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The Meridional Overturning Circulation of the Pacific Ocean: New Findings Present New Challenges for GCMs
Reference
McPhaden, M.J. and Zhang, D. 2002. Slowdown of the meridional overturning circulation in the upper Pacific Ocean. Nature 415: 603-608.

Background
Across the vast expanse of the Pacific Ocean, winds produce surface currents that flow away from the equator (Langenberg, 2002). The water in these currents eventually sinks and flows back along surfaces of constant water density at depths of 100 to 400 meters. Near the equator, the returning flows meet and rise to the surface, bringing with them cooler and more-highly-nutrient-laden water. Having only been recognized during the 1990s, the major characteristics of these Northern- and Southern-Hemispheric cellular circulations had not previously been well defined, particularly with respect to their temporal variability. The paper highlighted in this Journal Review thus significantly advances our knowledge of the subject, while at the same time raising a number of intriguing new questions.

What was done
The authors analyzed hydrographic data between 20S and 50N latitude for the period 1950-99 in the depth range of 50-400 meters in the tropical and subtropical Pacific Ocean, together with empirical wind data, to address the question of "whether the meridional overturning circulation in the upper Pacific may be changing on decadal time-scales."

What was learned
The authors discovered that the overturning circulation "has been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9N and 9S." They further note that "this reduction in equatorial upwelling of relatively cool water ... is associated with a rise in equatorial sea surface temperatures of about 0.8C." In addition, they point out that the onset of this change occurred at about the same time as the pronounced shift in the Pacific Decadal Oscillation, which occurred in 1976-77.

What it means
The ultimate significance of these several observations remains to be determined. As things stand currently, the authors note that the oceanic circulatory slowdown "can account for the recent anomalous surface warming in the tropical Pacific," which means it can also account for a significant portion of the slight warming of the globe over this period. On the other hand, they say that the slowdown they discovered may possibly "have been influenced by global warming." However, they also acknowledge that "natural variability" may have played a role in this regard, and that "the observed decadal changes may simply be the low-frequency residual of random or chaotic fluctuations in tropical ocean-atmosphere interactions that give rise to the ENSO cycle."

Much speculation thus abounds as to what is cause and what is effect in this regard; and more research will clearly be needed to sort things out. In addition, there are questions related to the impact of the new findings on earth's carbon cycle, as mediated by changes in the outgassing of CO2 from the equatorial Pacific Ocean and as influenced by changes in nutrient supply that affect phytoplanktonic productivity there.

All in all, things of a global change nature just got a little more complex for everyone, including climate modelers, who now have an important new phenomenon to replicate. As the authors put it, their results clearly provide "an important dynamical constraint for model studies that attempt to simulate recent observed decadal changes in the Pacific basin."

Reference
Langenberg, H. 2002. Oceanography: A slower flow. Nature 415: 594.


Reviewed 13 March 2002