an ocean circulation system that brings cool water from ocean depths to the surface has been slowing down since the mid-1970s, causing a decrease of carbon dioxide released into the atmosphere but an increase in sea surface temperatures along the Equator in the Pacific Ocean. This was discovered by scientists from the Washington dc-based National Oceanic and Atmospheric Administration (noaa). "Looking at records of the past 50 years, we found that the ocean currents flowing in the north-south direction have been slowing down in the tropical Pacific Ocean since the mid-1970s," said Michael McPhaden, a research scientist at noaa (www.noaanews.noaa.org).
Cool water hundreds of feet below the surface typically flows from the mid-latitudes to the tropics. These waters are eventually brought up to the surface along the equator. When the circulation slows down, the supply of cool water for equatorial upwelling decreases. During the study, a 25 per cent of this decrease was found, causing the sea surface temperatures in a band about 965 kms on either side of the Equator to rise considerably. Along with this, the amount of carbon dioxide released by the equatorial Pacific Ocean also decreased.
At present, the equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. These circulation changes are linked to the shifts in the climate of western North America, and can affect Pacific fisheries. They may be part of the naturally occurring ocean and atmosphere phenomenon called the Pacific Decadal Oscillation, which has a roughly 50-year cycle. But they also could be influenced by greenhouse gases.
The noaa researchers studied historical ocean data sets and wind records going back a half century. They focused their attention on this region as an outgrowth of their interest in the El Nio/La Nia cycle, which, over the past 25 years, has favoured stronger El Nios, and more frequent El Nios than La Nias. McPhaden notes that the system is driven by the trade winds, which have weakened since the 1970s. The trade winds have weakened because the sea surface temperatures have risen. "It's the same chicken-and-egg riddle we encounter with El Nio," he said.