On the low-frequency variability in the Indian Ocean

This paper presents the results of an investigation of low-frequency variability in the Indian Ocean (IO) primarily using satellite altimeter sea surface height (SSH) observations and expendable bathy-thermograph (XBT) data. We found that in most regions of the IO the low-frequency part of the SSH spectra (corresponding to signals with periods of a few months or longer) is concentrated in four frequency bands separated by substantial spectral gaps. These bands correspond to periods of ap-proximately 6 months, 12 months, 18-20 months and more than 30 months (hereafter referred to as the 30-month band). For both 18-20-month and 30-month bands the spectral density shows a dipole-like pattern with some degree of similarity; however, analysis of the spatial-temporal evolution of these signals suggests that the 18-20-month sig-nal is an internal mode of the IO, while that of the 30-month component propagates from the Pacific Ocean, in particular with a much stronger signal during the period from 1998 to 2003.