Tropical Climate Mean State and Variability during the Pliocene Warm Period (Invited)
Abstract
Past studies have shown that the mean climate state during the Pliocene warm period, about 3 - 4 million years ago, differed from present day climate in several ways: global temperature was about 3-4 degrees C warmer, the tropical thermocline was warmer and/or deeper, and meridional and zonal sea surface temperature gradients were reduced due to warmer high latitude temperatures but tropical sea surface temperatures that were similar to today. One of the most striking features of the Pliocene warm period is the El Niño-like (El Padre) mean state of the tropical Pacific, which is thought to have far-field impacts. In this study, we present a synthesis of new and published tropical Pacific data, detailing the mean state and higher frequency variability (e.g., using orbital scale records and measurements made on single foraminifera shells), for the purpose of meeting two main goals. First, we highlight important characteristics of the El Padre mean state, which include average Indo-Pacific warm pool temperatures that were similar and east Pacific cold tongue temperatures and cross-Pacific subsurface temperatures that were warmer than today. Because much of the paleotemperature data comes from Mg/Ca ratios measured in planktonic foraminifera, the impact of possible changes in Mg/Ca of seawater on paleotemperature estimates is addressed. We conclude that Mg/Ca-derived temperature estimates could be adjusted by no more than about 1 degree in order to account for seawater chemistry changes. Second, by examining orbital variability and temperature distributions based on single foraminifera analyses, we evaluate whether the cumulative strength of the many feedbacks that are involved in the generation of climate variability may be impacted by the mean state. Data indicate that the amplitude of orbital variability in surface temperature, and possibly the amplitude of ENSO variability, was reduced during the warm Pliocene compared to today. On orbital timescales, the reduction in the amplitude of variability could be explained by a possible reduction in orbital variations in CO2 forcing; this inference is supported by the single foraminiferal shell analyses from the west Pacific that show that the distribution of temperatures across seasons remained unchanged even as the mean temperature varied. On shorter, ENSO timescales, variability may have been affected by mean state conditions such as the reduced zonal gradient and the deeper/warmer thermocline that determine the strength of dominant feedbacks. In sum, while the mean state was radically different with reduced zonal gradients characteristic of El Padre, significant orbital, ENSO and seasonal variability was present, albeit reduced in amplitude in several datasets.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP52A..05R
- Keywords:
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- 4900 PALEOCEANOGRAPHY;
- 4954 PALEOCEANOGRAPHY Sea surface temperature