Mechanisms and impacts of enhanced rainfall over large islands in the Maritime Continent during the mid-holocene

Both paleoclimate proxy records and Community Climate System Model version 4 (CCSM4) simulations show that rainfall over large islands in the Maritime Continent (e.g., Borneo) increased substantially during the mid-Holocene relative to the pre-industrial time. The rainfall increase is hypothesized to be caused by enhanced local autumn insolation, but is also attributed to increased sea surface temperature in the Indo-Pacific warm pool (IPWP) and/or weakened El Nino Southern Oscillation (ENSO). Here, we performed numerical experiments based on the output of the CCSM4 simulations and examined the underlining mechanisms of the enhanced rainfall and how they may have impacted the surrounding regions. Our numerical experiments, performed with the Community Atmospheric Model version 4 (CAM4), show that a wetter climate in the mid-Holocene, mainly during July, August, September, and October (JASO), is a direct response to stronger insolation rather than being chiefly impacted by a weaker magnitude of ENSO. The results reveal that stronger insolation can enhance rainfall over large islands through the convection-moisture positive feedback. However, the same mechanism acting over the warmer South China Sea (SCS) would instead reduce the convection over the large islands. On the other hand, the island convection suppresses oceanic convection by weakening the low-level southerly wind over the SCS. The suppressed convection over the SCS during the mid-Holocene weakens the local Hadley circulation, but without dramatically shifting the latitudinal position of the Intertropical Convergence Zone (ITCZ).