Semiannual variability of middepth zonal currents along 5°n in the eastern indian ocean: Characteristics and causes

Abstract

Four-year (2014–17) zonal current data observed by a mooring at (58N, 90.58E) in the eastern Indian Ocean show a strong semiannual cycle in the middepth (~1200 m) with distinct vertical structure. This pronounced middepth semiannual variability, however, is inconsistent with the local wind forcing, which shows a predominant annual cycle. The underlying causes for this unique middepth variability along 58N were elucidated with the addition of a reanalysis product and a continuously stratified linear ocean model. The results suggest that the observed seasonal variability in the middepth zonal flow at 58N is primarily caused by boundary-reflected Rossby waves forced by the remote semiannual winds along the equator. Contribution from the locally wind-forced Rossby waves is much less. The theoretical Wentzel–Kramers–Brillouin ray paths further verify that the strong semiannual variability of the middepth signals over a moored region in the eastern Indian Ocean is largely a manifestation of the steep angles of propagating energy of the long Rossby waves at semiannual time scale. The annual signals are only significant in the upper and western sections (758–808E) as a result of the smooth trajectories of Rossby waves forced by local annual winds. Further analysis reveals that the middepth zonal currents along 58N are expected to be associated with equatorial symmetric Rossby waves at semiannual period. Consequently, similar zonal flows should also exist in the middepth near 58S.