Vertical Propagation of Middepth Zonal Currents Associated With Surface Wind Forcing in the Equatorial Indian Ocean

Abstract

The vertical propagation of middepth (200-1,500 m) zonal velocity in the equatorial Indian Ocean (EIO) and its relationship with surface wind forcing are investigated. We focus on semiannual time scales, using in situ mooring observations and an oceanic reanalysis product that can reasonably well reproduce the structure and magnitude of the observed middepth zonal velocity in the EIO. The pronounced semiannual cycle in observed middepth zonal currents indicates a clear upward phase propagation with particularly robust signatures at depths of 500-1,000 m between 3°S to 3°N, 70 to 85°E. The main characteristics of the middepth currents are consistent with vertically propagating first-meridional-mode Rossby wave at semiannual period. Theory suggests that equatorial Kelvin waves, forced by zonal wind variations in the western half of the basin, transfer energy downward and eastward, reflecting into Rossby waves at the eastern boundary. These reflected Rossby waves transfer energy downward and westward, contributing greatly to observed phase propagation at middepths in the EIO.

Semiannual Variations in 1,000‐dbar Equatorial Indian Ocean Velocity and Isotherm Displacements from Argo Data

“…Our results are consistent with previous studies (e.g., Luyten & Roemmich, ; Huang, Han, et al, ; Huang, McPhaden, et al, ) that have demonstrated that strong semiannual variations in deep zonal velocities are in part the result of a vertically propagating, first meridional mode Rossby wave. The zonal velocity semiannual signals presented here have been described previously using Argo data (Davis, ) and have been revisited more recently with a focus on the middepth (500–1,500 m) equatorial Indian Ocean (Huang, Han, et al, ; Huang, McPhaden, et al, ). However, we provide a more complete picture of the semiannual variations by including analyses of the meridional velocity (which are novel to the best of our knowledge) and isotherm displacements, both of which indicate the presence of a vertically propagating Rossby wave (Figures ) as well.…”

“…West of 65°E, the westward propagation becomes unclear, aligning roughly with the beginning of the region (around 60°–65°E) in the western Indian Ocean where the spatial structure of all three fields is less coherent and amplitudes are smaller (Figure ). The zonal phase speed of the anomalies ( c px ), estimated from the slope of the zonal velocity anomalies in Figure b, is 0.56 m·s −1 , similar to that ( c px = 0.57 m·s −1 ) found by Huang, McPhaden, et al () for their middepth zonal velocities. Calculation of the zonal wavenumber ( k = ω / c px ) yields a zonal wavelength of ~8,800 km, consistent with the larger‐than‐basin‐scale wave suggested by the phase lines in Figure .…”

“…Using Argo data, here we provide independent confirmation of these results and, in addition to the zonal velocities, expand upon the work of Huang, McPhaden, et al () by examining 1,000‐dbar meridional velocity and 0‐ to 2,000‐dbar isotherm displacements as well. We find that there are also clear variations in the meridional velocity and isotherm displacements, especially in the eastern Indian Ocean, consistent with the presence of a vertically propagating semiannual Rossby wave there.…”

“…Along 2.5°N (Figure a) and 2.5°S (Figure b), phase lines generally slope downward and westward from ~200 dbar at the eastern basin boundary down to 2,000 dbar between 55°E and 60°E. They largely follow the Wentzel–Kramers–Brillouin raypath of a first meridional mode Rossby wave (Figures a and b; dashed orange lines) that has been initiated via reflection at the Indian Ocean's eastern boundary (see, e.g., Figure 7 of Huang, McPhaden, et al, ) rather than by direct wind forcing at the surface. Phase propagation is overall upward and westward (perpendicular to the phase contours) as expected for a vertically propagating Rossby wave.…”

“…Recently, Huang, McPhaden, et al () presented evidence from a combination of moorings and ocean reanalysis data from ORAS4 in support of the Luyten and Roemmich () and McPhaden () observations. They showed that semiannual variations in equatorial Indian Ocean middepth (500–1,500 m) zonal currents are due in part to vertically propagating Rossby waves that develop as a result of surface‐wind forced Kelvin waves reflecting off the Indian Ocean's eastern boundary.…”