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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.

Pallas-Sanz, E, Johnston TMS, Rudnick DL.  2010.  Frontal dynamics in a California Current System shallow front: 1. Frontal processes and tracer structure. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc006032   AbstractWebsite

The three-dimensional dynamics in a shallow front are examined using density and current data from two surveys 100 km offshore of Monterey Bay, California. Survey 1 is forced by down-front winds, and both surveys have considerable cross-front density gradients and flow curvature. The maximum Rossby numbers on the dense side reached maxima of +0.60 in survey 1 and +0.45 in survey 2. Downwelling occurs in regions of confluence (frontogenesis) associated with potential vorticity (PV) change and thermal wind imbalance. Streamers of particulate matter and PV are advected southeastward by the frontal jet and downward. Nonlinear Ekman currents advect dense water over light water in the presence of down-front winds, which leads to upwelling along the front and downwelling on the light side of the front. At sites of active ageostrophic secondary circulation (ASC), induced by frontogenesis or Ekman effects, the observed cross-front ageostrophic velocity is consistent with the diagnosed vertical velocity. Furthermore, in survey 2, ageostrophic divergence may play an important role at the curved front, presumably counteracting quasi-geostrophic frontogenesis due to isopycnal confluence. Downward frictional vertical PV flux below the surface extracts PV from the pycnocline and reinforces the frontogenetic vertical PV flux. PV destruction at the surface is inferred from a low PV anomaly below the mixed layer in survey 2. Since the magnitude of the frontogenetic ASC is only twice the magnitude of Ekman suction, external forcing may have a considerable impact on the vertical heat and PV fluxes.

Pallas-Sanz, E, Johnston TMS, Rudnick DL.  2010.  Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006474   AbstractWebsite

This is the second paper investigating the three-dimensional dynamics from two consecutive, quasi-synoptic surveys of a shallow front in the California Current System. The mesoscale vertical velocity (w) is obtained by solving a generalized omega equation using density and horizontal velocity observations. Highly nonlinear dynamics emerge from the ageostrophic forcing terms for w in an adiabatic generalized omega equation. The two main processes driving w are (1) wind-induced cross-frontal ageostrophic circulation (survey 1) and (2) ageostrophic kinematic deformation during frontogenesis (surveys 1 and 2). The horizontally averaged heat fluxes are positive in the whole water column with maxima at similar to 50 m, which warms (cools) the upper (lower) water column and upwells (downwells) light (dense) water. Wind-induced currents interact with the front, cooling the upper ocean and creating a divergent potential vorticity (PV) flux at the Ekman layer base which weakens the vertical heat and PV fluxes in survey 1. Vertical velocity extrema reach similar to 10 m d(-1) in both surveys. A diabatic omega equation is derived, which introduces an important new idea: the relation of the frictional w with the vertical diffusivity of the differential ageostrophic vorticity. This term is not found in the quasi-geostrophic omega equation. By including vertical mixing, |w| is enhanced by a factor of 2 in the upper similar to 100 m and reduced below. This effect is pronounced when the wind blows in the direction of the frontal jet, but it is sensitive to the vertical mixing parameterization.