Elevated mixing at a front

Johnston, TMS, Rudnick DL, Pallas-Sanz E.  2011.  Elevated mixing at a front. Journal of Geophysical Research-Oceans. 116

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3-dimensional circulation, azores front, california current system, fine-structure, gulf-stream, intensive, part ii, submesoscale transition, surveys, upper ocean, vertical circulation


The mesoscale, submesoscale, and microscale structure of a front in the California Current was observed using a towed vehicle outfitted with microconductivity sensors. Thirteen >60 km cross-front sections from 0 to 350 m in depth were covered in 3.5 days. Objectively mapped data are fit via the Omega (omega) equation to obtain vertical velocity. A composite cross-front section shows elevated mixing on the dense side within 10-20 km of the front. Water downwells and gradients are elevated there: i.e., Rossby number (Ro), horizontal strain (alpha), spice gradients, and microscale thermal dissipation (chi). Thermal eddy diffusivity (K(T)) reaches 10 (3) m(2) s (1) and increases 3-10x from the anticyclonic to the cyclonic side with a depth mean of similar to 10 (4) m(2) s (1). The spatial structure of K(T), Ro, and alpha are similar on the dense side, suggesting an energy cascade from the mesoscale via the submesoscale to the microscale. However, it is unclear whether frontogenesis, internal wave blocking by elevated vorticity, or internal wave trapping by large a produces the elevated mixing. The mean turbulent heat flux opposes the mean restratifying, mesoscale heat flux of 10Wm(-2) and may allow the front to persist. Turbulent nitrate fluxes are 0.1-0.3 mmol m(-2) s(-1). Chlorophyll fluorescence and beam transmission reveal a <6 km wide, similar to 100 km long alongfront streamer which is a deep biomass maximum. Time scales for mixing and nutrient fluxes are 0.3-3 days, which are similar to phytoplankton growth rates and the time scale for frontal evolution.






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