Inter-linkages of SE Asian, Indian and Indonesian-Australian monsoonal subsystems on orbital and suborbital timescales

Abstract:

The SE Asian, Indian and Indonesian-Australian monsoonal subsystems are closely inter-linked, but show substantial differences in the spatial and temporal distribution of precipitation, mainly due to contrasting land-sea distribution and high latitude control. We explore changes in these subsystems in relation to high latitude climate variability on suborbital and orbital timescales, focusing on the last deglaciation and the long-term Miocene evolution. Our main proxies are δ¹⁸O and Mg/Ca based salinity and temperature reconstructions in combination with sedimentary and geochemical runoff signatures. Key issues are the synchroneity of monsoonal precipitation changes in relation to northern and southern hemisphere insolation and the response of individual subsystems to atmospheric CO₂ and global ice volume variations.

In contrast to northern hemisphere monsoonal records, the deglacial intensification of the Australian summer monsoon paralleled southern hemisphere climate evolution. We hypothesize that intensification of the summer heat low over the Australian continent through enhanced greenhouse forcing accentuated the southward pull of the Intertropical Convergence Zone (ITCZ). Additional forcing mechanisms including the variability of the Walker circulation and Indian Ocean Dipole, the heat and moisture transfer from the tropical Indian Ocean and deglacial sea-level changes remain highly debated.

High-resolution Miocene records from the South China Sea (ODP Site 1146) indicate that the latitudinal displacement of the ITCZ also impacted the long-term development of the SE Asian summer monsoon. Antarctic ice growth episodes at 14.6, 14.2, 13.9, and 13.1 Ma coincided with surface warming and freshening, implying high sensitivity of tropical rain belts to the inter-hemispheric temperature gradient. However, comparable records of the long-term evolution of the Indian and Indonesian-Australian monsoonal subsystems that would allow testing of this hypothesis are still missing. High-resolution sedimentary archives recently recovered during IODP Expedition 346 (Asian Monsoon) and to be drilled during IODP Exp 353 (Indian Monsoon) will enable direct comparison of the three monsoonal subsystems and reconciliation of linkages between marine and land records.