A current meter mooring with current meters at 135m, 280m, 983m, 1996m and 3995m was deployed at 93ZE in the eastern equatorial Indian Ocean during February – December, 2000 with the objective of understanding the intra-seasonal variability of the currents in this region. The analysis of the current data showed the mean zonal currents of magnitude, 3.7 cm/sec. towards east and the mean meridional current of magnitude, 1.2 cm/sec towards south with maximum recorded speeds of about 40 cm/sec. However, the interesting feature of the current variability was a 10-20 day oscillation in the observed current components. This periodicity was found to coincide with a similar periodicity in the wind field over this region.
The structure of the vertical modes was investigated by performing EOF analysis on the observed currents. The first three EOF modes of both u and v - components explain 90-95% of the variability. The first baroclinic mode had the maximum amplitude around 135 m for both the components. However, the reversal of direction was around 2000m for the zonal current and around 800m for the meridional current. It is not clear whether the shallow depth of reversal of the meridional flow is related to the stronger 10-20 day oscillations of the v-component that is found to be influenced by the winds. The third mode of both u and v components had less magnitude but opposite directions.
Separation of barotropic and baroclinic modes was also done by vertically averaging the observed currents at every time step (daily) to get the barotropic time series. The deviation of this vertical average from the observed currents then represents the baroclinic part. The zonal barotropic currents were westward during February - April and eastward during May – September. These flow directions coincide with the mild easterly / no winds during the spring and the Indian summer monsoon (SW) winds during summer respectively. In addition to the 10-20 day oscillation, a low frequency oscillation of 3-4 months is also visible in the zonal barotropic component. The meridional barotropic time series showed a stronger 10-20 day oscillation. The baroclinic time series, on the other hand, was significant only in the top three current meters (upper 1000m) and showed a 10-day periodicity both in the zonal and meridional components. |
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