Interannual variability in the Indian Ocean using altimeter and IX1-expendable bathy-thermograph (XBT) data: Does the 18-month signal exist?

The dominant frequency bands in altimeter sea surface height (SSH) variability over the whole Indian Ocean and in temperature profiles near the Sumatra-Java coast are identified and analysed using spectral analysis techniques. We find that in most regions of the Indian Ocean, the low-frequency part of the SSH spectra (corresponding to signals with periods from six months to six years) is concentrated in five frequency bands separated by substantial spectral gaps: semi-annual, annual, 18-20 months, 3 years, and 4-6 years. The existence of semi-annual, annual, 2-3-year, and 4-6-year periodical signals is well known; however, the 18-20-month signal has not previously been described. Further investigation of temporal and spatial characteristics of this later signal point to its relationship with the Indian Ocean dipole events: the signal is particularly strong between 1994 and 2000; it develops near the Sumatra coast and propagates to the Bay of Bengal and into the Indonesian Throughflow. Copyright 2006 by the American Geophysical Union.