Submesoscale Fronts Are Torqued and Energized by Surface Gravity Waves, Turbulence, Larger Scales, and Time Evolution

The budgets of linear momentum, angular momentum, and energy during a high-resolution simulation of submesoscale frontogenesis are computed. This 20 km x 20 km x 160 m Large Eddy Simulation explicitly resolves submesoscale and boundary layer turbulence down to the 5 m x 5 m x 1.25 m grid. Important effects of surface gravity waves are included through integration of the wave-averaged, or Craik-Leibovich equations. The budget analysis is aided by writing the momentum equations in terms of the forces that directly pertain to the energy budget. The results show that the typical energy source of submesoscale frontogenesis—frontal buoyancy production—is joined by energy from surface waves and from the ambient nearly-geostrophic and confluent current. These amount to 24% and 47% of the buoyancy production, respectively, nearly doubling the total energy production. Likewise, the typical energy sink for the overturning (ageostrophic secondary circulation) is not limited to generation of the along-front current (−69% of the buoyancy production); work done altering the currents of the Stokes-Ekman spiral and dissipation due to the small-scale turbulence are as important as −38% and −16% of the buoyancy production, respectively. This typical front is not equilibrated but actively evolving in magnitude with the submesoscale flow on a timescale of hours. Because of the surface waves, turbulence, interactions with larger scales, and unbalanced acceleration, the hydrostatic balance and thermal wind balance are poor approximations for describing submesoscale frontogenesis.