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Johnston, TMS, Rudnick DL, Pallas-Sanz E.  2011.  Elevated mixing at a front. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007192   AbstractWebsite

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.

Johnston, TMS, Cheriton OM, Pennington JT, Chavez FP.  2009.  Thin phytoplankton layer formation at eddies, filaments, and fronts in a coastal upwelling zone. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:246-259.   10.1016/j.dsr2.2008.08.006   AbstractWebsite

On two cruises in August and September 2003 (hereafter cruises 2 and 3) during wind relaxations and transitions to upwelling conditions, thin layers of phytoplankton were observed in or a few meters below the stratified transition layer at the mixed layer base and in regions of sheared flow on the flanks of eddies, filaments, and fronts near Monterey Bay, California. On an earlier cruise in August (cruise 1), no thin layers were found after a prolonged wind relaxation. Chlorophyll concentrations and shear were both an order of magnitude less than on cruises 2 and 3. Our vertical profiles were made using a fluorometer mounted on a conductivity-temperature-depth package, which was lowered from the ship as slowly as 0.25 m s(-1) every 10 km on five similar to 80-km cross-shore transects. Remotely sensed sea-surface temperature, chlorophyll, and currents are required to understand the temporal and spatial evolution of the circulation and to interpret the quasi-synoptic in situ data. Decorrelation scales are similar to 20km from lagged temperature and salinity covariances. Objectively mapped sections of the in situ data indicate the waters containing thin layers were recently upwelled at either the Point Sur or Point Ano Nuevo upwelling centers. Spatially limited distributions of phytoplankton at the coastal upwelling centers (similar to 40 km alongshore, 20 km cross-shore, and 30 m thick) were transformed into thin layers by current shear and isolated from wind-driven vertical mixing by the stratification maximum of the transition layer. Vertically sheared horizontal currents on the flanks of the eddies, filaments, and fronts horizontally stretched and vertically thinned phytoplankton distributions. These thin, elongated structures were then observed as thin layers of phytoplankton in vertical fluorescence profiles at four stations on cruise 2 and eight stations on cruise 3. Light winds during relaxations did not mix away these thin layers. On cruise 2, thin layers were found at eddies at the inshore and offshore ends of a 100 km-long filament, while broader subsurface chlorophyll maxima were found along the filament. This result suggests that higher-resolution sampling along and across a filament may find thin layers forming and dissipating along its length. On cruise 3, thin layers were found at three adjacent stations across an upwelling front and may have extended continuously for > 20 km, but neither species composition nor bio-optical data are available to confirm this conjecture. The thin layers were 1-5 m thick in the vertical at full width half maximum and had peak fluorescence values from 7-30 mg m(-3). (Bottle chlorophyll samples showed fluorometer chlorophyll readings may have been 1.3-1.5 x too large, but the scatter in this relation was considerable especially at the larger fluorescence values detected in thin layers.) While sheared currents thinned an initially thick subsurface chlorophyll maximum into thin layers, the peak values in these thin layers exceeded concentrations in the upwelled source waters and were unexplained by our data. (C) 2008 Elsevier Ltd. All rights reserved.