Karen Braun is a Reuters market analyst
CHICAGO, April 26 (Reuters) – The Pacific Ocean has been stirring with activity over the last two weeks in the much-anticipated switch from El Niño to La Niña, the cold phase of tropical Pacific surface temperatures.
Commodity markets have been closely watching this transition because its timing might help determine whether the United States will have a hot and dry summer or whether the Atlantic hurricane season will be more active than normal, among several other global weather scenarios.
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Since the warm-phase El Niño hit its peak last November, sea surface temperatures in the tropical Pacific Ocean have been steadily declining, but the cool-off over the last two weeks has been anything but steady.
A cold pool that is characteristic of La Niña has been lingering beneath the surface of the tropical Pacific Ocean for quite some time now. Over the last two weeks, it finally replaced warm surface waters in the Eastern Pacific, aided by strong La Niña-supporting trade winds (http://reut.rs/1SnQWks).
As a result, the temperature trend in the Niño 1+2 region in the Pacific Ocean, off the coast of Peru, has changed drastically over the past two weeks. Between April 6 and April 13, the 1+2 sea surface temperature dropped 1.5 degrees Celsius, and an additional 0.9 degree was shaved off in the following week. The temperature anomaly in the 1+2 region now sits at minus 0.6 degree Celsius, which is effectively La Niña territory (http://reut.rs/1SnQBOU).
To highlight the significance of the 1.5-degree drop, there were only three of 1,373 weeks since weekly records began during which the 1+2 region lost a greater amount of heat.
When considering the 2.4-degree decline over the last two weeks, the Niño 1+2 region has never before lost so much heat in that amount of time, smashing the old record of 2 degrees.
Sea surface temperatures in the Niño 3 region, directly to the west of 1+2, underwent their first big downturn last week, losing 0.6 degree Celsius. In the 3 region, only six weeks since 1990 have recorded larger heat losses, and another nine tallied cool-offs that were identical to last week’s.
It is important to remember that sea surface temperatures in the Niño 3.4 region are most closely linked with resulting global climate patterns, so the “mini La Niña” in the Eastern Pacific would be unlikely to have huge consequences on its own. But it may be a sign of things to come.
The rapid cooling of the eastern Niño regions not only proves just how cold the subsurface cold pool really is but that it also means business. As soon as the trade winds return to normal or even increase, the Nino 3.4 and 4 regions can respond and cool off almost immediately.
The relatively benign trade wind forecast for the next 10 days implies that equally rapid cooling for the western Niño region is not expected in the immediate future, and that the mini La Niña could even retreat somewhat.
But it also means that El Niño will not escalate and that La Niña conditions could still easily be in place by the start of June.
