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

TitleInfluence of surface forcing on near-surface and mixing layer turbulence in the tropical Indian Ocean
Publication TypeJournal Article
Year of Publication2014
AuthorsCallaghan AH, Ward B., Vialard J.
JournalDeep-Sea Research Part I-Oceanographic Research Papers
Date Published2014/12
Type of ArticleArticle
ISBN Number0967-0637
Accession NumberWOS:000345820100008
Keywords(ASIP); Air-Sea Interaction Profiler; boundary-layer; breaking waves; convection; Convectively-driven mixing; dissipation; Dissipation decay time; Dissipation of turbulent kinetic energy; diurnal cycle; driven; microstructure; mixed-layer; Near-surface turbulence; north-atlantic; Remnant layer; restratification; scale; Wind-driven mixing

An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy (epsilon) in the tropical Indian Ocean during a single diurnal cycle, from about 50 m depth to the sea surface. This dataset is one of only a few to resolve upper ocean epsilon 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 similar to 5 m s(-1) and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), epsilon values were on the order of 10(-7)-10(-6) W kg(-1) at a depth of 0.75 m 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, epsilon 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 epsilon characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant epsilon 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 epsilon started decaying exponentially with an e-folding time of 47 min, about 30 min after the reversal of the total surface net heat flux from oceanic loss to gain. (C) 2014 Elsevier Ltd. All rights reserved.

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