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Mid-depth Zonal Velocity in the Southern Tropical Indian Ocean: Striation-like Structures and their Dynamics


In this study, the upper ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the mid-depth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of mid-depth zonal velocity in the equatorial and southern tropical Indian Ocean. Specifically, the eastward time-mean flows are located at the equator, 2°S, 5°S, 8°S, 13°S, 16°S, 18°S−19°S, and 21°S−22°S, with a meridional scale of ∼300 km. The generation mechanisms of the striation-like zonal velocity structure differ between the near-equatorial and off-equatorial regions. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the high potential vorticity (PV) of Antarctic intermediate water and low PV of southern Indian Ocean Subantarctic Mode Water lead to strong baroclinic instability, which increases the eddy kinetic energy in the mid-depth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations.