This wave, called a Kelvin wave, took four years to build. It has just started to breach the surface. It is so large that it has substantially raised the height of the sea surface for thousands of miles along the equator in the central Pacific ocean. As it pushes towards the west coast it will impact global ocean currents and weather.
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.
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.