Global climate and Indonesian Throughflow during the Middle Miocene Climate Transition : a modeling approach

Abstract

The current study focuses on the Middle Miocene Climate Transition (MMCT), a main global cooling step during the Cenozoic (66-0 Ma). This transition, likely triggered by changes in the Earth's orbital configuration and a decrease in atmospheric CO2 concentration, entailed major expansion of the Antarctic ice sheet, cooling of the surface and deep ocean, and global eustatic sea level fall during the interval 15-13 Ma. A central aim of this study is to assess, by means of the global coupled model Community Climate System Model version 3 (CCSM3), the ocean response to atmospheric CO2 dropdown and Antarctic ice sheet expansion during the MMCT. In particular, it is investigated whether the combined effects of the CO2 decrease and Antarctic ice sheet expansion could explain the cooling of surface and deep waters across the MMCT inferred from proxy data, the separate effects of these two forcings on surface and deep water temperatures, and the mechanisms these forcings were triggering that explain their modeled effects on ocean temperatures. Ocean gateways are relatively narrow channels of water separating two main ocean basins. Changes in the bathymetry of ocean gateways alter the water properties of the basins they connect and this can have regional to global scale climate effects. An example of ocean gateway is the Indonesian gateway, the tropical passage connecting the Pacific and Indian oceans, which has a significant influence on the climatic states of those oceans. The origin of the West Pacific Warm Pool, for example, the most extensive warm surface water mass on Earth, has been suggested to be linked to narrowing of the Indonesian gateway. A further aim of this study is that of modeling the characteristics of the Indonesian Throughflow during the MMCT by means of CCSM3, providing estimates of volume transport, analyzing the vertical structure of the waterflow, the direction of waterpaths, the relative contributions of North and South Pacific water to the Indonesian Throughflow, the control mechanisms of its seasonal variability, and examining whether a similar structure to the present-day West Pacific Warm Pool existed during the MMCT in the Indonesian Throughflow area. The model boundary conditions employed in our experiments include Middle Miocene global vegetation, topography, and bathymetry - comprising a geographic reconstruction of Southeast Asia for 15 Ma - as well as Antarctic ice sheet configurations, sea levels, and atmospheric CO2 concentrations characterizing the periods before (Middle Miocene Climatic Optimum) and after the transition (Middle Miocene Glaciation).