DATE2021.10.13 #Press Releases

Identified the "trigger" for the "Madden-Julian Oscillation."

Disclaimer: machine translated by DeepL which may contain errors.

-Atmospheric waves over the equator are the key to the development of giant cloud swarms that affect weather around the world.

Japan Agency for Marine-Earth Science and Technology

Ochanomizu University

The University of Tokyo

Summary

Daisuke Takasuga, Postdoctoral Researcher at the Research Center for Environmental Change Prediction, Global Environment Division, Japan Agency for Marine-Earth Science and Technology, Tsubasa Kamiyama, Assistant Professor at Ochanomizu University, Hiroaki Miura, Associate Professor, and Tamaki Suematsu, Project Associate Professor at The University of Tokyo, have analyzed how the Madden-Julian Oscillation (MJO), which is observed in the tropics as a giant cumulus cloud swarm, occurs in the tropical Indian Ocean. As a result of analyzing how the Madden-Julian Oscillation (MJO) occurs in the tropical Indian Ocean, they discovered for the first time that the MJO is triggered by the amplification of atmospheric "waves" (periodic atmospheric fluctuations) traveling at an altitude of about 10 km above the surface.

The MJO is not only a major factor influencing daily weather in the tropics, but is also related to the onset and termination of the El Niño phenomenon, which causes abnormal weather on a global scale, and to the occurrence of tropical cyclones in the mid-latitudes, including Japan. On the other hand, the formation of cloud formations, which characterize the MJO, is a complex interplay of water vapor, temperature, and wind fluctuations, and a clear understanding of these processes has not yet been achieved.

In this study, we proposed a completely new way to explain the occurrence of the MJO by focusing on wind fluctuations in particular through the analysis of data obtained from in-situ observations of the MJO and simulations to calculate tropical atmospheric motion. Specifically, the study revealed for the first time that mixed Rossby-gravity waves, atmospheric waves with a period of several days observed far above the tropical Indian Ocean prior to the onset of the MJO, are transmitted downward into the atmosphere when they are amplified over the western Indian Ocean, and as a result, wind fluctuations are promoted near the ground, which generates the cloud swarms of the MJO in the Indian Ocean. This has been clarified. They also showed that the trigger of this process, the "amplification of mixed Rossby-gravity waves over the western Indian Ocean," is caused by the influence of atmospheric circulation (Walker circulation) that exists over the Indian Ocean throughout the year.

Since the MJO itself is a 1-2 month cycle phenomenon dominated by cumulonimbus clouds, the interpretation of the MJO has tended to focus on the fluctuations in the lower layers of the atmosphere, where cumulonimbus clouds begin to form, on a scale of a few dozen days. This finding is the first demonstration of the existence of a species that generates the MJO in the far sky. This finding is expected to be useful not only for forecasting the timing of MJO occurrence, but also for evaluating and improving the reproducibility of simulations used for weather forecasting and climate prediction.

Figure: Satellite image of the Madden-Julian Oscillation (MJO) (December 29, 2006 image: prepared according to the procedure shown in Miura et al. [2007, Science]). A huge cloud swarm associated with the MJO, which originally originated in the western Indian Ocean, is approaching Indonesia from the eastern Indian Ocean and then moving eastward toward the Pacific Ocean. The green star near the equator on the left edge of the image indicates the location where the field observation data used in this study was acquired (Gan Island).

These results were published in Geophysical Research Letters, a journal published by the American Geophysical Union, on October 13 (Japan Standard Time). This work was also supported by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (20J00605, 20H05728, 19K23460, 20K14554, 16H04048, 20B202, 20H05729, 21K13991).

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