July 2018 ENSO Update: Dog days
Author: Emily Becker
The chance that El NiƱo conditions will be in place across the tropical Pacific by the fall is about 65%, and close to 70% by the winter, continuing the El NiƱo Watch from last month. Here in the U.S., with sultry weather from coast to coast, itās hard to think about winter right nowā¦but thatās our job! After a trip through the tropical Pacific, weāll get into what El NiƱo could mean for global weather and climate this winter.
A splash in the pool
The tropical Pacific is still well within neutral limits. The temperature of the ocean surface in the NiƱo3.4 region has edged above average, with June 2018 coming in about 0.11 degrees Celsius above the long-term average, based on our best-quality data set. Most climate models predict that the NiƱo3.4 region temperatures will reach the El NiƱo threshold (0.5Ā°C above the long-term average) by the fall.
Much of the rest of the equatorial Pacific is warmer than average now, as well, with only the NiƱo1+2 region (the farthest east region, next to the coast of South America) still cooler than average.
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. Itās likely the surface will continue to warm over the next few months, helping to support the forecast for El NiƱoās 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 winterās La NiƱa. 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.
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 NiƱo. Donāt forgetāthe āSOā part of ENSO stands for āSouthern Oscillation.ā
Weāll keep an eye on these indexes and the other indicators of the atmospheric component of ENSO while weāre sweating it out over the summer and into the fall. El NiƱo is a coupled system, meaning it requires both the NiƱo3.4 region to be more than 0.5Ā°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 NiƱo conditions will have arrived.
Dreaming of winter
Soā¦ who cares? Why do we even spend so much time and energy on El NiƱo? Because, to quote my blog-brother Tom, itās a seasonal forecasterās best friend. When the normal location and intensity of thunderstorms and winds in the Pacific basin are disturbed, itās like a cannonballer sending waves through a pool: the effects are felt around the globe. Because El NiƱo and La NiƱa are generally predictable months in advance, forecasting an El NiƱo 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 donāt see every expected effect every time El NiƱo 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. (Hereās the corresponding image for temperature.)
Also, El NiƱo is never the only player on the field (check out the index page, under āPosts about things other than ENSOā). That said, a forecast for an El NiƱo winter can give us an early glimpse of our winter weather, even while weāre sweltering under summerās heat domes.