June 25, 2019
by Kevin Krajick, Columbia University
The tropical Pacific Ocean (Australia and South America in gray, left and right). Top map shows what climate models say sea-surface temperatures should be doing in response to rising greenhouse gases, including pronounced warming of waters along the equator. Bottom map shows what the waters are actually doing; the equatorial waters are remaining relatively cool. Credit: Seager et al., Nature Climate Change 2019
State-of-the-art climate models predict that as a result of human-induced climate change, the surface of the Pacific Ocean should be warming—some parts more, some less, but all warming nonetheless. Indeed, most regions are acting as expected, with one key exception: what scientists call the equatorial cold tongue. This is a strip of relatively cool water stretching along the equator from Peru into the western Pacific, across quarter of the earth’s circumference. It is produced by equatorial trade winds that blow from east to west, piling up warm surface water in the west Pacific, and also pushing surface water away from the equator itself. This makes way for colder waters to well up from the depths, creating the cold tongue.
Climate models of global warming—computerized simulations of what various parts of the earth are expected to do in reaction to rising greenhouse gases—say that the equatorial cold tongue, along with other regions, should have started warming decades ago, and should still be warming now. But the cold tongue has remained stubbornly cold.
This troubles many scientists, because the cold tongue plays a key role in global climate. For example, it affects the El Niño-Southern Oscillation, a natural cyclic strengthening and weakening of the trade winds that causes cooling and warming of the eastern Pacific surface every two to seven years. ENSO is the world’s master weather maker; depending on which part of the cycle it is in, its echoes in the atmosphere may bring heavy rains or drought across much of the Americas, east Asia and east Africa. Whether the cold tongue warms will likely affect weather across huge regions. Resulting shifts could affect world food supplies and outbreaks of dangerous weather. But our predictions of those shifts rest on climate models.