As Pacific Ocean Cools Rapidly, La Nina Threatens Early Return

Forecasts for an El Nino this winter have given way to the prospect of more La Nina-like conditions as sea surface temperatures in the central-eastern Pacific cool rapidly.

Surface temperatures in the critical area of the Pacific have fallen to 0.2 degrees Celsius below average, down from 0.7 degrees above average in the week centred on June 28. The rapid cooling has forced meteorologists to reassess the outlook for the northern hemisphere winter.

Until June, most forecasters were predicting a mild or moderate El Nino between December 2017 and February 2018.

But the rapid cooling of the sea’s surface in July and August now points to a shift towards more neutral conditions, or even La Nina developing.

The U.S. National Oceanic and Atmospheric Administration (NOAA) has cut the probability of El Nino between December and February from 44 percent in its May forecast to just 16 percent in August.

At the same time, it has doubled the probability of La Nina between December and February from 14 percent to 28 percent.

The latest runs of NOAA’s forecast models point to La Nina conditions developing by the end of 2017 and into early 2018.

Surface temperatures in the central-eastern Pacific area are monitored by meteorologists because they correlate with a broader set of oceanic and atmospheric changes known as the El Nino-Southern Oscillation (ENSO).

But the evolution of ENSO remains notoriously hard to predict. The warm phase forecast earlier this year proved to be very weak, failing to reach the threshold to qualify as a full El Nino, and then fizzled early.

Something similar could happen with the current predictions of La Nina. Even if La Nina conditions develop in the next few months, it remains hard to predict their intensity or duration.

Despite the rapid cooling of surface temperatures in the central-eastern Pacific, most of the other components of a La Nina episode are absent or only weakly present at the moment.

“Sea surface temperatures have cooled during the past several weeks, yet have remained within the neutral range,” the Australian Bureau of Meteorology reported at the end of August. “Other indicators of ENSO … are also at neutral levels.”

The most likely outcome this winter is for ENSO to remain in a fairly neutral or slightly cool phase.