018 El Nino Update: Dog Days Are Here

The temperature of the water below the surface of the Pacific is elevated, too. The downwelling Kelvin wave that was initiated a few months ago has sloshed to the east across the Pacific, bringing warmer waters toward the surface. It’s likely the surface will continue to warm over the next few months, helping to support the forecast for El Niño’s development.

Fans & sprinklers

The atmosphere over the tropical Pacific is demonstrably neutral right now. Cloud cover and rain over the central Pacific are still reduced compared to average, a hold-over from this past winter’s La Niña. On the other hand, atmospheric conditions over Indonesia have returned to near-average. Both the Southern Oscillation Index and the Equatorial Southern Oscillation Index were near zero during June, reflecting neutral conditions. These indexes compare the surface air pressure in the central-eastern Pacific to the pressure in the western Pacific, a useful metric for the Walker circulation. [Summary of decision process in determining El Niño conditions. NOAA Climate drawing by Glen Becker and Fiona Martin.] If the pressure in the eastern Pacific is relatively lower (more rising air than average, resulting from warmer-than-average sea surface temps) and the pressure to the west is relatively higher (less rising air, due to cooler waters), it can be another indication the Walker circulation has weakened, representing the atmospheric component of El Niño. Don’t forget—the “SO” part of ENSO stands for “Southern Oscillation.” We’ll keep an eye on these indexes and the other indicators of the atmospheric component of ENSO while we’re sweating it out over the summer and into the fall. El Niño is a coupled system, meaning it requires both the Niño3.4 region to be more than 0.5°C above average and for the atmosphere to respond to those warmer-than-average waters. When both conditions are met, and expected to continue for several months, El Niño conditions will have arrived.

Dreaming of winter

So… who cares? Why do we even spend so much time and energy on El Niño? Because, to quote my blog-brother Tom, it’s a seasonal forecaster’s best friend. When the normal location and intensity of thunderstorms and winds in the Pacific basin are disturbed, it’s like a cannonballer sending waves through a pool: the effects are felt around the globe. Because El Niño and La Niña are generally predictable months in advance, forecasting an El Niño during the winter means we can often anticipate certain U.S. weather patterns during the winter. Of course, the atmosphere is a huge, chaotic system, and we don’t see every expected effect every time El Niño shows up. More than half of the events since 1950 have some kind of wet signal over California, for example, but it can be confined to the southern part of the state in some events and shifted northward during others. (Here’s the corresponding image for temperature.) Also, El Niño is never the only player on the field (check out the index page, under “Posts about things other than ENSO”). That said, a forecast for an El Niño winter can give us an early glimpse of our winter weather, even while we’re sweltering under summer’s heat domes. Edited for WeatherNation by Meteorologist Mace Michaels