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OST-2A: Climate Change
INDIAN OCEAN CLIMATE VARIABILITY: ITS DECADAL MODULATION AND LINKS TO THE PACIFIC
H. Annamalai1, J. Potemra1, R. Murtugudde2 and J. P. McCreary1
 
1International Pacific Research Center, University of Hawaii Honolulu, Hawaii, USA
hanna@soest.hawaii.edu
 
2ESSIC, University of Maryland College Park, Maryland, USA
 
Recent studies suggest the existence of a natural mode of coupled climate variability in the Indian Ocean. This mode of interannual variability is known as the Indian Ocean Zonal Mode (IOZM) or alternatively referred to as the Indian Ocean Dipole Mode. In the present research, first we diagnose atmosphere and ocean model assimilated data sets for the period 1950-99. We demonstrate that there is no clear climate signal at interannual time scales in the eastern equatorial Indian Ocean that is significant above a red noise spectrum. We show that the IOZM is particularly active in the 1960s and 1990s while in other decades the signal is weak or absent. The pronounced activity in these two decades is related to the decadal variability (climate shift) present in the Indo-Pacific basins. The hypothesis proposed is the following: decadal variability acts to raise (lower) the thermocline in regions where the mean thermocline is deep, thereby strengthening (weakening) the existence of unstable modes of coupled climate variability, in our case leading to strong (weak) IOZM events. This hypothesis is tested with a series of sensitivity experiments using an ocean general circulation model. Finally, we show that the IOZM is not necessarily an Indian Ocean phenomenon but rather is a part of the Indo-Pacific warm pool variation that is strongly linked to the eastern Pacific in the 1990s but not in the 1960s.
 
OST-2A: Climate Change
THE INDIAN OCEAN DIPOLE INFLUENCE ON DARWIN PRESSURB AND SOUTHERN OSCILLATION
Swadhin K. Behera1 and Toshio Yamagata1,2
 
1IGCR/FRSGC
Showa-machi, Kanagawa, JAPAN
 
2Graduate School of Science, The University of Tokyo, Tokyo, JAPAN
behera@jamstec.go.jp
 
The Southern Oscillation (SO) is traditionally known as a seesaw in the atmospheric sea level pressure between the region that covers from the central to the eastern tropical Pacific and the region covering the tropical Asia-Pacific. Historically, the SO study has its root in the predictability of the Indian summer monsoon (Walker 1924). The interesting seesaw in the atmospheric pressure field was later linked to the well-known SST variability in the eastern tropical Pacific Ocean by Bjerknes (1969). This combined phenomenon, commonly known as the El Nio-Southern Oscillation (ENSO), is identified as the most dominant climate signal that affects the weather and climate worldwide. The Bjerknes hypothesis in light of these recent studies is that ENSO evolves as a self-sustained coupled oscillation in which warm/cold anomalies of SST in the tropical Pacific cause the anomalous strengthening/weakening of the trade winds that, in turn, drive the ocean currents associated with the SST anomalies. Although the oceanic component of ENSO is a key element that determines the SO, influences from other climate variations outside of the Pacific cannot be neglected in this regard (Zhang et al. 1997). In a previous study, Deser and Wallace (1987) showed that the major negative swings of the Southern Oscillation that occurred in 1963 and 1977 were not accompanied by strong El Nio events. The motivation of the present short article is due to the fact that the Indian Ocean Dipole (IOD) discovered recently (Saji et al. 1999; Webster et al. 1999; Behera et al, 1999; Iizuka et al. 2000) has suggested a new role of the Indian Ocean in determining the sea level pressure oscillation over the Asia-Pacific sector.
 
Statistical correlation between the IOD index and the global sea level pressure anomalies demonstrates that opposite polarity loadings are distributed between the western and the eastern parts of the Indian Ocean. The area of positive correlation in the eastern part even extends to the Australian region and the IOD index has a peak correlation of about 0.4 with the Darwin pressure index that is the western pole of the Southern Oscillation. This relation has undergone interdecadal changes; in last 50 years the correlation is highest during the most recent decade of 1990-99 and weakest during 1980-89.
 
OST-2A: Climate Change
THE INDIAN OCEAN DIPOE
Toshio Yamagata
 
Frontier Research System for Global Change Yokohama, JAPAN
yamagata@eps.s.u-tokyo.ac.jp
 
The El Nio/Southern Oscillation (ENSO) phenomenon has been discussed for more than a century and is now recognized as the important tropical ocean-atmosphere coupled phenomenon after the active TOGA decade. Recently, the Indian Ocean Dipole (IOD) phenomenon has been catalogued as another important manifestation of the tropical air-sea interaction. The impact of the IOD is not limited to the equatorial Indian Ocean; it influences the Southern Oscillation, the Indian summer monsoon rainfall and even the summer climate condition in Asia. The robust nature of the IOD is simulated using high-resolution coupled GCMs that also resolve ENSO in the Pacific. However, because of its inevitable short history of research, several issues related to its existence/nonexistence and dependence/independence of ENSO have been raised in the climate research community.
 
In this paper, using multiple datasets such as GISST 2.3b, sea surface height from SODA, rainfall and atmospheric data derived from NCEP-NCAR reanalysis, we demonstrate that the Indian Ocean Dipole (IOD) is a physical mode involving dynamics of the tropical Indian Ocean, and not an artifact. We believe that the concept of IOD has raised a new question, and a new possibility to regard the old problems such as the relation between the Indian summer monsoon rainfall and El Nio from a new angle, and to make a real advance in the predictability.







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