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 × 10
to
35 × 10 m
s
,
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 CO
from the
equatorial Pacific Ocean—at present the largest
oceanic source of carbon dioxide to the atmosphere.