Impact of Atmospheric Intraseasonal Oscillations on the Indian Ocean Dipole during the 1990s*

Effects of atmospheric intraseasonal oscillations (ISOs) on the Indian Ocean zonal dipole mode (IOZDM) are investigated by analyzing available observations and a suite of solutions to an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Data and model solutions for the period 1991–2000 are analyzed, a period that includes two strong IOZDM events, during 1994 and 1997, and a weak one, in 1991. Both the data analysis and model results suggest that atmospheric ISOs play a significant role in causing irregularity of the two strong IOZDM events and the premature termination of the weak one. Of particular interest is a basinwide, wind-driven oceanic resonance with a period near 90 days, involving the propagation of equatorial Kelvin and first-meridional-mode Rossby waves across the basin. Before the onset of the strong 1997 dipole, wind variability had significant power near 90 days, and the resonance was strongly excited. Associated with the resonance was a deepened thermocline in the eastern basin during August and early September, which reduced the upwelling in the eastern antinode region of the IOZDM, thereby delaying the reversal of the equatorial zonal SST gradient—an important indicator of a strong IOZDM—by over a month. A similar deepened thermocline in the eastern basin also contributed to the premature termination of the weak 1991 dipole. During the 1994 IOZDM, the winds had little power near 90 days, and the resonant mode was not prominent. The ISOs influenced the IOZDM through both surface fluxes and thermocline variability. They enhanced warming in the western antinode region during October, the peak phase of the IOZDM, intensifying its strength. During November, strong winds significantly cooled the western and central basin through upwelling and surface fluxes, cooling SST there and contributing to the early and quick termination of the 1994 event.