QUANTIFYING THE ROLE OF GEOGRAPHIC CHANGE IN CENOZOIC OCEAN HEAT TRANSPORT USING UNCOUPLED ATMOSPHERE AND OCEAN MODELS

Abstract

A series of five Cenozoic atmospheric general circulation model (AGCM) experiments has been performed using a new set of paleogeographic reconstructions for 55, 40, 33, 20 and 14 Ma. The five continental reconstructions incorporate the tectonic evolution of early Eocene to middle Miocene continental positions and topography. With all other model boundary conditions and forcings held constant, the series of AGCM experiments captures a <1°C decrease in annual mean temperature through the Paleogene and early Neogene. Regional and seasonal differences among the five experiments are small in magnitude, but are consistent with the imposed paleogeographic changes. From the AGCM experiments alone, it might be concluded that changes in continental positions had little impact on Cenozoic climate change. However, ocean configuration changes between 55 and 14 Ma, especially gateway openings and closures, are expected to produce significant changes in ocean thermohaline circulation, a system that cannot be simulated by the slab ocean model component of an AGCM. The nature of changes in ocean heat transport and thermohaline circulation arising from the evolution of early Eocene through middle Miocene ocean basin configurations is examined in a series of five global, three-dimensional ocean model experiments forced by output from the AGCM. The ocean model suggests that paleogeographic change throughout the Cenozoic has caused changes in the interhemispheric partitioning of heat transport and that the modern shape of the ocean heat transport curve has evolved in response to ocean basin evolution. The prediction of very low ocean heat transport in the Northern Hemisphere of the early and middle Eocene suggests a much more important role for atmospheric heat transport in the temperate polar climates of the Eocene than is generally acknowledged. Results suggest that Southern Hemisphere ocean heat transport decreased throughout the interval 55–14 Ma. The results also show that, in the absence of reliable coupled models for paleoclimate studies, full three-dimensional ocean models must be used in parallel with slab ocean AGCMs if we wish to understand the true effects of paleogeographic change on climate and the true nature of past ocean heat transport.