The Rigakubu News

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El Nino's Brother - Dipole Mode for India

Tomomi Higashizuka, Associate Professor, Department of Earth and Planetary Science

The Indian Ocean Dipole Mode (IOD) phenomenon occurs in the tropical Indian Ocean due to the interaction between the atmosphere and the ocean. When an IOD occurs, sea surface temperatures in the tropical Indian Ocean are above normal in the western part and below normal in the eastern part. IODs are known to cause extreme weather events not only in the countries surrounding the Indian Ocean but also in Japan, and are thought to have contributed to the recent record warm winter in Japan from 2019 to 2020. Therefore, if we can accurately understand the mechanism of IOD and predict the occurrence of IOD with high accuracy in advance, we can take countermeasures to mitigate the effects of extreme weather events, but we do not yet have a complete understanding of the IOD.

Then, why do sea surface temperatures in coastal areas of Indonesia become colder than normal during IODs? To find out the cause, we need to simulate the ocean, reproducing the Indian Ocean as realistically as possible, and then investigate the exact heat budget of the ocean surface layer. SSTs fluctuate in response to heat exchange with the atmosphere immediately above it, heat transport by ocean currents, and vertical mixing. In the tropics, the water near the sea surface is warmer because it is strongly heated by the sun, but this effect diminishes with depth, so there is cold water below the surface layer with relatively low water temperature. Vertical mixing, in which the warm water in the surface layer is stirred with the cold water below it, results in a decrease in the surface water temperature, which contributes to the occurrence of IOD through the following two mechanisms.

First, during IOD, the southeast trade winds overhead are stronger and the stirring effect of the winds is enhanced, so the vertical mixing causes the surface water temperature to decrease more than usual. In addition, in the eastern part of the tropical Indian Ocean, cold water is deeper than in other tropical regions, but during the IOD, the cold water is lifted by the upward flow. More specifically, the effect of the earth's rotation transports warmer near-surface water toward the left side of the wind direction in the southern hemisphere, so when the southeast trade winds strengthen, more near-surface water is transported offshore from the Indonesian coastal region, and to compensate, cooler water with relatively lower water temperatures moves more upward As a result, the surface layer moves upward and downward. As a result, the surface layer is cooled more efficiently by vertical mixing.

Figure: Schematic representation of the role of vertical mixing in the development of the Indian Ocean Dipole Mode (IOD) event. In the top map, the locations where SSTs are cooler than normal with IOD are shown in cold colors, and the locations where SSTs are warmer than normal with IOD are shown in warm colors; the darker the color, the more pronounced the trend.

The combination of these two effects indicates that vertical mixing is responsible for the strong SST decrease in the coastal areas of Indonesia associated with IOD. It is expected that this improved physical understanding of IOD will contribute to improved prediction of extreme weather events in the months ahead.

The results of this study were published in M. Nakazato et al. Scientific Reports 11, 22546 (2021).

(Press release, November 25, 2021)

Published in the March 2022 issue of Faculty of Science News

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