Influence of surface forcing on near-surface and mixing layer turbulence in the tropical Indian Ocean

Abstract

An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy (ε) in the tropical Indian Ocean during a single diurnal cycle, from about 50m depth to the sea surface. This dataset is one of only a few to resolve upper ocean ε over a diurnal cycle from below the active mixing layer up to the air-sea interface. Wind speed was weak with an average value of ~5ms-1 and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), ε values were on the order of 10-7-10-6Wkg-1 at a depth of 0.75m and when averaged, were almost a factor of two above classical law of the wall theory, possibly indicative of an additional source of energy from the wave field. Below this depth, ε values were closer to wall layer scaling, suggesting that the work of the Reynolds stress on the wind-induced vertical shear was the major source of turbulence within this layer. No evidence of persistent elevated near-surface ε characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant ε values at depths between the WWSL and the base of the mixing layer, characteristic of mixing by convective overturning. Within the remnant layer, depth-averaged values of ε started decaying exponentially with an e-folding time of 47min, about 30min after the reversal of the total surface net heat flux from oceanic loss to gain.