National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2021

Biweekly mixed Rossby‐Gravity waves in the equatorial Indian Ocean

Pujiana, K., and M.J. McPhaden

J. Geophys. Res., 126(5), e2020JC016840, doi: 10.1029/2020JC016840, View online (2021)

This study describes biweekly (defined as periods between 10 and 15 days) ocean variability in relation to equatorial wave theory using a comprehensive set of moored ocean current velocities as well as satellite-retrieved oceanic and atmospheric parameters in the Indian Ocean. Moored biweekly current velocities exhibit the meridional structure of mixed Rossby-gravity waves: Symmetric for meridional velocity and antisymmetric for zonal velocity around the equator with a decay scale of 228–309 km. The biweekly meridional velocity along the equator has average zonal and vertical wavelengths of 3,430 ± 550 km and 0.76 ± 0.15 km, respectively, with phase propagating westward at speeds of 2.8–3.1 m s−1 and upward at 54–56 m day−1. Conversely, wave energy is inferred to propagate eastward and downward. The range of best fit constants (1.4–2.0 m s−1) that result from the linear separation of horizontal and vertical wave structures suggests the second baroclinic wave mode is dominant over other low baroclinic modes in accounting for the velocity variability. The biweekly waves are forced by surface winds; satellite-derived wind stress, sea level, and surface current anomalies, regressed against the first principal component of the biweekly moored velocities, illustrate the basin-scale structure and temporal evolution of the waves.

Plain Language Summary. An extensive set of moored ocean current velocities permits evaluation of the three-dimensional flow field properties, dynamics, and energetics of biweekly waves in the Indian Ocean. We determine, for the first time, the meridional structure of the biweekly flow field from the multiyear in situ measurements and demonstrate that it is dynamically consistent with that of Rossby-gravity waves, with zonal velocity being antisymmetric and meridional velocity being symmetric about the equator. Our analysis finds that the waves have a decay scale of 228–309 km away from the equator in the meridional direction, a zonal wavelength of 3,430 ± 550 km, and vertical wavelength of 0.76 ± 0.15 km. Phase propagates westward at speeds of 2.8–3.1 m s−1 and upward at 54–56 m day−1, while wave energy flux is eastward and downward. A combination of moored velocities and satellite-derived data illustrates the basin-scale structure and temporal evolution of the waves in the Indian Ocean.

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