On the low-frequency variability in the Indian Ocean
Irina Sakova(1) , Richard Coleman(2) , and Gary Meyers(1)
CSIRO Marine and Atmospheric Research,
GPO Box 1538,
Hobart, Tasmania, 7001,
(2) University of Tasmania, Private Bag 78, Hobart, Tasmania. 7001, Australia
The satellite altimetry data available today, since the launch of TOPEX/Poseidon in August 1992, covers a time interval of more than 13 years. Such a long time series opens up new opportunities for analysing low-frequency processes in the ocean with periods of more than one year. In this work we present results for the Indian Ocean using spectral analysis and band-pass filtering of 1992-2004 sea surface height anomalies (SSH) from satellite altimeter data sets. Four dominant frequencies are identified at frequencies corresponding to a 6-month period and longer, separated by significant spectral gaps. Two of these frequencies constitute the well known semi-annual and annual signals, while the other two frequencies correspond to 18-20 months and more than 24 months. For each of these frequencies, the spatial distribution of the power spectral density of the SSH field is analysed. The results show westward and eastward propagation suggestive of Rossby and Kelvin waves; however, they also point to potentially new phenomena. The spatial distributions of the power spectral density provide important clues to the physics of the corresponding variability modes, such as where the signals are strongest. The spatial/temporal structure of identified variability modes is identified by inverse Fourier transforms of the sets of harmonics belonging to a particular band. One of the most interesting results of this analysis is the discovery of strong 18-20 month variations to the south-west of Sumatra that are opposite phase with sea level near the Cocos Islands. Another interesting phenomenon is propagation of the greater than 24 month signal from the Pacific to the Timor Sea and around Australia as far as the Great Australian Bight.