Contrasting the Indian and East Asian monsoons: implications on geologic timescales

The surface winds over the Arabian Sea and South China Sea are meaningful indicators for the strength of the Indian monsoon and East Asian monsoon, respectively. Paleo-monsoon variability has been studied through analysis of sediment records from these two monsoon regions. To facilitate interpretation of these records, we focus on the impacts of ‘internal’ and ‘external’ forcing of the monsoon system by contrasting the annual cycle and interannual variability of two subsystems: the monsoon over the Indian sector (40–105°E) and over the East Asian sector (105–160°E). Differences in the annual cycle within these subsystems arise primarily from the different land–ocean configurations that determines atmospheric response to the solar forcing. Thus factors that drive intensities of the monsoonal annual cycle share common features with the external (geographic and orbital) forcing that controls paleo-monsoon variability. We show that the differences in interannual variations between the two monsoon subsystems are primarily due to internal factors of the coupled atmosphere–ocean–land system, such as remote impacts of El Niño/La Niña and local monsoon–ocean interactions. The mechanisms that operate on interannual to interdecadal timescales may differ fundamentally from that on geologic/orbital timescales. The low-level flows over the East Asia and Australia are essentially established by geographic forcing. The amplification of the Australia summer monsoon during increased solar precession is likely caused by an enhanced East Asian winter monsoon, rather than following an enhanced Indian summer monsoon as on the interannual timescale. It is also found that El Niño influences the low-level flow moderately over the Arabian Sea but to a greater extent over the South China Sea. As such, large changes in the Pacific thermal conditions may significantly alter the intensity of the East Asian monsoon but not the Indian monsoon.