Response of the tropical Pacific to changes in extratropical clouds

A decrease in cloud cover over higher latitudes—a decrease in the extratropical albedo—especially over the Southern Ocean, can result in an extratropical and tropical warming with the intensity of the equatorial cold tongues in the Pacific and Atlantic basins decreasing. These results, obtained by means of a coupled ocean–atmosphere model of intermediate complexity that allow the prescription of atmospheric cloud cover, are relevant to future global warming, and also to conditions during the Pliocene some 3 million years ago. The mechanisms responsible for the response of the tropics to changes in the extra-tropics include atmospheric and oceanic connections. This tropical adjustment can be interpreted from the constraint of a balanced heat budget for the ocean: A change in the albedo of the Southern Hemisphere causes the ocean to lose less heat there, so that it has to gain less heat in the tropics. As a consequence the cold tongues are reduced, particularly in the eastern Pacific where a decrease in the zonal tilt of the equatorial thermocline significantly weakens the east-west sea surface temperature gradient. The total adjustment time scale of the equatorial Pacific to the extratropical perturbation is of the order of interdecadal to centennial time scales, and thus represents a new mechanism of rapid climate change.