Dual-Frequency Wind-Driven Mixed Rossby–Gravity Waves in the Equatorial Indian Ocean

Abstract

Abstract Frequency spectra of in situ meridional velocity measurements in the central equatorial Indian Ocean show two distinct peaks at “quasi biweekly” periods of 10–30 days. One is near the surface at frequencies of 0.06–0.1 cpd (periods of 10–17 days) and the other is in the pycnocline (∼100-m depth) at lower frequencies of 0.04–0.06 cpd (periods 17–25 days). Analysis of a wind-forced ocean general circulation model shows that variability in the two frequency bands represents wind-driven mixed Rossby–gravity waves. The waves share a similar horizontal structure, but the meridional scale of lower-frequency variability is about one-half of that of higher-frequency variations. Higher-frequency variability has its largest amplitude in the eastern basin while the lower-frequency variability has its largest amplitude in the central basin. The vertical wavelength of lower-frequency variability is smaller by a factor of 3–4 than that of higher-frequency variability. These results are consistent with expectations from linear mixed Rossby–gravity wave theory. Numerical simulations show that the primary driver of these waves is surface wind forcing in the central and eastern Indian Ocean and that dynamical instability does not play a major role in their generation. Significance Statement Spectra of meridional velocity in the central equatorial Indian Ocean from in situ measurements show two distinct peaks in the biweekly period band with different spatial structures. This study uses an ocean general circulation model to show that variability in these two bands is driven by surface winds that are themselves spatially structured. The variability in both period bands is consistent with linear mixed Rossby–gravity wave theory, but the spatial structures, including meridional trapping scale, vertical wavelength, and zonal distribution of energy, are very different.