Monsoons Of Arabian Sea Control Productivity And Carbon Export

December 17, 1996

San Francisco, Calif. -- Investigation of the oxygen and nutrient content of the Arabian Sea could help shed light on how monsoons influence ocean productivity and the carbon cycle, according to a Penn State oceanographer.

"The monsoons of the Arabian Sea create peculiar ocean currents in the area," says Dr. Raymond Najjar, assistant professor of meteorology.

In the summer, the monsoon blows from the southwest up the Arabian Sea towards Pakistan. Because of a combination of wind and the Earth's rotation, the summer monsoon pushes water away from the Somalian coast and the eastern edge of the Arabian peninsula. The water that leaves these coastal areas is replaced by water that wells up from depths and is very high in nutrients.

In the winter, the monsoon reverses and blows from the northeast.

"This monsoon does not have as much impact because it pushes water toward the Arabian Peninsula resulting in down welling, not up welling," says Najjar. "However, the strong winds cause turbulent mixing, which brings up some nutrients, but not on the scale of the summer monsoon."

Najjar is using data compiled by the National Oceanographic Data Center since 1900, to assess the average nutrient content of Arabian Sea surface waters on a monthly basis throughout the year.

"This annual cycle is distinct," Najjar told attendees today (Dec. 16) at the Fall meeting of the American Geophysical Society in San Francisco. "June through August there is a huge burst of nutrients in the area, especially off the Arabian coast. In January and February there is a lesser, but still noticeable, increase in nutrients."

In summer, the nitrate levels increase, as do nitrite levels, but the latter are somewhat more complicated to analyze, according to Najjar. The phosphate levels follow nitrate but silicate shows no regular pattern. The ocean temperature goes down, even though it is summer, because the water coming up from depth is much colder than the surface water. Water high in nutrients is usually low in oxygen and so the surface water contains less oxygen than expected from equilibrium with the atmosphere.

"The productivity of the Arabian Sea increases dramatically with an increase in nitrate," says Najjar. "This productivity is in the form of tiny plankton, which are the lowest link in the oceans food chain."

Summer fishing in the Arabian Sea has long been a productive and economical pursuit. Najjar however, is not interested in fishing, but in understanding the role the ocean plays in the climate system, especially in regulating the amounts of carbon dioxide, a greenhouse gas, in the atmosphere.

When nitrate increases productivity, the tiny organisms convert carbon dioxide in surface waters to organic forms of carbon. Some of this carbon leaves the surface layer of the ocean where it could once again become carbon dioxide and is stored in the deeper layers of the ocean.

"For every nitrate molecule removed, there are about eight carbon dioxide molecules removed or exported from surface waters," says Najjar.

"This removal of carbon from surface waters causes atmospheric carbon dioxide to be lower than it would otherwise be. By estimating the average decrease in nitrate after the summer monsoon, we can estimate the role this area of the ocean plays in carbon export and atmospheric carbon dioxide regulation."

*aem*

EDITORS: Dr. Najjar may be reached at (814) 863-1586.

Penn State

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