Maritime Continent Weakens Asian Tropical Monsoon Rainfall Through Australian Cross-equatorial Flows

Abstract:

Cross-equtorial flows (CEFs) play a pivotal role in mass, moisture and energy transport between the Northern and Southern Hemispheres, and serve as one of the crucial components of the Asian monsoon system, exerting a profound effect on the weather and climate anomalies in Asia. This study uses an atmospheric general circulation model to examine the relative effects of Maritime Continent (MC) orography, surface roughness, and land–sea contrast on the Australian CEF and Asian tropical monsoon rainfall during boreal summer. Four experiments are conducted: with islands, with islands without orography, with islands with ocean roughness and no orography, and with ocean only in the MC region. At the approximately 1° horizontal resolution of these sensitivity experiments, results indicate that the Australian CEF would be a homogeneous, thin southerly flow without the MC. The presence of the islands creates three CEFs originating from north of Australia to the South China Sea (SCS), Celebes-Moluccas and New Guinea, and the orography further strengthens, concentrates the Celebes-Moluccas CEF and shifts the New Guinea CEF to the east about 6 degrees.In addition, the presence of the islands over the MC directly weakens the moisture transportation and results in decreased monsoon rainfall by roughly a quarter over the SCS. Meanwhile, the rainfall over South Asia decreases by half. For the reduced rainfall over South Asia, it is mainly associated with three mechanisms. First, the decreased rainfall over the SCS could induce an anticyclonic circulation response over Bay of Bengal and then suppress the convective activity there. Second, the decreased rainfall over South Asia can weaken the Somali CEF, which in turn leads to less moisture transport into South Asia. Third, the presence of the islands can intercept the westward propagation of the quasi-biweekly oscillation.