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Birth of the Cool
Over the past 4 million years or so, tropical sea surface temperatures have experienced a cooling trend (see the Perspective by Philander). Herbert et al. (p. 1530) analyzed sea surface temperature records of the past 3.5 million years from low-latitude sites spanning the world's major ocean basins in order to determine the timing and magnitude of the cooling that has accompanied the intensification of Northern Hemisphere ice ages since the Pliocene. Martínez-Garcia et al. (p. 1550) found that the enigmatic eastern equatorial Pacific cold tongue, a feature one might not expect to find in such a warm region receiving so much sunlight, first appeared between 1.8 and 1.2 million years ago. Its appearance was probably in response to a general shrinking of the tropical warm water pool caused by general climate cooling driven by changes in Earth's orbit.
Abstract
The cold upwelling “tongue” of the eastern equatorial Pacific is a central energetic feature of the ocean, dominating both the mean state and temporal variability of climate in the tropics and beyond. Recent evidence for the development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline. We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis. In addition, we show that sub-Antarctic climate changed in its response to Earth’s orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.
