Unformatted text preview: in the EEP. The Walker circulation enhances
upwelling of cold water in the East Pacific, causing a powerful
positive feedback, the Bjerknes (21) feedback, which tends to
maintain the La Niña phase, as the SST gradient and resulting
higher pressure in the EEP support east-to-west trade winds.
This normal state is occasionally upset when, by chance, the
east-to-west trade winds slacken, allowing warm water piled up in
the west to slosh back toward South America. If the fluctuation is
large enough, the Walker circulation breaks down and the Bjerknes
feedback loses power. As the east-to-west winds weaken, the
Bjerknes feedback works in reverse, and warm waters move more
strongly toward South America, reducing the thermocline tilt and
cutting off upwelling of cold water along the South American coast.
In this way, a classical El Niño is born.
Theory does not provide a clear answer about the effect of global
warming on El Niños (19, 20). Most climate models yield either a
tendency toward a more El Niño-like state or no clear change (22).
It has been hypothesized that, during the early Pliocene, when the
Earth was 3°C warmer than today, a permanent El Niño condition
We suggest, on empirical grounds, that a near-term global
warming effect is an increased likelihood of strong El Niños. Fig. 1B
shows an absence of warming in recent years relative to 1951–1980
in the equatorial upwelling region off the coast of South America.
This is also true relative to the earliest period of SST data,
1870–1900 (Fig. 3A). Fig. 7, which is published as supporting
information on the PNAS web site, finds a similar result for linear
14290 www.pnas.org cgi doi 10.1073 pnas.0606291103 trends of SSTs. The trend of temperature minima in the East
Pacific, more relevant for our purpose, also shows no equatorial
warming in the East Pacific.
The absence of warming in the EEP suggests that upwelling water
there is not yet affected much by global warming. Warming in the
WEP, on the other hand, is 0.5–1°C (Fig. 3). We suggest that
increased temperature difference between the near-equatorial
WEP and EEP allows the possibility of increased temperature
swing from a La Niña phase to El Niño, and that this is a
consequence of global warming affecting the WEP surface sooner
than it affects the deeper ocean.
Fig. 3B compares SST anomalies (12-month running means) in
the WEP and EEP at sites (marked by circles in Fig. 3A) of
paleoclimate data discussed below. Absolute temperatures at these
sites are provided in Fig. 8, which is published as supporting
information on the PNAS web site. Even though these sites do not
have the largest warming in the WEP or largest cooling in the EEP,
Fig. 3B reveals warming of the WEP relative to the EEP [135-year
changes, based on linear trends, are 0.27°C (WEP) and 0.01°C
The 1983 and 1998 El Niños in Fig. 3B are notably stronger than
earlier El Niños. This may be partly an artifact of sparse early data
or the location of data si...
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This document was uploaded on 03/15/2014 for the course MEA 570 at N.C. State.
- Spring '08