Scale interaction between MJO and the Diurnal variability over the Maritime Continent during Austral Summer

Abstract

This study aims to investigate the scale interaction between the diurnal variability and MJO over the Maritime Continent during Austral summer. As the first part for this study, the impact of MJO on the diurnal cycle of rainfall over the western MaritimeContinent during the Austral summeris examined. For this purpose, Cyclostationary Empirical Orthogonal Function (CSEOF) analysis is applied to the Tropical Rainfall Measuring Mission (TRMM) rain rate and the Japanese Reanalysis Analysis-25 (JRA-25) data for the period of 1998-2008. Real-time Multivariate MJO (RMM) index by Wheeler and Hendon (2004) is adopted to define the intensity and the phase of MJO. It is demonstrated that hourly maximum rain rate over the domain tends to increase when convectively active phase of MJO approaches the Maritime Continent. In contrast, hourly maximum rain rate tends to decrease when convectively suppressed phase of MJO resides over the region. The rain rate change due to MJO over the ocean is quite different from that over land. During the mature stage of MJO over the Maritime Continent, diurnally varying rain rates over the ocean indicate maximum values while terrestrial rain rates show minimum values throughout the day. Thus, precipitation becomes more intense in the morning over the Java Sea and is weakened in the evening over Borneo and Sumatra during the mature stage of MJO. During the decaying stage of MJO over the Maritime Continent, the diurnal cycle of precipitation weakens significantly over the ocean but less than over the land. Analyses suggest that the anomalous lower level winds accompanied by MJO interact with the monsoonal flow over the Maritime Continent. Westerlies induced by MJO convection in the mature stage are superimposed on the monsoonal westerlies over the equator and increase wind speed mainly over the Java Sea due to the blocking effect of orography. Mountainous islands induce flow bifurcation,causing near-surface winds to converge mainly over the oceanic channels between two islands. As a result, heat flux release from the ocean to the atmosphere is enhanced by the increased surface wind resulting in instability as described in the wind-induced surface heat exchange (WISHE) mechanism. This may contribute to heavy rainfall over the Java Sea in the morning during the mature stage. On the other hand, convergence and vertical velocity over the islands, which play important roles in inducing nighttime rainfall, tend to be weak in the evening during the mature stage of MJO. Strong westerlies arising from MJO and the seasonal flow during the mature stage tend to interrupt convergence over islands. This interruption of convergence by MJO gives rise to decreased rain rates over the land regions. As the second part of the study, modeling experiments are conductedto investigate the impact of the diurnal cycle on MJO during the Australian summer.Physical initialization and a nudging technique enable usto assimilate the observed TRMM rain rate and atmospheric variables from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis 2 (R2) into the Florida State University Global Spectral Model (FSUGSM), resulting in a realistic simulation of the MJO. Model precipitation is also significantly improved by TRMM rain rate observation viathe physical initialization.We assess the influence of the diurnal cycle on the MJO by modifying the diurnal component during the model integration. Model variables are nudged toward the daily averaged values from R2. Globally suppressing the diurnal cycle (NO_DIURNAL) exerts a strong impact on the Maritime Continent. The mean state of precipitation increases and intraseasonal variability becomes stronger over the region. It is well known that MJO weakens as it passes over the Maritime Continent.However, the MJO maintains its strength in the NO_DIURNAL experiment, and the diminution of diurnal signals during the integration does not change the propagating speed of the MJO.Diminishing the diurnal cycle in NO_DIURNAL seems to consume less moist static energy (MSE), which is required to trigger both diurnal and intraseasonal convection. Thus, the remaining MSE may play a major role along with larger convective instability and stronger lower level moisture convergence in intensifying the MJO over the Maritime Continent in the model simulation.