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One of the major drivers in the severity of any winter are the sea surface temperatures. That makes sense when you consider 70 percent of the earth is composed of water. A known body of water with a major influence is the tropical Pacific, where the state of the southern oscillation comes into play, especially during winter. The El Niño (Southern Oscillation ENSO), is a yearly recurring climate pattern that centers on changes in the sea surface temperatures of the central and eastern tropical Pacific Ocean. On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal.

This oscillating warming and cooling, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.

  • El Niño:  A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.  Over Indonesia, rainfall tends to become reduced while rainfall increases over the central and eastern tropical Pacific Ocean.  The low-level surface winds, which normally blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start blowing the other direction (from west to east or “westerly winds”).  In general, the warmer the ocean temperature anomalies, the stronger the El Niño (and vice-versa).

  • La Niña:  A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.  Over Indonesia, rainfall tends to increase while rainfall decreases over the central and eastern tropical Pacific Ocean.  The normal easterly winds along the equator become even stronger.  In general, the cooler the ocean temperature anomalies, the stronger the La Niña (and vice-versa).

  • Neutral:  Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average.  However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).

The ENSO state is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.  We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate.

So, by now, you might have noticed that while “ENSO” is a nice catchall acronym for all three states, that acronym doesn’t actually have the word El Niño La Niña in it.  Why is that?  Well, that is a fluke of history.  Before El Niño or La Niña was even recognized, South American fisherman noticed the warm-up of coastal waters occurred every so often around Christmas, impacting their catch. They referred to the warming as “El Niño,” (niño being Spanish for a boy child) in connection with the Christmas holiday.

Sir Gilbert Walker discovered the “Southern Oscillation,” or large-scale changes in sea level pressure across Indonesia and the tropical Pacific.  However, he did not recognize that it was linked to changes in the Pacific Ocean or El Niño.  It wasn’t until the late 1960s that Jacob Bjerknes and others realized that the changes in the ocean and the atmosphere were connected, and the hybrid term “ENSO” was born.  It wasn’t until the 1980s or later that the terms La Niña and Neutral gained prominence.

The big news floating around the weather world Friday was that the El Niño, that's been bordering on "a super Niño", is weakening. The trades are also pushing it west, where it will likely be classified as a Modoki El Niño. In other words, its warmest waters are expected to be found in the central Pacific in a region called 3.4. Back in September, it was east based with the warmest waters off the South American coast.

Pressure gauges indicate El Niño is weak 

A tool that some forecasters use to determine the status of El Niño and La Niña events is the Southern Oscillation Index, or what is commonly referred to as the SOI. The index measures pressure differences in the southwest Pacific, and when the gauge turns substantially positive, a La Niña event is likely happening or on the way. The reverse is also true, and when figures are markedly negative, an El Niño event is likely in progress.

Recent values show a streak of positive figures that have impacted the SOI count. In fact, at times, during the last 30 and 90 days, the figure has weakened below what is typically considered an El Niño event.

El Niños are known to exist when figures are at -8 on the index scale, and La Niñas exist when the average figure is +8 or greater. There is a huge disconnect between the SOI and a similar index called the ONI. The latest SOI values were -7.56 for the last 90 days and -2.78 for the last 30 days. The latest daily contribution Friday was positive (+4.05) which, at a glance, argues against the fact there is even so much as a weak El Niño.

At the rate the SOI is progressing, this entire El Niño episode has the potential to be over and done by April 1st. This is a significant rise, and it's bound to have implications on the winter. The big unknown is how fast this El Niño ends and how far into the central Pacific the warmth migrates. The rapid decline we are seeing is somewhat unprecedented, and I think it has the potential to be a big winter wildcard going forward.

As we let the El Niño issue simmer the next week or two, we do have some weather concerns this weekend, centered on a clipper like disturbance that slides SE through central Iowa Saturday. That puts the area in the right quadrant of the system where lift is maximized. That means light to at times moderate rains will commence Saturday morning in the south and then progress NE into the rest of the region Saturday afternoon. The rains will end Saturday night, only to be followed by blustery and colder weather Sunday and especially Monday.

Here's what models are indicating for rainfall through Saturday night.



Temperatures will be just a touch too warm for snow, aside for a few wet flakes towards the end of the event in the north Saturday night. These are the projected snow totals off the GFS and EURO.



Behind the system, blustery and cold conditions are expected later Sunday and especially Monday, when highs will hold in the upper 20s to low 30s.

The cold is in and out in a hurry as temperatures quickly rebound by the middle of next week. Most of the period Wednesday through Christmas Eve Day will see highs in the range of 45 north to 50 south. That's mighty balmy by pre-Christmas standards. Here's the daily temperature departures for the next 10 days ending December 25th.

After Saturday's light rain, conditions appear dry the remainder of next week as we close in on Christmas weekend. For now, I still don't see any real cold air in the pattern until the first of the year. Happy shopping and roll weather...TS.



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