Annual Rossby Waves Below the Pycnocline in the Indian Ocean

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 f2m2/N2) 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. Plain Language Summary Argo floats periodically move up and down between the sea surface and (nominally) 2,000‐m depth and provide valuable in situ measurements of ocean temperature and salinity with unprecedented density. This study collects and processes these observations and examined annual variability in the Indian Ocean. Results showed that energy, which is likely supplied by winds at the sea surface, propagates vertically from near the surface to the middepth, following the dynamics of Rossby waves. Vertical propagation was observed in the Arabian Sea and the tropical South Indian Ocean north of 20°S. Also, results showed that the angle of the propagation path of energy is steeper at higher latitudes, meaning that energy given by surface winds penetrates deeper at higher latitudes (down to 1,000 m at 5°N/S but to 2,000 m at 10°N/S). These results give a clue to understand how middepth currents are energized and eventually heat/material tran