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November 14, 2018 – Author: Emily Becker – The surface of the tropical Pacific East-Central is near 1 ° C above the long-term average! We have not yet seen the atmospheric reaction that characterizes El Niño conditions. Forecasters estimate that about 80% of the chances El Niño will form soon and continue in the spring.
Where do I start
Let's run the numbers! The four ENSO monitoring regions in the tropical Pacific were warmer than average in October. The Niño3.4 region was 0.8 ° C warmer than average using the ERSSTv5 dataset with the long-term trend suppressed, comfortably above the El Niño threshold of 0.5 ° C above the average.
However, to qualify as El Niño conditions, we must have evidence that the atmosphere reacts to the warm temperatures of the sea surface. In short, the average atmospheric regime over the tropical Pacific, Walker's circulation, is powered by warm air rising in the very warm waters of the far western Pacific and Indonesia. This air moves to the east at high levels in the atmosphere, descends into the eastern Pacific and completes the circulation by moving west near the surface, thus forming the trade winds.
During El Niño, warmer than average waters in the eastern / central Pacific lead to higher than normal air, clouds and precipitation in this region. This weakens Walker's circulation as a whole, which means that upper and lower level winds are slower than normal. For a more in-depth treatment of Walker's circulation and his role in ENSO, check out Tom's post on the subject.
We monitor the atmosphere of the tropical Pacific in different ways. The Southern Oscillation Index measures the difference in sea level pressure between Darwin, Australia and Tahiti. When this is negative, it indicates that the pressure is above average in Darwin and below average in Tahiti. (A lower pressure accompanies the rise of the air than the average.) A negative Southern Oscillation index indicates that the El Niño-y atmosphere … for the moment, it is positive, 0.4.
In the same vein, the Equatorial Southern Oscillation Index measures the pressure differences between the western equatorial Pacific and the eastern easterly. The EQSOI was slightly negative in October (-0.3), unconvincing evidence of an atmospheric reaction.
Another way to monitor the atmospheric circulation in the Pacific is to look at the amount of clouds or rainfall in the equatorial region. If Walker's circulation has weakened, we expect more clouds than average in the central Pacific and fewer clouds over Indonesia. However, cloud cover in the central Pacific was slightly below the average of last month. The amount of cloud / precipitation is also important as it results in teleconnections that impact the North American Pacific region. There is currently no indication that we are seeing these teleconnections even though ocean conditions are favorable.
Light my fire
All we have just said is … The El Niño conditions have not started yet. Most computer models predict that sea surface temperatures will remain above average, with the anomaly (deviation from the long-term average) remaining above the El Niño threshold of + 0 , 5 ° C until spring.
However, confidence in the model's predictions is related to the substantial amount of water warmer than the average below the Pacific surface. The average October was the fifth highest since 1979. These waters will provide a warmer water source to the surface for the next few months.
So we have the first two requirements of our El Niño decision tree. The atmosphere is expected to react to warmer surface temperatures … When that happens, the El Niño conditions will be there. In 2014, the ocean was above the El Niño threshold for several months, but the atmosphere was slow to react. There are some key differences this time around (the sea surface temperatures are a little warmer and the temperatures below the surface are much warmer), so we do not predict an identical result. In addition, one thing you can bet on is that nature will never behave exactly the same way twice.
For all your latest ENSO news, watch this space!
ENSO Blog
A blog on El Niño and La Niña monitoring and forecasting and their impacts.
Warning:
The ENSO blog is written, edited and moderated by Michelle L 'Heureux (NOAA Climate Prediction Center), Emily Becker (CPC Contractor), Nat Johnson (NOAA Geophysical Fluid Geophysics Laboratory), and Tom DiLiberto and Rebecca Lindsey (contractors of the NOAA Climate Program), with periodic guest contributors.
The ideas and explanations found in these articles should be attributed to the ENSO blog team and not to NOAA (the agency) itself. These are blog posts, not communications from official agencies; if you cite these articles or the comments section, you must attribute the content cited to the blogger or commentator, and not to NOAA, CPC or Climate.gov.
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