A NASA study suggests changing winds and currents in the Indian Ocean during the 1990s contributed to the observed warming of the ocean during that period. The findings, published in a recent issue of Geophysical Research Letters, have potential implications for long-term regional climate variability.

"Establishing this correlation provides an important missing piece to the global ocean-warming puzzle and provides vital information for regional governments and climate modelers," said Dr. Tong Lee, study author and researcher at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "These findings from satellite data also advance space exploration by increasing understanding of how complex planetary system elements, such as winds and currents, in our home planet interact to drive climate change. Such technologies, which have been demonstrated to be critical in understanding Earth's climate system, may someday prove useful in studying climate systems on other planets."

Lee's findings are based on sea level measurements from NASA's Topex/Poseidon oceanographic satellite, sea-surface temperature data from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer satellite, and wind data from the European Space Agency's European Remote Sensing satellites. Collected between 1992 and 2000, the combined data reveal weakening of southeasterly trade winds over the South Indian Ocean caused a major circulation of this ocean to wane by nearly 70 percent of its average strength.

The atmosphere heats the upper Indian Ocean. The circulation of this ocean counteracts the atmospheric heating by exporting warm surface water and importing colder subsurface water.

The slowdown of this circulation tends to prevent warm surface water from exiting and colder subsurface water from entering the upper Indian Ocean, raising its average temperature. During this period, the average sea-surface temperature of the Indian Ocean increased by approximately 0.25 Celsius (0.45 Fahrenheit).

"This is a very important and intriguing element of climate observations, suggesting convincingly that a major piece of the world ocean has significantly changed its circulation during the last decade," said Professor Jochem Marotzke, director of the Max Planck Institute for Meteorology in Germany. "While it is too early to say whether the underlying cause is natural variability or human-induced global warming, this result poses an interesting challenge to global climate modelers all over the world," he added.

Multi-decadal warming of the Indian Ocean in the past has affected the North Atlantic climate and was blamed for a devastating drought along the southern edge of the Sahara Desert in the 1970s and '80s. Understanding the cause of this warming and predicting its future evolution are major challenges to the climate community, as the ocean's warming is tied into a much larger global cycle of events. This research suggests the Indian Ocean is subject to the same type of long-term ocean-circulation oscillations that drive weather and climate patterns in the Pacific and Atlantic Oceans.

"The waning wind and ocean currents of the Indian Ocean might be a manifestation of decadal and longer climate variability. This could have significant effects on the ocean's ability to absorb atmospheric carbon dioxide and on the region's marine food web," Lee said.

Like vegetation, the ocean is a natural carbon dioxide "sink" that absorbs variable amounts of carbon dioxide from the atmosphere, depending on winds, ocean currents and temperatures. The weakened wind and ocean currents, along with rising ocean temperatures, could hamper the Indian Ocean's ability to absorb carbon dioxide. Ocean phytoplankton, the base of the marine food web, relies on the nourishment brought up by cooler, nutrient-rich subsurface water to survive and reproduce. The slowed cycling of warm and cold water could also bring fewer nutrients from the depths of the ocean to the surface, resulting in a decrease in the region's biological productivity.