Equatorial deep jets and abyssal mixing in the Indian Ocean

Abstract

Vertical profiles of horizontal currents and hydrographic measurements from three cruises along 80.5°E from the coast of Sri Lanka to 6°S between December 1990 and September 1994 are used to investigate the scales of the Indian Ocean deep jets as well as internal wave parameters and dissipation at the equator. The deep jets at 80.5°E have a vertical wavelenght of 660 sm (stretched meters) and amplitudes exceeding 10 cm s-1 in zonal velocity. They are observed throughout the water column and their flow direction reverses at 2° off the equator. The vertical positions of the jets differ among the cruises and are consistent with a flow reversal between the data collected in winter and summer. During September 1994, the jets were less pronounced. Due to the meridional distribution of their zonal velocity and the phase relationship between zonal velocity and vertical displacement, the jets are best described as nondispersive first-mode equatorial Rossby waves. The hydrographic data revealed thick layers of low stratification with vertical scales of 15-55 m in the upper 2000 m of the water column. They are found primarily within 1° of the equator and there is some evidence of correlation between the vertical position as well as the extent and the high strain zones of the deep jets. At vertical wavenumbers larger than those of the deep jets, shear and strain levels are five times larger than at off-equatorial locations and the compliant internal wave range ("roll-off range") begins at a smaller wavenumber (kc ≈ 0.02 cpsm). An estimate of the average dissipation rate within the deep jets yielded ∈ = 7.5 × 10-10 W kg-1 between 500- and 2000-m depth. The elevated finescale internal wave field appears to be the main cause for the existence of the low stratification layers.