An attempt has been made to identify tropical waves with periods less than 20 days in the Indian Ocean by spectrum analysis of radiosonde data for five stations, widely scattered over the Indian Ocean area. The data used is for the period from January to September, 1973, which was further divided into two five-month seasons, overlapping in May due to consideration of the monsoonal shift in the mean flow. Three spectral peaks in the 10-17 day, 6-7 and 4-5 day ranges have been detected. However, only the 10-17 day fluctuations seem to be prominent and appear consistently at all stations in both seasons. Cross-spectrum analysis reveals a basically in-phase structure in the vertical in the lower troposphere and reversed phase at upper levels. Evidences also indicate a northwest-southeast tilt in the Northern Hemisphere and a northeast-southwest tilt in the Southern Hemisphere. Inter-station cross-spectra and theoretical considerations suggest an estimated horizontal wavelength of approximately 3,000 km with a westward propagation direction. Due to the limited number of available stations and the quality of data, these results are considered tentative and further study in this area is suggested.

https://calhoun.nps.edu/bitstream/10945/17247/1/spectrumanalysis00mala.pdf

Jonathan Thomas. Malay

Papers

Spectrum analysis of tropical waves in the Indian Ocean.