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In the equatorial Pacific, the prevailing easterly surface winds drive an east-west tilting of the thermocline (the region of sharp vertical temperature gradients separating warmer water in the upper ocean from cooler water below). The tilting causes cool subthermocline water to upwell in the east. The resulting gradient in SST between the Cold Tongue in the east and the Warm Pool in the west drives an east-west overturning circulation in the atmosphere, known as the Walker cell, which enhances surface easterlies and produces further upwelling. This positive feedback, first recognized by Bjerknes (47), controls both the time-mean state (6-8) and the interannual (ENSO) variability (72) of the tropical Pacific. By "weakening of the Bjerknes feedback," we mean either a weakening of the atmospheric response to a given SST anomaly or a weakening of the oceanic response to a given surface stress anomaly. We focus mostly on this latter effect, which can be achieved by making the mean thermocline deeper and/or reducing the temperature difference across it (11).
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-2) and mean thermocline depth (∼120 m) (fig. S5) place the Eocene simulation near the point marked "A" in their figure 4a, i.e., near modern values. The ENSO period associated with these conditions is ∼5 years, matching those found in Eocene simulations and proxy records.
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Today, maximum SST variability is found farther east, in the "Niño 3" (150°W to 90°W) and "Niño 3.4" (170°W to 120°W) regions. Given the different basin geometry in the Eocene simulation, a shift in location is not surprising. Today, the Niño 3.4 region brackets the zone where the Cold Tongue impinges on the Warm Pool. Thus, it strongly affects precipitation and hence diabatic heating and atmospheric circulation, making it the preferred region for studies of extratropical ENSO impacts. Precisely the same role is played in the Eocene simulation by our "Eocene Niño" region.
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Lake sediments are affected by a number of seasonally varying factors, including temperature, insolation, snowmelt, runoff, and surface wind speed (49), whose interannual variations are recorded by changing layer thickness. Here, we concentrate on surface temperature and precipitation results.
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50
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0347782512
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M. H. acknowledges L. C. Sloan for mentorship and support by NSF ATM9810799 and the Packard Foundation. This study would not have been conducted without B. Otto-Bliesner and E. Brady having first demonstrated ENSO in their Cretaceous simulation. We authors also thank F. F. Jin, Z. Y. Liu, M. A. Cane, and S. G. Philander for sharing their ideas. The CCSM model, computer time, and graphics tools (NCAR Command Language, NCL) were provided through NCAR by the NSF. Support for both authors was provided by Dansk Grundforskningsfond through the Danish Center for Earth System Science.
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