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Slowdown of the meridional overturning circulation in the upper Pacific Ocean


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 × 106 to 35 × 106 m3 s-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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Figure 1: Mean circulation and potential vorticity averaged over 50 years (1950–99) in the upper pycnocline of the Pacific Ocean.
Figure 2: Meridional transports in the pycnocline and smoothed sea surface temperatures over the past 50 years.
Figure 3: Decadal differences in the tropical Pacific between 1990–99 and 1970–77.


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We thank J. Potemra for advice on Indonesian throughflow calculations. We also thank W. Kessler, G. Johnson and R. Kleeman for comments on an earlier version of this manuscript. This work was supported by the NOAA Office of Oceanic and Atmospheric Research and by the Joint Study for the Atmosphere and Ocean (JISAO) at the University of Washington.

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Correspondence to Michael J. McPhaden.

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McPhaden, M., Zhang, D. Slowdown of the meridional overturning circulation in the upper Pacific Ocean. Nature 415, 603–608 (2002).

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