Hansen et al 2006 Global Temperature Changes

This normal state is occasionally upset when by

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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 existed (23). 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 (EEP)]. 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.

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