Large Enhancement of Monsoon Depression Intensification by the Madden-Julian Oscillation
Monsoon low-pressure systems are atmospheric vortices with outer diameters of about 1,000 km that produce abundant rainfall. They are most well known in South Asia, Earth’s most energetic monsoon system, but are distributed throughout the global tropics. About one-third of these vortices intensify into the strong storms known as monsoon depressions, which produce extreme rainfall. The RGMA Monsoon Extremes project collaborated with researchers at Florida State University to analyze a large dataset of low-pressure system properties and determine how conditions during initial vortex genesis altered the peak intensity later achieved by the storms.
The intensification of weak low-pressure systems into monsoon depressions was found to be greatly enhanced in observations by the Madden-Julian Oscillation (MJO), the dominant planetary-scale mode of variability of the tropical atmosphere. Depressions were two to three times more likely to develop from low-pressure systems that formed during the convectively active phase of the MJO compared with the inactive phase. The research showed that the MJO preconditioned the large-scale atmosphere to enhance the low-level vorticity and moisture content so that low-pressure systems forming in this background state would later achieve greater intensities. This work provides a new understanding of the dynamics governing the distribution of tropical vortex strengths.
The intensification of weak low-pressure systems into monsoon depressions was found to be greatly enhanced by the MJO, the dominant planetary-scale mode of variability of the tropical atmosphere. Depressions were two to three times more likely to develop from low-pressure systems that initially formed during the convectively active phase of the MJO, as compared to the inactive phase of the MJO.