Detecting "Fluctuations" in Ensemble Spaces Preceding Stratospheric Sudden Warmings

1. Key Points

• While the existence of the phenomenon of "stratospheric sudden warming" in which the temperature in the winter stratosphere rises by several dozen degrees over several days, and the effects of these phenomena on the local climate in which we live are known, predicting the occurrence of these events is difficult.

• Through the detection of ensemble "fluctuations" using "spread"—a statistic representing the size of the disparity between ensembles via ensemble atmospheric reanalysis data^※1—we have discovered that these fluctuations increase in a precursory manner several days before the occurrence of stratospheric sudden warming events.

• By detecting ensemble fluctuations just before stratospheric sudden warming events, it is possible to provide early warning information before these events.

2. Overview

Researchers Akira Yamazaki of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Assistant Professor Shunsuke Noguchi of Kyushu University have detected the occurrence of precursory "fluctuations" expressing uncertainty in atmospheric conditions a few days before two stratospheric sudden warming events—one in the Northern Hemisphere and the other in the Southern Hemisphere, in December 2018 and September 2019, respectively. It has been clarified that ensemble atmospheric reanalysis data can detect these fluctuations by using a statistical value called "spread." Stratospheric Sudden warming is a difficult phenomenon to be predicted, and attempts to clarify their mechanisms and predictability have gained attention. Detecting fluctuations just before the occurrence of these phenomena has the potential to provide new insights and clues that past research could not. The study successfully detected fluctuations occurring before actual stratospheric sudden warming events. This achievement was made possible by JAMSTEC's atmospheric ensemble reanalysis data that reconstructs weather phenomena occurred in the past years.

The result was published in the American Meteorological Society's Monthly Weather Review on October 18 (Japan time). The study was assisted by JSPS grants-in-aid (20H01976, 18K13617, 19H05702) and by the ArCS II project.