Bimonthly Variability in Zonal Velocity at Middepths of the Equatorial Indian Ocean

Abstract Intraseasonal variability in zonal velocity at middepths (between 450 and 800 m) of the equatorial Indian Ocean is investigated using in situ velocity measurements for the period from 2014 to 2019 at 0°, 83°E and the output of a wind-forced ocean general circulation model (OGCM). The spectral analysis of observed zonal velocity indicates that the highest energy peak at middepths on intraseasonal time scales is at the period of about 57 days between 450- and 800-m depths. The OGCM is able to simulate the spectral peak qualitatively. The statistical analysis using model output shows that the meridional structure, zonal wavelength, and vertical wavelength of 57-day variability in zonal velocity are consistent with those of a free, first meridional mode Rossby wave in an ocean at a state of rest. Two possibilities are discussed as the energy source of middepth 57-day variability. The first is that variability near the surface propagates to middepths as a wind-forced linear Rossby wave. This possibility is partly supported by the analysis of wave rays, which connect the spectral peak of surface zonal velocity with the analysis region at the middepths. The second possibility is the supply of energy by nonlinear advection, which is supported by the results obtained from the zonal kinetic energy budget. It is found that the forcing by advection propagates to the west at a similar phase speed to that of zonal velocity, suggesting a resonant forcing. Significance Statement Previous studies examined intraseasonal variability in zonal velocity near the surface in the equatorial Indian Ocean; reported elevated energy at the periods of about 30, 60, 90, and 180 days; and proposed their generation mechanisms. In contrast, intraseasonal variability in zonal velocity at middepths has not been fully investigated. The current study examines zonal velocity variability in the central equatorial Indian Ocean below the pycnocline using in situ observations and model output and found that the highest energy peak on intraseasonal time scales at middepths is at the bimonthly period (about 57 days). The horizontal and vertical structures of detected variability are examined, and their energy source is discussed.