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El Niño has been showing signs of weakening since November, but this will apparently not diminish its impact on Canada's winter weather. Here's why.
OUT OF THIS WORLD | Earth, Space And The Stuff In Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

El Niño peaks, but hangs in as major player for Winter 2016

Scott Sutherland
Meteorologist/Science Writer

Thursday, December 31, 2015, 3:55 PM - El Niño has been showing signs of weakening since November, but this will apparently not diminish its impact on Canada's winter weather. Here's why.

Anyone keeping a close eye on the behaviour of this year's intense El Niño, and especially how it has been developing in comparison to the 1997-1998 "super" El Niño, very likely took notice of an important trend over the past 90 days.

Specifically, that temperatures in the central equatorial Pacific Ocean - what forecasters call the Niño 3.4 region - were beginning to drop slightly after reaching a peak in mid-November.

As this Niño 3.4 region is the one forecasters use to gauge the strength of an El Niño or La Niña event, especially in comparison to other events, seeing temperatures begin to fall there is a definite sign that this El Niño is starting to weaken, and right on schedule.

ENSO forecast modelling "plumes" from Mid-May to Mid-Dec 2015. Credit: Earth Institute/Columbia University/NOAA Climate Prediction Center

The animation above tracks the mid-month El Niño observations and forecasts, from May to December, keeping the three-month Oceanic Niño Index (ONI) periods sync'd to preserve the progression of the forecasts.

FYI: NOAA's Oceanic Niño Index is recorded as a three-month "running" average, written as DJF (December-January-February), JFM (January-February-March), FMA (February-March-April), etc, up to OND (October-November-December) and NDJ (November-December-January) to complete a full year's worth of records.

As this shows, the forecasts began to pick up on the trend that this event would reach its peak sometime in November or December, level off through the remainder of winter and then weaken through spring and early summer. Since these three-month ONI values capture more data than a weekly average, the mid-November peak in the weekly data means that we've yet to see the highest ONI value this year. Most likely it will be the OND (October-November-December) value computed in early January, or possibly the NDJ (November-December-January) value, depending on how things develop over the next few weeks.

Weakening, but not out of the game yet

Temperatures may have peaked in the central Pacific, and we may be seeing the signals of a weakening pattern, but this strong El Niño is still setting up to match or even best the "super El Niño" from 1997-98.

The three-month average temperature in the Nino 3.4 region was already up to +2.0oC as of the last ONI value, and they will likely reach at least +2.3oC (or possibly significantly higher, given the forecaster consensus) once all the values from the last three months are tallied and combined (for reference, the super El Niño reached a value of +2.3oC in the last two periods of 1997, even after it reached a peak weekly value in late November).

However, as the NASA animation below shows, this isn't just about the intensity, but also the extent of the current El Niño.

The animation compares sea surface height anomalies - how much higher or lower the sea surface is than normal - measured in 2015 by the Jason-2 satellite with those from 1997 as seen by the Topex/Poseidon satellite. Since El Niño represents a shift of higher sea surface waters from the western Pacific to the central and eastern Pacific, this is one way to measure the intensity and extent of an El Niño event from space.

At this time of year, as the 1997-98 pattern reached its peak, the region of higher sea surface (and thus higher temperatures) was mostly concentrated in the eastern Pacific, closer to the coast of South America.

The current El Niño pattern, on the other hand - even as it breaks records for weekly averages and is setting up to rival the overall intensity of the pattern 18 years ago - is spread out over a much wider area of the ocean.

Credit: NASA

Therefore, even with the temperatures in the Niño 3.4 region currently dropping, the overall amount of water that has been shifted eastward and the amount of water that has been heated to these temperatures guarantees that we'll still be feeling the effects of this pattern for some time.

No time to relax?

While forecasters are projecting a return to "ENSO-neutral" conditions - neither El Niño or La Niña - by late spring or early summer, that may not be the end of strange weather effects coming out of the equatorial Pacific.

"Looking ahead to summer, we might not be celebrating the demise of this El Niño," NASA JPL climatatologist Bill Patzert said in a statement on Tuesday. "It could be followed by a La Niña, which could bring roughly opposite effects to the world’s weather."

In 1998, after the super El Niño had subsided, the pattern went neutral in April and May, and then dipped directly into a La Niña pattern right after, where stronger winds over the equatorial Pacific push the surface waters towards the west with even more vigor than usual, causing cooler than normal sea surface temperatures in the central and eastern parts of the Pacific. While it wasn't the strongest La Niña pattern on record, it persisted for over two and a half years.

There's no telling, right now, exactly what the Pacific Ocean is going to do once this El Niño finally settles down. The size of this pattern means that it will certainly behave differently than the 1997-98 pattern did, and it remains to be seen how the excess heat in the world's oceans - which are much warmer in 2015 than they were in 1997 or 1998 - will factor into all of this.

Sources: Golden Gate Weather | NOAA | Columbia UniversityNASA

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