Mixed Rossby gravity waves at mid-depths of the equatorial Indian Ocean

This study examines mixed Rossby gravity (MRG) waves at mid-depths (500-2000 m) of the Indian Ocean, using multi-year velocity time series obtained from current meter moorings at 77°, 83°, and 93°E along the equator over the period 2000-2019. These data are analyzed in combination with a high-resolution wind-forced ocean general circulation model. The spectrum of observed meridional velocity showed elevated energy over a wide range of periods from about 10 to 100 days with the spectral peak at a period of about 30 days. The model was able to simulate the characteristics of the observed spectrum. Further diagnostics determined that the detected variability is generally consistent with theoretical MRG waves in a resting ocean. Statistical analysis and a model sensitivity experiment identified distinct variations at three periods, where meridional velocity has sizable energy. The 14-day variability is wind-driven and has a long zonal (∼3300 km) and vertical wavelength (∼4200 m). The 28-day variability is excited by the dynamical instability of the background flow in the equatorial western Indian Ocean near the surface and propagates to the study area. It is characterized by a shorter zonal (∼1100 km) and vertical wavelength (∼2800 m) compared to 14-day variability. The 43-day variability has a zonal wavelength (∼820 km) comparable to the 28-day variability, but does not show the tendency of propagation and is likely generated in situ through nonlinear interactions. These results show that various processes contribute to the excitation of MRG waves at mid-depths of the Indian Ocean.