1. NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington 98115, USA
2. Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington 98115, USA

Correspondence to: Michael J. McPhaden¹ Correspondence and requests for materials should be addressed to M.J.M. (e-mail: Email: mcphaden@pmel.noaa.gov).

Abstract

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.