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El Nino and the coming winter
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The latest El Nino temperature number for the central Pacific Region is the warmest peak since the El Nino of 2002 at +1.7C and now places the 3.4 region in the strong category. I suspect this is the Kelvin Wave reflection we have been expecting and that for the first part of the winter the El Nino will be in the moderate category sliding down to the weak category by the end of winter maybe even near-neutral.
During a "typical" El Nino in the winter we would ascribe the following traits: a split flow jet stream pattern (66% of El Nino events), a stormier than normal West Coast, an active Sub-tropical Jet Stream California to the Mid-Atlantic and/or Texas to Florida, and a lot of mild air across the country. The tendency for 2-3 jet stream branches has already been seen the past 30 days or so.
The problem with these attributes of a "typical El Nino" is that there really is no such thing as a "typical" El Nino. One size does not fit all, they are not all the same so you can't just "cookie cutter" a winter forecast based on their being an El Nino advisory. It is also difficult to predict how strong an El Nino will be for the December through March period and how the water patterns will behave.
The El Ninos of 1982 and 1997 are the benchmarks for the past 50 years. But there are different ways each El Nino develops. Some come on the heels of another one, while others like the current one follow a La Nina. Some come on strong during the summer, while others form late in the year. Some are "West" based and some are "East" based depending on where the biggest SST anomaly is located in the central Pacific, others are basin wide. The upward motion in the tropics associated with where the warm sea surface temperatures are located will have different impacts on the jet stream and weather along with where cool pools are located.
Each nuance has a ramification for how the weather will play out across the hemisphere. And when talking about "an El Nino winter" we are ignoring the fact that other factors in the oceans, atmosphere, cryosphere, stratosphere, volcanic aerosols and solar cycle all affect how the winter will play out. Factors like stratospheric warmings, Kelvin waves MJO/SOI, EPO, the QBO, the AO/NAO PDO, Japanese Dipole, IOD, and AMO/Atlantic tripole. The level of these indices and their oscillations, phasing and timing all go into making a winter. So the El Nino is just one atmospheric teleconnection. The El Nino can dominate but it does not always do so. See the slideshow for the composite or average affects of the "typical" El Nino.
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