Time variability of Indian Ocean equatorial currents

As part of the INDEX program we have obtained two records from current meters each of 6-month duration, from a single depth (500 m) and location (0°, 55°40′E) in the equatorial Indian Ocean. Spectral analysis for periods shorter than 500 h reveals features consistent with other equatorial measurements in the Indian and Atlantic oceans, features which follow from the disappearance of the mid-latitude inertial peak and its replacement by a sum of equatorially trapped waves. Certain comparisons with a model spectrum of such waves due to Eriksen ( Journal of Geophysical Research, 85, 3285–3303, 1980) are noted. Longer-period variations cannot be analyzed statistically with such short records. Nevertheless, it is interesting that in the spring of two different years the zonal velocity changes abruptly (in 1 to 2 weeks) from near zero to 25 cm s −1 westward. There is a clear need for much longer measurements to examine the possible relationship between this variability and seasonal forcing due to the monsoons. Spectra of the higher frequency motions are not discernably different before and after this transition.