Thursday, March 23, 2017

The diurnal cycle of tropical cloudiness and rainfall associated with the Madden-Julian Oscillation

The Madden Julian can lead to flooding rains over the United States and North America. (Credit: FEMA)

A study published as an early online release in the Journal of Climate reports that the Madden Julian Oscillation, or MJO, moderates tropical rainfall and cloudiness through its influence on cloud type population distribution.

Findings of this study suggest MJO’s behavior varies on a case-by-case basis particularly over the Maritime Continent, the region including many islands and peninsulas in Southeast Asia. These variations may be important in understanding the propagation of the MJO, which is known to be an influential atmospheric phenomenon moving eastward in the Pacific that can increase chances of extreme events such as tropical monsoons, cyclones, and thunderstorms around the world and the United States. 

This study was supported by the CPO Climate Variability and Predictability program.

Read the paper


This study examines the diurnal cycle of rainfall and cloudiness associated with the Madden-Julian Oscillation (MJO) using TRMM rainfall rate and ISCCP multi–level cloud fraction data. There are statistically significant differences in diurnal cycle amplitude and phase between suppressed and enhanced envelopes of MJO convection. The amplitude of the diurnal rainfall rate and middle to deep cloudiness increases within enhanced MJO convection, especially over the ocean. However, the differences in diurnal cycle amplitude between enhanced and suppressed MJO is generally smaller than the differences in daily-mean values, so that its relative contribution to total rainfall or cloudiness variance within enhanced MJO convection becomes smaller. Near the coastlines of islands within the Maritime Continent, the diurnal cycle amplitude tends to increase 5–10 days prior to the arrival of the peak enhanced MJO convection, but this relationship is weaker over the interior areas of larger islands where the climatological diurnal amplitude is already large. Within enhanced MJO convection, the diurnal rainfall peak is frequently delayed by about 3 hours and cloud height decays at slower rate compared to suppressed conditions. More stratiform rainfall occurs following the peak convective rainfall within enhanced MJO convection, delaying the total rainfall peak by a few hours due to its greater horizontal extent. The results of this study suggest that the MJO modulates both the amplitude and phase of diurnal cycle of tropical rainfall and cloudiness by influencing cloud type population distribution and associated rainfall rates.

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