Tightly linked oceanic meridional and zonal temperature gradients - a fundamental climate constraint for the Miocene, Pliocene and other epochs

Abstract:

Two major characteristics define the mean climate of the tropics – pronounced zonal (east-west) and meridional (equator to mid-latitudes) gradients in sea surface temperature (SST). These gradients control the atmospheric zonal and meridional circulations and thus Earth’s climate. Previous studies have shown that global cooling, which started in the late Miocene and accelerated since the early Pliocene, was accompanied by the gradual strengthening of these gradients. Here, we carefully analyze temperature trends over the past 5 million years, including a newly generated SST record from the Southern Pacific Ocean (ODP site 1125), and show that changes in these gradients followed each other very closely, reflecting a tight relationship between these two climatic variables. To investigate this relationship we conduct simulations with a comprehensive climate model (CESM) wherein we systematically reduce the meridional temperature gradient by modifying the latitudinal distribution of cloud albedo or atmospheric CO₂ concentrations. We find that the zonal SST gradient in the Pacific adjusts accordingly, such that changes along the equator match the imposed changes in the meridional gradient, with the two variables linked by upper-ocean circulation. The meridional temperature distribution is mapped onto the ocean vertical thermal stratification and then onto the east-west SST distribution along the equator. These findings are accounted for by a simple model of tropical climate informed by our simulations. Consequently, the tight relationship between the two SST gradients posits a fundamental constraint on past climates that can help resolve current debates on the state of the tropics during the early Pliocene, including the prevalence of permanent El Nino-like conditions (El Padre).