Impacts of Insolation and Soil Moisture on the Seasonality of Interactions Between the Madden-Julian Oscillation and Maritime Continent

This study investigates the seasonality of the interaction of the Madden-Julian Oscillation (MJO) with the Maritime Continent (MC). Beside their seasonal east-west migration with the monsoons, observations show that the MJO amplitude and precipitation over the MC islands exhibit semiannual variability, with apparent strengthening of MJO signal during March and September, when solar insolation is strongest over the MC region. Furthermore, during the high-insolation months, soil moisture shows wetter conditions during the early phases of the MJO. Motivated by these results, a series of regional convection permitting simulations are performed for the November 2014 MJO event as a case study under the following conditions: (i) increased solar insolation to mimic the local summer, (ii) reduced initial soil moisture, and (iii) the two conditions combined to isolate their local effects from their impacts on the large-scale circulation. Results show that increased insolation increases precipitation including that associated with MJO moisture convergence over the MC region. On the other hand, decreased initial soil moisture slightly increases precipitation over the MC islands and reduces it over the surrounding waters, but such effect is short lived as the increased precipitation gradually restores the soil moisture to wetter conditions. Using a moisture budget analysis that isolates the MJO and non-MJO signals and additional idealized simulations, the MJO response to high insolation is demonstrated to be related to an increase in the basic state moisture over the MC region rather than a direct response of the MJO to the insolation.