TRR 181 Seminar "Ocean turbulence measurements and the effect of bottom enhanced mixing in coastal upwelling regions" by Marcus Dengler (GEOMAR)

The TRR 181 seminar is held by Marcus Dengler (GEOMAR Kiel) on May 27, 11 am.

Ocean turbulence measurements and the effect of bottom enhanced mixing in coastal upwelling regions

Abstract

Turbulence plays a key role in oceanic energy budgets and transport of heat, salt, dissolved gases, and nutrients in the ocean. Observations of turbulence are required to assess and improve how mixing is represented within regional and global ocean models. The most direct, and practical, estimator of the strength of turbulence in the ocean is based on in situ measurements from microstructure shear probes. It requires instrumentation that can accurately measure the time- (milli-seconds to seconds) and length- (milli-meters to meters) scales within the viscous subrange of ocean turbulence.  In the past 20 years, observations have shown that turbulence in the deeper ocean is enhanced above sloping or rough bottom topography. Using microstructure shear data from a multi-cruise program to the Mauritanian and Peruvian  upwelling region, we show that dissipation rates of turbulent kinetic energy increase by about an order of magnitude between 50 to 100m above the bottom of the upper continental slopes. Bottom intensified turbulence here is generated by the interaction of barotropic and low-mode baroclinic tides with sloping topography. According to the continuity equation, this setting leads to a diapycnal downwelling velocity in the bottom enhanced turbulent layer, which must be balanced by diapycnal upwelling in the bottom boundary layer. Indeed, a deliberate tracer release in the bottom boundary layer at 250m depth in the Peruvian upwelling regions suggest upward diapycnal velocities of about 0.5-1 m/day.  These results suggest that nutrient transport due to diapycnal upwelling in the bottom boundary layer may be an important contribution to sustaining elevated productivity in upwelling regions.