Seasonal and interannual variations of subsurface zonal currents in the eastern equatorial Indian Ocean

Variations of subsurface zonal currents in the eastern equatorial Indian Ocean are investigated by examining 6 years' data (December 2000 - November 2006) from acoustic Doppler current profiler (ADCP) mooring at 0S, 90E. The analysis indicates the presence of robust subsurface eastward currents during both boreal winter and summer. These subsurface eastward currents are generated by eastward pressure gradient associated with the equatorial wave dynamics. During boreal winter, the generation of eastward pressure gradient is caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterly winds. On the other hand, the downwelling Rossby waves generated by the reflection of the spring downwelling Kelvin waves in the eastern boundary as well as the upwelling equatorial Kelvin waves triggered by easterlies create an oceanic state that favours the generation of the eastward pressure gradient during boreal summer. In addition, these subsurface eastward currents reveal a distinct seasonal asymmetry. The maximum eastward speed of 63 cm/s is observed in April and secondary maximum of 49 cm/s is seen in October. The zonal transport per unit width within the depth where these subsurface eastward currents are defined exhibits similar variations: reaching maximum eastward transport of 35 m2/s in April and secondary maximum of 29 m2/s in October. Moreover, the subsurface eastward current during boreal summer undergoes significant interannual variations: it was absent in 2003, but it was anomalously strong during 2006.