TOPNEWS FRONTIER Newsletter No.12 Oct.2000

Relationship between Ocean Climate Changes in the Indian Ocean and Hot Summers

Dr.Yamagata Toshio Yamagata
Program Director, Climate Variations Research Program

By October of this year, it will be three years since the Frontier Research System for Global Change was established. I would like to introduce the hottest topic in the ocean climate research related to the tropical Indian Ocean, which was undertaken by the Climate Variations Research Program over these past three years.

This summer was very hot. Although it was not as hot as the summer of 1994, it is certain that the record will remain as one of the hottest summers. The tropical Pacific Ocean was still under the influence of last year's La Nina until the beginning of this summer, and it was expected as usual in La Nina years that four seasons would appear clearly but such a hot summer was not expected. As of September, the warm water anomaly in the western tropical Pacific has crossed the date line and extends into the central Pacific. If cold surges that blow from this winter through early spring next year bring westerly wind bursts in the western tropical Pacific, it would not be surprising to have a new El Nino in the first year of the new millennium. Next year, we will probably experience a different kind of summer. As human beings, we do not physically feel climatic variations as anything more than dramatic daily changes in weather. It is without doubts that more and more research will be done to fill the scale gap in understanding unusual weather and climatic variations.

The P-J pattern that appears in the atmospheric pressure distribution, which was proposed by the late Professor Nitta of the University of Tokyo, and the variations of its pattern are important in order to understand the summer in Japan. This P-J pattern, is made up of an atmospheric updraft above the ocean near the Philippines and the atmospheric downdraft to the north near Japan, and as what happened this summer, is affected by the sea surface temperature in the tropical western Pacific Ocean. However, our recent research indicates that it is also influenced by the sea surface temperature in the further distant tropical Indian Ocean. It is thought that this is what happened in 1994. On the other hand, it has been thought that the sea surface temperature variations in the Indian Ocean are determined almost completely by the phenomena in the Pacific. However, the major contribution made by my coworkers, Drs. Vinayachandran, Saji, and Behera, has shown that an independent coupled ocean-atmosphere phenomenon may exist even in the tropical Indian Ocean, and it is clear that this also affects the western Pacific and east-Asian summer climates.

This coupled ocean-atmosphere phenomenon in the Indian Ocean comes from the sea surface temperature variation, which shows the east-west dipole pattern, and the variation in the tropical zonal winds that connect the zonal temperature gradient. In other words, it is just like an El Nino in the Pacific although it occurs in the Indian Ocean. Last fall, our epoch-making article on the Indian Ocean Dipole (IOD) was published in the British journal, Nature. Since then, the IOD was discussed widely at CLIVAR/WCRP and other international conferences as providing a new viewpoint, and it has been very encouraging for our new research group to be recognized by world climate researchers. And just as there are an infinite number of research articles on El Nino and Southern Oscillation, there will no doubt be wide-ranging research into this newly recognized phenomenon, including into longer-term climate variation and paleoclimate fields.

It will be possible to predict those climate changes in the tropical Indian Ocean and the atmosphere if a system can be completed for assimilating, in a real time basis, sea level data from ocean satellites, sea water temperature and salinity data obtained by TRITON Buoys, and XBT and XCTD data collected by the Volunteer Observation Ships (VOS), into a coupled atmosphere-ocean model. Countries surrounding the Indian Ocean and East and Far East Asia are the most densely populated in the world; this climate prediction system would have, therefore, an unlimited impact on society. Furthermore, if the Pacific, Indian, and Atlantic Ocean prediction systems were able to be combined, a real Tropical Ocean-Global Atmosphere (TOGA) program would be completed. This would make a major contribution to the international community, even just in agriculture, fisheries, water resource management, disaster prevention, national land planning, and health applications. The successful hindcasting of the past IOD index using general ocean circulation models, based on our cooperative research with Dr. Vinayachandran now at CAS/IIS in India (Fig. 1), and the successful simulation of the IOD phenomenon in a coupled atmosphere-ocean general circulation model in our cooperative research with Dr. Matsuura and Mr. Iizuka (also working in the Frontier Research System for Global Change) of the National Research Institution for Earth Science and Disaster Prevention have propelled us greatly to the above goal.

The Climate Variations Research Program has developed models of various levels and, with the leading young researchers from Japan and overseas gathered in our research program, we share not only research ideas but also multi-cultural richness on daily basis. We will continue our efforts hand-in-hand to build a unique center of excellence that produces cosmopolitan researchers that contribute greatly from Asia to world climate research community

Fig. 1 Dipole mode index based on an ocean general circulation model (blue), which corresponds with the black line denoting the index based on the observational data. The red line shows similar index calculated using the sea level data from the TOPEX/POSEIDON satellite.

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