A biweekly mode in the equatorial Indian Ocean

The National Institute of Oceanography, Goa deployed moorings with several subsurface current meters at 0◦, 93◦E (in February 2000) and 0◦, 83◦E (in December 2000) in the eastern Indian Ocean. Observed meridional current at all depths has a 10-20 day (or biweekly) variability that is distinct from longer period (20-60 day) subseasonal variability. Lags between different instruments suggest the presence of groups of westward and vertically propagating biweekly waves with zonal wavelength in the range 2100 to 6100 km. We use an ocean model forced by high resolution scatterometer wind stress to show that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by subseasonal variability of winds. We demonstrate that quasi-biweekly fluctuations of surface meridional wind stress resonantly excite ocean waves with westward and upward phase propagation, with typical period of 14 days and zonal wavelength of 3000-4500 km. The biweekly wave is associated with fluctuating upwelling/downwelling in the equatorial Indian Ocean, with amplitude of 2-3 meters per day located 2◦-3◦ away from the equator. Possible reasons for eastward intensification of biweekly energy is discussed.