Relative importance of meridional and zonal sea surface temperature gradients for the onset of the ice ages and Pliocene-Pleistocene climate evolution

1 During the early Pliocene (roughly 4 Myr ago), the ocean warm water pool extended over most of the tropics. Subsequently, the warm pool gradually contracted toward the equator, while midlatitudes and subpolar regions cooled, establishing a meridional sea surface temperature (SST) gradient comparable to the modern about 2 Myr ago (as estimated on the eastern side of the Pacific). The zonal SST gradient along the equator, virtually nonexistent in the early Pliocene, reached modern values between 1 and 2 Myr ago. Here, we use an atmospheric general circulation model to investigate the relative roles of the changes in the meridional and zonal temperature gradients for the onset of glacial cycles and for Pliocene-Pleistocene climate evolution in general. We show that the increase in the meridional SST gradient reduces air temperature and increases snowfall over most of North America, both factors favorable to ice sheet inception. The impacts of changes in the zonal gradient, while also important over North America, are somewhat weaker than those caused by meridional temperature variations. The establishment of the modern meridional and zonal SST distributions leads to roughly 3.2°C and 0.6°C decreases in global mean temperature, respectively. Changes in the two gradients also have large regional consequences, including aridification of Africa (both gradients) and strengthening of the Indian monsoon (zonal gradient). Ultimately, this study suggests that the growth of Northern Hemisphere ice sheets is a result of the global cooling of Earth's climate since 4 Myr rather than its initial cause. Thus, reproducing the correct changes in the SST distribution is critical for a model to simulate the transition from the warm early Pliocene to a colder Pleistocene climate.