RESULTS Frontier Newsletter No.14 Mar.2001

Introduction of the research results in the FRSGC
In the FRONTIER Newsletter, we introduce research results of the FRSGC upon occasion. This time, we introduce an example of research results contributed by a researcher, Dr. Swadhin K. Behera, and co-author, Program Director, Dr. Toshio Yamagata, of the Climate Variations Research Program.

Subtropical dipole mode events in the Indian Ocean

Swadhin Behera(Climate Variations Research Program) and Toshio Yamagata(Program Director,Climate Variations Research Program)

Fig. 1a
Composite of SST and wind anomalies for January-March of the six strong positive subtropical dipole events during 1968, 1974, 1976, 1981,1982 and 1993. SST anomalies are derived from GISST data and wind anomalies are derived from NCEP-NCAR reanalysis data.

Climate variability in the Pacific-Indian sector has potential societal impact on the large human population living in the surrounding regions.

One of the major goals of the Climate Variation Research Program is thus to identify new and emerging climate signals in this sector. Such a pursuit of scientific endeavor in the ever increasing world of climate research is extremely challenging. In 1999, we discovered the coupled ocean-atmosphere phenomenon in the tropical Indian Ocean. This well-known Indian Ocean Dipole ever since its discovery has drawn much attention from the researchers world-wide.

In continuing our pursuit for unraveling the nature's mysteries, we recently discovered another independent climate signal in the southern Indian Ocean. In a recent article (Behera and Yamagata; GRL, 2001) we describe this climate signal as the subtropical dipole mode events that generally evolve during the austral summer with opposite polarity in the SST anomalies of the subtropical region. The evolution of SST anomalies during subtropical dipole events is found to be linked with the strengthening/weakening of the atmospheric subtropical high normally observed in the sea level pressure.

In a positive dipole event, which is associated with above normal rainfall in many parts of southern Africa, the subtropical high is strengthened along with the strengthening and westward shift of the west coast trough observed in the western Australia during summer. The resultant stronger than normal winds off the coast of Australia cause cooling of SST in the southeastern part (Fig. 1a) mainly through the enhanced evaporation.

Fig. 1b Same as Fig. 1a but for the anomalies of the coupled general circulation model.

On the other hand reduced evaporation due to reduced wind speed allows increased influx from the atmosphere thus warming the southwestern part. The increased moisture advection from the southeastern part by the stronger easterlies then helps in the enhanced convective activity over southern Africa.

Though changes in heat fluxes could contribute significantly, those changes can not explain completely the SST anomalies associated with the subtropical dipole. This suggests the role of ocean dynamics and mixed layer processes in the evolution of the SST anomalies. However such an investigation is difficult in the southern Indian Ocean because of data sparseness. Fortunately, the subtropical dipole is remarkably well simulated in a coupled ocean-atmosphere general circulation model (Fig. 1b).

In a cooperative research with Dr. Matsuura and Mr. Iizuka (also affiliated with the Frontier Research System for Global Change) of the National Research Institution for Earth Science and Disaster Prevention, we are now investigating the role of oceanic processes in the formation of anomalous SST by analyzing subsurface data from the coupled model simulation.

Fig. 2 SST and wind anomalies for January-March for the year 1981.

The subtropical dipole mode in the Indian Ocean is generally accompanied by similar dipole mode events in the subtropical southern Atlantic and Pacific Oceans, for example during February of 1981 (Fig. 2).

Using the coupled model results and the NCEP-NCAR reanalysis data, we are also analyzing the possible connections among the dipole mode events in three basins. Understanding such processes will not only increase our knowledge of the otherwise less understood climate variability in the Southern Ocean but also enhance predictability of our complex climate system.

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