Temperature oscillations in the upper thermocline region - A case study on internal waves off Kalpeni island in the southern Arabian Sea

Abstract

Temperature data were collected for 12 h in the thermocline region at an oceanic station off Kalpeni island (11-degrees 15'N and 69-degrees E) from the stationary ship using a thermistor chain. These data were Fast Fourier Transformed (FFT) to examine the spectral characteristics of the temperature oscillations. The power spectra of temperature fluctuations at 11 depths in the upper thermocline from 80 to 100m with 2m interval, were computed for studying the short period internal waves. Power spectral density was higher at 90m at all frequencies compared to that at above or below this depth. An abrupt change in the degree of coherence occured at 88m depth, in the sense that the oscillations above 88m showed very low degree of coherence, while below 88m depth the oscillations show very high degree of coherence. The low frequency oscillations (0.01667 to 0.2167 cpm) had higher coherence and very small phase shift whereas, high frequency oscillations (> 0.2167 cpm) had rather poor coherence and a large phase difference. Analysis of the coherence relations of the power spectral densities showed that the internal waves of periods longer than 10 min had higher coherence below 88m depth. The energy density spectra revealed a relation of f-2.7 fall off.