Mechanisms for the Dipolar Patterns of Rainfall Variability over Large Islands in the Maritime Continent Associated with the Madden-Julian Oscillation

Before the eastward-propagating rainy envelope of a Madden-Julian oscillation (MJO) arrives at the Maritime Continent (MC), some islands in the MC experience dipolar patterns of rainfall variability with opposite signs of rainfall anomalies in two neighboring regions within an island. Similar incoherent rainfall anomalies are also observed after the MJO passed the MC. The mechanisms for these dipolar patterns of rainfall anomalies are investigated by using observed and reanalysis data. It is found that the response of rainfall in the MC depends on the direction of wind anomalies and the availability of atmospheric moisture in different phases of the MJO. The low-level wind anomalies over the MC are easterlies in MJO phases 1-3, which cause above-normal rainfall over the mountainous areas in Java, and in western Borneo, western Sumatra, and western Malay Peninsula, respectively. In phases 5-6, the low-level wind anomalies are westerlies and the positive rainfall anomalies are over the eastern part of the islands. Two physical mechanisms are responsible for this phenomenon of the dipolar patterns of rainfall anomalies: 1) the monsoonal damping effect on rainfall over elongated narrow islands-an inverse relationship between the intensity of the diurnal cycle of sea breezes and valley breezes and the large-scale monsoonal wind speed, and 2) the wake effect on rainfall over large and wide islands-above-normal rainfall on the downwind wake side of an island or mountain range with respect to large-scale wind anomalies.