The maps above show sea surface temperature anomalies in the Pacific from winter and fall of 2015. In the case of the oceans, satellite radiometers-such as the Advanced Very High Resolution Radiometer ( AVHRR) on NOAA weather satellites and the Moderate Resolution Imaging Spectroradiometer ( MODIS) on NASA's Terra and Aqua satellites-detect the strength of infrared and microwave emissions from the top few millimeters of the water. Sea surface temperatures are measured from space by radiometers, which detect the electromagnetic energy (mostly light and heat) emitted by objects and surfaces on Earth. Since the late 1970s, satellites have provided a global view of ocean surface temperatures, filling in the gaps between those singular points where floating measurements can be made. (The western Pacific grows cooler than normal.) By March 2016, cooler water begins moving east, sparking a mild La Niña in the eastern Pacific late in 2016, while the western Pacific begins to warm again.įor hundreds of years, the temperature near the water surface has been measured by instruments on ships, moorings and, more recently, drifters. Note the warm water in the depths starting to move from west to east after March 2015 and peaking near the end of 2015. It shows temperature anomalies that is, how much the temperatures at the surface and in the depths ranged above or below the long-term averages. The visualization above shows a cross-section of the Pacific Ocean from January 2015 through December 2016. These in situ instruments (more than 3,000 of them) record temperatures and other traits in the top 300 meters of the global ocean. This mass, referred to as the 'western Pacific warm pool,' extends down to about 200 meters in depth, a phenomenon that can be observed by moored or floating instruments in the ocean: satellite-tracked drifting buoys, moorings, gliders, and Argo floats that cycle from the ocean surface to great depths. While easterly winds tend to be dry and steady, Pacific westerlies tend to come in bursts of warmer, moister air.īut as an El Niño pattern develops and trade winds weaken, gravity causes the warm water to move east. The typically strong high-pressure systems of the eastern Pacific weaken, thus changing the balance of atmospheric pressure across the eastern, central, and western Pacific. The circulation of the air above the tropical Pacific Ocean responds to this tremendous redistribution of ocean heat. It also reduces the upwelling of cooler, nutrient-rich waters from the deep-shutting down or reversing ocean currents along the equator and along the west coast of South and Central America.

This allows great masses of warm water to slosh from the western Pacific toward the Americas. Easterly trade winds (which blow from the Americas toward Asia) falter and can even turn around into westerlies. That change is intimately tied to the atmosphere and to the winds blowing over the vast Pacific. During an El Niño event, the surface waters in the central and eastern Pacific Ocean become significantly warmer than usual.

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