The Madden-Julian Oscillation (MJO) is a pulse of clouds and precipitation that travels eastward across the tropics every month or two, on average. Predicting whether and how this oscillation travels over the Maritime Continent islands—including Sumatra, Borneo, New Guinea, and the Philippines—is challenging because the factors that influence it in this region are not well understood. Now scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory and the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory have analyzed observations and model simulations. They discovered that the semi-annual variation in the amount of sunlight the region receives plays a role in enhancing MJO precipitation over the Maritime Continent islands.
Limitations in understanding and simulating the variability of the MJO propagation across the Indo-Pacific Maritime Continent have been an impediment to MJO prediction. The MJO can influence hurricanes and atmospheric rivers, so challenges in its prediction can have implications for global predictions of extreme weather events. This study examines the roles of the semi-annual variation of solar radiation and soil moisture on the MJO propagation across the Maritime Continent islands. The relationship between the effects of the annual monsoonal cycle and semi-annual variability due to sunlight exposure provides important insight for interpreting variations in the predictability of MJO events.
Researchers investigated the seasonality of the interaction of the MJO with the Maritime Continent. Using observations, they showed that the MJO amplitude and precipitation over the Maritime Continent islands exhibit semi-annual variability. The MJO signal appeared to strengthen during March and September when sunlight, or solar insolation, is strongest over this region.
Next, the researchers performed a series of regional convection-permitting simulations for the November 2014 MJO event as a case study under various insolation and soil moisture conditions. Insolation and soil moisture are two factors that influence convection over land. Results showed that increased insolation increases precipitation, including precipitation associated with MJO moisture convergence over the Maritime Continent region. Using a moisture budget analysis that isolated MJO and non‐MJO signals, along with additional idealized simulations, the MJO response to high insolation is demonstrated to be related to an increase in the basic state atmospheric moisture due to the high insolation over the Maritime Continent region.