El Nino In Decline

El Nino’s effect on global temperatures and weather systems has been dramatic in the past few months, achieving records for individual months and, with a significant contribution from the Pacific “Blob,” is bound to make 2015 globally the warmest year of the instrumental era.

Many were expecting the current El Nino to start in 2014. Certainly the signs were there. 2014 started out a lot like 1997 the last great El Nino event. There were some strong westerly wind bursts, big Kelvin waves traversing the Pacific as well as warming in Eastern Pacific. But it didn’t happen, no one knows why, perhaps the westerly wind bursts were too weak and they certainly ended abruptly, perhaps the extreme cold phase of the Pacific Decadal Oscillation, or the extreme cold in South East Tropical Pacific inhibited it. In the event, not all the necessary conditions were maintained for the long-awaited El Nino to commence in 2014.

As can be seen in Fig 1 (click on image to enlarge) the start of the current El Nino took place in May-June 2015.

Since then the warm water has been travelling westward across the Pacific. However, Fig 2 shows the Upper Ocean Heat Anomaly was increasing since February and then declining since December.

This decline is seen in the latest data from the El Nino 3.4 region, Fig 3 (the Nino 3.4 SST anomaly index is an indicator of central tropical Pacific El Nino conditions. It is calculated with SSTs in the box 170°W – 120°W, 5°S – 5°N) which shows the El Nino temperature threshold was reached around May and increased until mid-December.

Such a peak can be seen in other comparison regions, Fig 4. It marks the start of the end of the current El Nino.

Sea surface temperature across the equatorial Pacific basin have cooled roughly one-half a degree over the last four weeks. At the same time, a large pool of cold water beneath the surface in the western Pacific has been expanding eastward, nearly doubling in size over the past two months.

In general terms El Ninos and La Ninas are predictable. The first such prediction was made in 1986 by Mark Cane of the Lamont-Doherty Earth Observatory. Currently about 20 groups make such predictions using models that have demonstrated skill for predictions of up to 6-9 months.

The consensus of some 25 models is for a decline in sea surface temperature in the Pacific. The El Nino is expected to remain strong through the Northern Hemisphere in the winter with a transition to neutral conditions expected during late spring or early summer 2016.

Another indication that the current La Nina is waning comes from observations of trade winds. During the previous “super” El Ninos in 1982 and 1997, western trade winds were highly reversed but abruptly returned to normal within one month at the end of both of those years. Although these winds have not been as anomalously strong in 2015, the same rapid jump occurred last month.

The consensus among international climate models is that by August, sea surface temperature anomalies will be near zero, and are likely to keep declining heading towards La Nina conditions.

Exactly how long the El Nino conditions persist will determine how warm 2016 will be. If neutral conditions are reached in the Spring and La Nina by the final quarter then 2016 will be somewhat cooler than 2015 though perhaps a little warmer than 2014. With the advent of La Nina, though, 2017 could be much cooler than many recent years.

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