Upper layer diapycnal mixing and nutrient flux in the subtropical frontal region of the Indian sector of the Southern Ocean

Upper layer diapycnal mixing in the Subtropical Front (STF) was estimated using microstructure shear profiles collected from the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2012. Observations were made in the northern and southern boundary of the highly mesoscale turbulent STF, which is characterized by the presence of the dynamic Agulhas Return Current. During the observational period, the STF was populated with alternating cyclonic and anticyclonic eddies. In this mesoscale turbulent region, the average eddy diffusivity at the base of the euphotic zone was 5.5 x 10(-5) m(2) s(-1). The average diapycnal nitrogen flux at the base of the euphotic zone, calculated using direct turbulence measurements, and nitrate (NO3) and nitrite (NO2) concentrations, was 6.4 x 10-5 mu mol m(-2) s(-1). The satellite-derived primary production in the STF was similar to 1000 mg C m(-2) day(-1). The observed diapycnal nutrient flux could only sustain < 1% of the production observed in the region. Analysis of satellite-derived ocean currents, sea level anomalies, and thermohaline distribution further shows that despite the study area is a highly mesoscale turbulent region, the primary supply of nutrients is a result of advection (vertical or zonal) rather than vertical mixing.