海洋研究の歴史を語る 50周年記念コラム

2021.11.09 UP

Finding the Indian Ocean Dipole~From curiosity driven science to societal applications~

Swadhin Behera
Director, Application Laboratory

Many parts of the world are now facing a variety of extreme climate and weather events such as scorching heatwaves, widespread wildfires, persistent droughts and deadly torrential rainfall. In the summer of 2021, intense heat waves in Canada and USA, widespread wildfire in California, Greece and France, and torrential rains and floods including mudslides in Japan hit the headlines of newspapers across the globe. All these extreme events are likely to get worse as global surface temperatures continue to rise, according to the 6th IPCC report released in autumn 2021. While global warming is increasingly cited as a reason for such extreme events, these extreme events of shorter durations are actually linked to long term changes in climate through the intermediate process of climate variations - which is a natural variation of earth’s climate system on seasonal to multi-year timescales. The most prominent mode of such a climate variation is the El Niño/Southern Oscillation (ENSO) phenomenon observed in the tropical Pacific. This phenomenon has world-wide teleconnections leading to a cascade of weather and climate events in many parts of the world. For example, droughts and heatwaves in India, Indonesia and several other parts of the world are believed to be associated with the El Niño phenomenon. But El Niño and its counterpart La Niña are not the only players modulating the extreme climate and weather events. The Indian Ocean Dipole (IOD), the Atlantic Niño/Niña and several other modes of climate variations are also responsible for some of those extreme events. The IOD is linked to widespread forest fires and severe droughts in Indonesia and Australia and floods in East Africa and India besides many other climate extremes.

These climate phenomena are discovered while trying to understand some of the natural abnormalities in the ocean-atmosphere systems. For example, the El Niño was discovered while trying to explain the story of dead fishes in coastal waters off Peru around the Christmas time of some the years witnessed by local fishermen for centuries. The unusual warming of coastal water leading to devastating marine ecosystem there was later linked to the basin-wide variation in water volume, sea surface temperature (SST) and associated atmospheric pressure variation as a part of the ENSO cycle through an ocean-atmosphere coupled process famously known as the Bjerknes feed-back mechanism. Since then the ENSO is recognized as an ocean-atmosphere coupled phenomenon of tropical Pacific.

Few such stories are heard about any unusual behavior of the Indian Ocean. In fact, historically researchers believed it to be a passive ocean basin with no apparent processes leading to ocean-atmosphere interactions and generation of inherent coupled climate phenomena such as ENSO in the tropical Pacific. That misunderstanding was dispelled, however, following the discovery of the IOD, which was discovered while scientists at JAMSTEC led by Prof. Toshio Yamagata try to understand the unexpected hot summer in Japan and unusual cooling of the eastern tropical Indian Ocean during summer and fall of 1994. Such an unusual nature of the eastern Indian Ocean was also noticed by a few other researchers who were unaware of the coupled IOD phenomena at that time. For example, Dr. Gary Meyers of CSIRO, Australia exchanged information about eastern Indian Ocean cooling with Prof. Yamagata during a meeting in Brisbane. While Prof. Yamagata was trying to put all the pieces of the puzzle together, a new framework of climate research was initiated in Japan. As a part of the initiative, JAMSTEC (traditionally engaged in developing observational instruments, in situ ocean observations, and data analysis) began a new research center called the Frontier Research System for Global Change (FRSGC) together with the then space agency NASDA (presently JAXA) besides having two Japan-US bilateral research centers IPRC (International Pacific Research Center) at the University of Hawaii and IARC (International Arctic Research Center) at University of Alaska Fairbanks. This initiative stemmed from the discussions against the backdrop of the famous Kyoto climate summit held in December 1997, leading to the Kyoto Protocol, to understand and predict the climate variation and change. That addition in JAMSTEC’s research direction led to discovery of several ocean-atmosphere phenomena and helped in developing numerical model-based prediction and projection systems for climate variations and change.

In early 1998, just after the establishment of the FRSGC, I joined one of its four departments on the invitation of its director Prof. Toshio Yamagata together with two other Indian colleagues Drs. Vinayachandran and Saji Hameed. At that time, his department was focusing on understanding climate variations while another department headed by Prof. Syukuro “Suki” Manabe, who won the Nobel Prize for Physics 2021, focused on the modeling of climate change. In retrospect, I feel we were lucky to work in such an environment interacting with experts like Prof. Manabe. It was a wonderful time for young researchers like us at FRSGC. Working under the guidance of Prof. Yamagata we investigated the unusual 1994 event. During the investigations, we discovered an unusually weak equatorial Yoshida-Wyrtki jet, intense coastal upwelling off Java and Sumatra, strong southeasterlies, strong evaporation and unusually weak atmospheric convection over eastern Indian Ocean all of which contributed to the formation of a positive IOD during spring and summer of 1994. It was concluded that all those processes are part of a Bjerknes feedback mechanism in the Indian Ocean leading to the formation of the IOD. We published several research articles at that time discussing one or the other aspect of this newly found phenomenon and one of the articles written by Saji et al (1998), published in the journal Nature, has been cited more than 4000 times as of now.

The earlier miss-conception of a passive nature of the Indian Ocean affected the early research on IOD following the publication of our initial papers. Some studies suggested IOD to be a statistical artifact and questioned its independent existence in the absence of ENSO. This misconception was removed with several follow-up studies, which to a great extent supported by IOD’s own natural variability during the last couple of decades. We have seen IOD developments in opposite phases of ENSO as well as in absence of ENSO. For example, the positive IOD evolved together with several El Niños but also with several La Niñas (e.g. together with El Niño in 1997 as well as with La Nina in 2007). This cannot be explained physically if both phenomena have to be part of one process, namely the Walker circulation. Most importantly, many IODs formed in the absence of an ENSO, e.g. the positive IOD event of 2019 (Fig. 1), which I believe has settled the debate of the physical existence of IOD independent of ENSO.


Fig. 1 Anomalies of sea surface temperature and surface wind during August-November of 2019 depicting the occurrence of a positive IOD event in the tropical Indian Ocean. Adapted from Behera et al., (2020), “Tropical and Extratropical Air-Sea Interactions”, Elsevier (https://doi.org/10.1016/B978-0-12-818156-0.00001-0)

After its discovery, we realized that the IOD has global impacts through its direct and indirect connections to local weather and climate variations. Several studies showed the pathways (Fig. 2) of its teleconnections from Sumatra to Europe in the Northern Hemisphere and to La Plata basin in the Southern Hemisphere. Through changes in the atmospheric circulation, IODs affect rainfall variability in India during the Indian summer monsoon, the summer climate condition in East Asia, the East African rainfall, the Sri Lankan Maha rainfall, the Australian rainfall and even the rainfall over La Plata basin in far off South American Continent. Recent studies have shown the relatively stronger impacts of IOD on the stream flows in western part of Indonesia. Most importantly we found that IOD influences the Southern Oscillation, the atmospheric component of ENSO, thereby influencing the ENSO variability and its predictability.


Fig. 2. Schematic of the positive IOD related surface temperature and rainfall anomalies together with some of the teleconnection pathways during boreal summer. Blue (orange) color means colder (warmer) than normal temperature and clouds (hashed lines) indicate wetter (drier) than normal condition. Adapted from Yamagata et al., (2015), World Scientific Series on Asia-Pacific Weather and Climate, (https://doi.org/10.1142/9789814696623_0001).

After realizing its global teleconnections and associated socio-economic impacts, Prof. Toshio Yamagata directed the group for developing a global ocean-atmosphere coupled climate model to predict IOD, in collaboration with several European climate research centers (CMCC in Italy, LOCEAN in France and MPI in Germany) under an EU-Japan collaborative framework using the Earth Simulator. The support and involvement of CMCC director Dr. Antonio Navarra and Drs. Sebastien Masson, Silvio Gualdi, Simona Masina, Marco Giorgetta and several other colleagues from European climate centers in addition to JAMSTEC’s Dr. Jing-Jia Luo are noteworthy in the development of the SINTEX-F (SINTEX-Frontier abbreviated from the original SINTEX model developed in Europe and adopted on the Earth Simulator at FRSGC) model. The state-of-the-art SINTEX-F is a leading climate prediction model in the world now with a very high success rate of predicting climate phenomena like ENSO, IOD, subtropical dipoles and coastal Niños/Niñas as well as sea ice (https://www.jamstec.go.jp/e/about/press_release/20190225/).

Unlike ENSO the IOD predictability is affected by several processes and phenomena of the Indian Ocean basin, which is quite narrow compared to the tropical Pacific Ocean. Also, being landlocked to the north, the Indian Ocean experiences the seasonal monsoon winds that reverse directions twice in a year. In addition, the basin is dominated by Madden-Julian Oscillation and tropical cyclones. These limit the sustenance of linear dynamics and ocean signals affecting the predictability of IOD at longer lead times. Nevertheless, SINTEX-F has been successful in predicting the IOD, sometimes even one year ahead (2019 event is the most recent example; https://www.jamstec.go.jp/e/about/press_release/20200406).

Over the years, our curiosity driven analyses of some of the unusual patterns of sea surface temperature around the world oceans have led to the discovery of several other climate phenomena; the subtropical southern Indian Ocean dipole (SIOD), the Ningaloo Niño/Niña (coastal Niños/Niñas off Western Australia), the California Niño/Niña, the Dakar Niño/Niña, the Chile Niño/Niña in addition to another mode of climate variation in the tropical Pacific now popularly known as the ENSO Modoki (https://www.jamstec.go.jp/apl/ertdf2-1405/index_en.html). These climate phenomena influence the regional climate in adjacent continental regions besides the regional oceanic processes. Therefore, by predicting those phenomena using SINTEX-F and using that information we started developing climate applications for societal benefits. A new laboratory was developed from that perspective.

The Application Laboratory in JAMSTEC was established in 2009 to apply climate predictions for societal benefits. After its formation, several application studies were developed together with domestic and international collaborations. One such project was developed under the framework of SATREPS jointly supported by Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA) to develop application studies based on prediction of climate variation in the southern African region together with several institutions from South Africa. Since subsistence farmers in many parts of southern Africa are highly dependent on nature, they are extremely vulnerable to the risks of climate extremes. Therefore, the first SATREPS project, led by Prof. Toshio Yamagata from JAMSTEC and Prof. George Philander of Princeton University, visiting University of Cape Town at that time, aimed to clarify mechanisms of long-term modulation as well as generation of climate modes, such as the SIOD, and their influences on the regional climate and crop production of southern Africa. By analyzing prediction data together with in situ observations, we not only deepened our understanding of predictability of climate variations but also improved parameterization schemes for cloud and precipitation processes to reduce model biases (https://www.jamstec.go.jp/apl/satreps_sa/e/index.html).

In a subsequent SATREPS project supported by Japan Agency for Medical Research and Development (AMED) and Japan International Cooperation Agency (JICA) besides Applied Center for Climate and Earth System Science (ACCESS) and several other centers from South Africa, the climate predictions were used in human health applications. The aim of this project was to apply the climate prediction system developed in the previous project for developing an infectious disease early-warning system (iDEWS) by incorporating climate prediction. This project, in which I was a co-PI, was jointly developed by Institute of Tropical Medicine in Nagasaki University, JAMSTEC, ACCESS and a few other institutes/universities in South Africa (https://www.jamstec.go.jp/apl/i-dews/index_en.html). Under the project, a malaria early warning system was developed and has been successfully implemented through the establishment of an iDEWS bureau in South Africa.

Curiosity driven fundamental research often leads to new research areas and useful applications in later years, though appears useless initially. The mRNA research of Dr. Katalin Kariko is a good example of this. The technology she developed was long ignored but became the savior of the world providing a platform for quick vaccination against the COVID-19. In the Application Laboratory at JAMSTEC, we have discovered several climate phenomena and underlying processes out of our curiosity to understand unknown sea surface temperature patterns in global oceans. Those fundamental research in turn led to societal applications in agriculture, health and water management, successes of which encourage us to continue in the direction of discovering climate processes and develop further applications for the benefit of the society.