The Interannual Variation of Intraseasonal Oscillation Linked with the Indian Ocean Dipole Mode

Abstract

Influence of the Indian Ocean Dipole Mode (IOD) on the interannual variation of the activity of intraseasonal oscillation (ISO) on submonthly (6-30-day) time scales during boreal fall is studied using ECMWF reanalysis data from 1958 to 2001. There is high negative correlation between the IOD and ISO activity over the southeastern Indian Ocean. The disturbances that cause this high negative correlation propagate westward slowly while maintaining a symmetric structure with respect to the equator and have the first baroclinic vertical structure. These disturbances are identified as convectively coupled equatorial Rossby waves. It is also shown that the interannual variation of ISO activity has a high correlation coefficient (0.850) with maximum negative relative vorticity anomalies at 850 hPa produced by cyclonic disturbances over the southeastern Indian Ocean, while it has a low coefficient (0.119) with the number of cyclonic disturbances. It is concluded that whether the equatorial Rossby waves produce strong cyclonic disturbances is a key factor determining the interannual variation of ISO activity over the southeastern Indian Ocean. The reason strong cyclonic disturbances are produced and their relation with the IOD are discussed in this paper.