El Niño: More than Just Pacific Sea Surface Temperature Anomalies

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

By Justin Grieser, Climate and Society 2014

As we enter the second half of 2014, climate forecasters continue to watch the Pacific for the development of an El Niño event. An El Niño Watch has been in place since March, but conditions still have yet to fully develop.

At the International Research Institute for Climate and Society’s (IRI) June climate briefing, chief forecaster Tony Barnston noted that although the Pacific SST pattern appears El Niño-like, the atmosphere isn’t quite cooperating in tipping the Pacific into a full-on El Niño.

Surface temperature anomalies in the Pacific, week ending July 5. Warmer colors indicate above-normal temperatures. (Source: IRI Map Room)

Surface temperature anomalies in the Pacific for the week ending July 5. Warmer colors indicate above-normal temperatures
(Source: IRI Map Room)

El Niño, is a pattern of natural climate variability characterized by warmer-than-average sea surface temperatures (SST) in the tropical Pacific. It occurs when the easterly trade winds along the equator stall or even reverse direction, warm waters of the western Pacific sloshing eastward toward South America. That shift can greatly alter seasonal temperature and precipitation patterns across the globe.

But there also has to be what Barnston describes as “atmospheric participation” to officially declare El Niño is here.

The main reason is that El Niño is a “coupled” phenomenon between the ocean and atmosphere. For an El Niño to develop, the atmosphere must produce westerly wind bursts near Indonesia to push the Pacific warm pool eastward. In recent weeks, these winds – typically seen during budding El Niño conditions – have stalled. The lack of a gradient, or contrast, between SST in the central and western Pacific means we also aren’t getting a contrasting atmospheric pressure pattern that would reinforce the westerly winds needed to push warm surface waters toward South America.

The latest map of weekly SST anomalies above shows warmer-than-average SSTs off the coast of South America. Yet in the central equatorial Pacific, where El Niño conditions are typically measured, SST anomalies are still at the borderline El Niño threshold of at least 0.5ºC above normal.

This might be seem like cause for alarm for those breathlessly waiting for El Niño, but a stalled atmospheric pattern during summer has happened before. At the June climate briefing, IRI researchers said 1986 saw a similar pattern, when westerly wind bursts temporarily slackened in the summer months. Later that year, El Niño arrived with a vengeance.

During an El Niño, easterly trade winds reverse direction, allowing warm water in the western Pacific to move eastwards, which leads to enhanced rainfall over South America. (NOAA)

During an El Niño, easterly trade winds reverse direction, allowing warm water in the western Pacific to move eastwards, which leads to enhanced rainfall over South America.
(Source: NOAA)

On the flip side, there’s also a slight chance the whole thing could fall apart similar to what happened in 2012. The IRI predicts only a 20-30% chance of this happening, however, and most climate models are still forecasting a weak to moderate El Niño by the fall.

Which way will 2014 go? While a strong El Niño event looks less likely, the atmosphere over the tropical Pacific is likely to eventually tip the scales to El Niño conditions. The Australian Bureau of Meteorology reports that easterly trade winds weakened in late June, and the Southern Oscillation Index – a measure of pressure differences between the western and central Pacific – has turned negative, a sign an El Niño may be getting back on track. For an El Niño to develop, these conditions will have to persist for several weeks.

So stay tuned for the latest details and forecast in the IRI’s next climate briefing on July 17.




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