Large Enhancement of Monsoon Depression Intensification by the Madden-Julian Oscillation
Researchers find that a well-known planetary-scale oscillation causes some precipitating atmospheric vortices to achieve greater intensities.
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, site of Earth’s most energetic monsoon system, but monsoons 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 Regional & Global Model Analysis (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 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.
William R. Boos
University of California
The Regional & Global Model Analysis (RGMA) Monsoon Extremes team was supported by the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, under Award Number DE-SC0019367. The collaboration also used support from National Aeronautics and Space Administration (NASA) grants NNX17AG72G and NNX16AD83G; National Science Foundation (NSF) award number 1606296; and the Earth System Science Organization, Ministry of Earth Sciences, Government of India (Grant nos. IITM/MM-864 II/Univ California USA/INT-3 and MM/SERP/ FSU/2014/SSC-02/002) under its Monsoon Mission.
Karmakar, N., Boos, W.R., Misra, V. “Influence of intraseasonal variability on the development of monsoon depressions.” Geophysical Research Letters 48(2), e2020GL090425 (2021). [DOI:10.1029/2020GL090425]