THE ROLE OF MIOCENE PALEOGEOGRAPHY, SEA LEVEL, AND THE DEVELOPMENT OF THE WESTERN PACIFIC WARM POOL ON THE EVOLUTION OF THE EAST ASIAN MONSOON

Recent isotopic and biotic data from Ontong Java Plateau suggest that the northward drift of Australia and New Guinea progressively restricted Indonesian throughflow (ITF) during the late middle Miocene. ITF plays an important role in modulating inter-basin fresh water flux, heat transport, and the volume of the Western Pacific Warm Pool (WPWP). As the Indonesian Seaway became progressively restricted in the Miocene, a proto-WPWP may have developed during low-stands in eustatic sea-level. Today's WPWP is a center for deep convection, affecting the Walker and Hadley circulation of the tropical atmosphere and ENSO. The formation of a proto-WPWP would have had important consequences on tropical western Pacific and Indo-African-Asian climate, with possible teleconnections to the mid to high latitudes. In order to test the possible effects of progressive Indonesian closure, we have begun running a suite of GCM experiments, designed to test the response of the climate system to the formation of a proto-WPWP in the context of evolving Miocene geography, including the evolution of Indonesian and East Asian topography and the uplift of the Himalaya/Tibetan Plateau, between 11 and 7 Ma. Preliminary results from our climate model simulations show that a proto-WPWP would have increased western Pacific convection, strengthened the western Pacific easterlies, and contributed to Saharan aridity. However, East Asian monsoon intensity (over land) appears little affected by the presence of a WPWP. This may be an artifact of our paleogeography, which includes high (~3000 meter) elevations over Indonesian and eastern Asia, limiting the advection of equatorial moisture over land. The results of the our GCM simulations will be presented in the context of the hypotheses that 1) a proto-WPWP became established as the Indonesian Seaway became increasingly restricted during the late middle to late Miocene; 2) the growth of a proto-WPWP had a first order affect on tropical Pacific climate and contributes to the monsoonal intensity usually associated with vertical tectonics over southern Asia.