Annual Rossby Waves Below the Pycnocline in the Indian Ocean

Abstract

Annual variability in pressure and horizontal velocity below the pycnocline of the Indian Ocean is examined using in situ observations obtained from Argo floats and conductivity-temperature-depth sensors. Pressure and velocity at 1,000-m depth show significant annual variability in the Arabian Sea and between 10°S and 20°S in the South Indian Ocean. Wavenumbers were estimated by fitting a straight line to annual harmonic phase by amplitude-weighted least squares fit. Results showed that vertical wavenumber tends to decrease poleward in both hemispheres, and vertical wavelength is more than tripled from about 5,000 m at 5°N/S to 16,000 m at 20°N/S. The inverse radius of deformation squared (defined as f 2 m 2 /N 2 ) does not show clear dependence on latitude, because poleward increase in the magnitude of the Coriolis parameter (f) compensates for poleward decrease in vertical wavenumber (m), and Brunt-Väisälä frequency (N) varies little meridionally in the analysis domain. According to the dispersion relation of quasi-geostrophic Rossby waves, these meridional structures in m and deformation radius result in steeper angle of ray trajectory at a higher latitude in the longitude-depth plane. Energy estimated from in situ observations shows a consistent pattern with this expectation. The line of constant phase of annual pressure harmonic is steeper in angle at higher latitudes, which is also expected from the uniform deformation radius and constant N. This result indicates that energy penetrates deeper at higher latitudes.