A sea-surface temperature rise of just five degrees in the eastern tropical Pacific can have global consequences.
Evaporation and convection increase over these warmer waters. Heavy rains that normally fall across the western tropical Pacific shift eastward, flooding the normally arid Peruvian and Ecuadorian coasts. In the western tropical Pacific, the normally reliable monsoon rains desert Indonesia and New Guinea. Fires burn the parched forests of eastern Australia. The effects of this warmer water even extend beyond the tropical Pacific. Floods strike California and the southeast United States. Southeastern Africa and northern Brazil suffer through droughts. On the brighter side, northwestern Canada and the northeastern United States enjoy mild winters.
These were the conditions that prevailed during the powerful El Niño of 1997–1998. Such climate disruptions, part of a larger ocean and atmospheric perturbation called El Niño-Southern Oscillation, or ENSO, take place to some degree every two to seven years.
Now imagine the tropical Pacific as a giant bathtub. In a normal year, warm water concentrates in the western end of the Pacific. In the east, a swath of chilly water, called the cold tongue, extends from the Peruvian coast west to the international dateline. Steady winds from the west, called trade winds, maintain this temperature gradient by pushing surface currents westward and then away from the equator. Cold water rises to the surface along the equator to take its place, feeding the cold tongue.
During El Niño, these east-west trade winds slacken then begin to reverse. Bursts of wind from the west become steadier, pushing the warmer water back to the east. Fabrice and his colleagues wanted to calculate the extent to which this eastward flow of warm water was responsible for the rise in the surface temperature of the eastern tropical Pacific.
First, however, Fabrice and his colleagues needed information on the surface currents themselves. Buoys can track currents directly, but there are too few to provide a complete picture. Fabrice therefore joined with Gary Lagerloef, also of Earth and Space Research, to develop models that calculate surface currents from satellite data. These data include sea-surface height (water flows “downhill”), and wind speed and direction. They then used information collected from buoys to verify these models. This project, called OSCAR (Ocean Surface Currents Analysis Real-time), now generates up-to-date information on surface currents over the tropical ocean and makes these data available to scientists and the public on a web site. Soon these maps will cover the entire ocean.
Fabrice and his colleagues now had maps of surface currents from OSCAR, information on air-sea heat flux that a colleague at the Woods Hole Oceanographic Institution calculated from satellite data, and rough estimates of the effects of the upwelling of cold water, which are notoriously difficult to quantify. They plugged the numbers into heat budget equations then compared the temperatures they calculated to the sea-surface temperature changes they observed. The comparisons weren’t perfect, but Fabrice is reasonably confident that their analysis describes what is happening in the real world. They show that during an El Niño, most of the warming of the eastern tropical Pacific is indeed due the shift in surface currents.