Mixing rates across the Gulf Stream, Part 1: On the formation of Eighteen Degree Water

Microstructure profiles taken in February 2007 across the Gulf Stream (GS) measured the temporal and spatial variability of the intense mixing that forms Eighteen Degree Water (EDW). Strong winds, gusting to 60 kt, and heat fluxes up to 1000 W m produced moderate-to-strong mixing in the surface mixed layer and the entrainment zone, as well as in the thermocline. In the limit of a vertically balanced heat budget, EDW formation is driven primarily by surface heat loss to the atmosphere across a region extending O(100) km south from the GS core, where entrainment heat fluxes based on dissipation rates were relatively low, O(10~100) W m. Near the GS core, much larger entrainment fluxes, O(100~1000) W m, contribute significantly to cooling the mixed layer, but less so to overall EDW formation due to its smaller volume. Below the mixed layer near the GS, diapycnal diffusivities in the thermocline averaged about O(10) m s , and are approximately 10 times levels previously observed in the GS during other seasons. Banded shear structures in velocity profiles, dominated by shoaling phase coherence and clockwise rotation, indicate that downward-propagating near-inertial waves are responsible for much of this enhanced subsurface mixing.