Assessing the role of the ocean–atmosphere coupling frequency in the western Maritime Continent rainfall

Abstract

High-frequency interactions between the ocean and atmosphere have the potential to affect lower frequency or mean state climate in various regions. Here we examine the importance of sub-daily air-sea interactions over the Maritime Continent region to the rectification of longer timescale variation. In order to determine the importance of these high-frequency interactions, we conducted two regional ocean–atmosphere coupled simulations over the Maritime Continent where exchanges between the oceanic and atmospheric components are performed either every hour (i.e. resolving diurnal changes) or every day. We find that coupling frequency has a significant influence on mean sea surface temperature (SST) and the mean state and diurnal cycle of rainfall over certain regions of the western Maritime Continent where air-sea interactions are strong during the Asian monsoon season, with little effect in other regions or seasons. Without sub-daily air-sea interactions, the mean SST along the southwest off Sumatra is ~ 2 °C warmer during the period from June to October as a result of a deepening of thermocline along the coast. This deepening is linked to anomalous downwelling equatorial eastward propagating Kelvin waves triggered by westerly anomalies in the eastern equatorial Indian Ocean. In addition, the mean rainfall in the vicinity of ocean warming increases, thereby producing an enhanced barrier layer that also provides a positive warming feedback. Although the coupling frequency has little impact on the timing of the rainfall diurnal cycle, suppression of sub-daily coupling significantly changes the diurnal rainfall amplitude causing a relative decrease (increase) in amplitude over the coast of Northwestern (Southwestern) Sumatra during the South Asian monsoon season.