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Slowdown of the meridional overturning circulation in the upper Pacific Ocean

M. J. McPhaden1 and D. Zhang2

1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, 98115
2Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, 98115

Nature, 415(7), 603–608 (2002).
Copyright ©2002 by Macmillan Publishers Ltd. Further electronic distribution is not allowed.


Decadal temperature fluctuations in the Pacific Ocean have a significant effect on marine ecosystems and the climate of North America. The physical mechanisms responsible for these fluctuations are poorly understood. Some theories ascribe a central role to the wind-driven meridional overturning circulation between the tropical and subtropical oceans. Here we show, from observations over the past 50 years, that this overturning circulation has been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9°N and 9°S. This reduction in equatorial upwelling of relatively cool water, from 47 × 10to the 6 to 35 × 10to the 6 mto the 3 sto the minus 1, is associated with a rise in equatorial sea surface temperatures of about 0.8°C. Another effect of the slowing circulation is a reduction in the outgassing of CO2 from the equatorial Pacific Ocean—at present the largest oceanic source of carbon dioxide to the atmosphere.

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