Huge cumulonimbus clouds in the tropical Indian Ocean: caused by atmospheric waves in the sky

A research group composed of Postdoctoral Researcher Daisuke Takasuka of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Research Institute for Global Change Research, Research Center for Environmental Modeling and Application, Ochanomizu University Assistant Professor Tsubasa Kohyama, University of Tokyo Associate Professor Hiroaki Miura, and Specially Appointed Professor Tamaki Suematsu analyzed how the Madden-Julian oscillation (MJO), which is observed as a huge cumulonimbus cloud group in the tropical zone, occurs in the tropical Indian Ocean, and found that the amplification of atmospheric "waves" (periodic atmospheric fluctuations) traveling at an altitude of about 10 kilometers was the trigger.

The MJO is a remarkable meteorological phenomenon in the tropics. It is observed as a phenomenon where, after appearing as a huge cumulonimbus cloud group that stretches for thousands of kilometers horizontally, mainly in the Indian Ocean, its clouds move over the Pacific Ocean over tens of days at a speed of about 5 meters per second.

The clouds of the MJO cause heavy rainfall in the tropics, sometimes causing heavy rainfall disasters in the countries it passes along the way. In addition, changes in atmospheric flow throughout the tropical zone to balance the maintenance of the huge clouds of the MJO will affect the occurrence and termination of the El Nino phenomenon, which causes extreme weather on a global scale. Japan is no exception, and in the past, MJO had caused abnormally high temperatures and heavy snowfall. In contrast, the formation of cloud groups that characterize the generation of MJO is realized by the complex relationship of fluctuations in water vapor, temperature, and wind; however, the development process has not been clarified.

The research group realized that the mixed Rossby gravity wave could play a decisive role by individually and carefully observing multiple data and reports when observing MJO in the field, and through analysis of the data obtained during field observations of MJO and simulations to calculate the motion of the tropical atmosphere, they focused on wind fluctuations and proposed a completely new perspective to explain the occurrence of MJO. Specifically, the mixed Rossby gravity wave, which is a multi-day atmospheric wave observed above the tropical Indian Ocean before the occurrence of MJO, is transmitted to the lower part of the atmosphere when it is amplified over the western Indian Ocean. As a result, for the first time, they clarified the perspective that wind fluctuations are promoted near the ground and MJO clouds are generated in the Indian Ocean. Moreover, they showed that the amplification of the mixed Rossby gravity wave over the western Indian Ocean, which triggers this process, is triggered by the influence of the Walker circulation that exists over the Indian Ocean throughout the year.

Takasuka commented on the groups result, saying, "We clarified the mechanism of MJO generation from a new aspect based on model experiments and analysis of one observation case; however, in the future, we will perform detailed analysis and simulation for various MJO cases. By doing so, I would like to clarify under what conditions the proposed mechanism is likely to function, which will lead to better predictions of MJO and related meteorological and climatic fields."