Moored Flux and Dissipation Estimates from the Northern Deepwater Gulf of Mexico

Results from a pilot program to assess boundary mixing processes along the northern continental slope of the Gulf of Mexico are presented. We report a novel attempt to utilize a turbulence flux sensor on a conventional mooring. These data document many of the features expected of a stratified Ekman layer: a buoyancy anomaly over a height less than that of the unstratified Ekman layer and an enhanced turning of the velocity vector with depth. Turbulent stress estimates have an appropriate magnitude and are aligned with the near-bottom velocity vector. However, the Ekman layer is time dependent on inertial-diurnal time scales. Cross slope momentum and temperature fluxes have significant contributions from this frequency band. Collocated turbulent kinetic energy dissipation and temperature variance dissipation estimates imply a dissipation ratio of 0.14 that is not sensibly different from canonical values for shear instability (0.2). This mixing signature is associated with production in the internal wave band rather than frequencies associated with turbulent shear production. Our results reveal that the expectation of a quasi-stationary response to quasi-stationary forcing in the guise of eddy variability is naive and a boundary layer structure that does not support recent theoretical assumptions concerning one-dimensional models of boundary mixing.

Magnetic viscosity due to collapsing flux cells

Reconnective annihilation of magnetic field leads to the formation of magnetic flux cells with small scales, followed by enhanced transverse plasmons occurring in a thin current sheet with a very small vertical extent. The analysis here focuses on the nonlinear interaction between the flux and plasmons. The transverse plasmon field is modulationally unstable in the Lyapunov sense. When the initial pumping wave amplitude attains the threshold of instability, this instability occurs with a high growth rate. Nonlinear development of modulational instability eventually results in self-similar collapse, due to nonlinear equilibrium, giving rise to a spatially intermittent, collapsing magnetic flux, very similar to a turbulent pattern. The Maxwell stress tensor from the turbulence flux determines the anomalous magnetic viscosity, i.e. the parameter α. It is shown that the instability is responsible for the alternation of outburst or quiescent states in astrophysical accretion disks. When the instability occurs, the parameter α is large. In the quiescent state, the instability is suppressed, leading to a smaller, collapse-quenching value of α.