Stronger than normal trade winds drove water westward in the equatorial Pacific ocean, building a large deep pool of exceptionally warm water in the western Pacific ocean. The height of the sea surface is normally about 0.5m (about one and a half feet) higher in the western Pacific than the eastern Pacific at the equator because of the warm pool. This is part of a normal cycle in the ocean that, according to climate models, has been amplified by warming by greenhouse gases produced by human activities. Four years is unusually long in this normal cycle, allowing more time to develop an exceptionally large, deep pool of warm water. Sea level was about 4 inches higher than normal before it began moving east.
The speeding up of the trade winds over the past 15 years has deepened the warm pool leading to the development of stronger and stronger November typhoons in the Philippines. Typhoon Haiyan, which devastated the Philippines this November, was the strongest typhoon in Philippine history and in contention for the strongest recorded on earth. Its explosive development was enabled by the deep warm pool.
Increasingly negative numbers in the lower panel indicate strengthening trade winds.
This January very intense winter storms, fueled by outbreaks of Siberian air interacting with warmer than normal waters in the western Pacific, developed large westerly wind fields that reached deep into the tropics. These westerly winds unleashed the warm pool and it began moving east with the winds. When the west winds stopped, the sea surface height difference between the eastern and western Pacific continued to drive it east.
As the Kelvin wave and the warm pool moves east it warms the air over the central Pacific lowering the air pressure near Tahiti. At the same time, the water near Darwin Australia may cool a little as the warm pool moves away from its normal center near Indonesia. The air pressure rises are Darwin. The difference in air pressure, normalized for seasonal effects, between Tahiti and Darwin, called the Southern Oscillation Index has just turned significantly negative. If this trend continues, it will be the classic sign of a developing El Nino event.
Global models are now in agreement that an El Nino is beginning. The spring months are the hardest months to make El Nino forecasts, but the size and strength of the Kelvin wave is so great this time that the forecast is almost surely correct. This Kelvin wave is bigger and stronger than the one that led to the "super El Nino" of 1997 - 1998.
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