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

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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 30 m s–1, and heat fluxes up to 1000 W m–2 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) to O(100) W m–2. Near the GS core, much larger entrainment fluxes, O(100) to O(1000) W m–2, contribute significantly to cooling the mixed layer, but less so to overall EDW formation due to its smaller volume. Relationships between observed dissipation rates and the atmospheric and local shear forcing scales are examined for this limited data set and compared with empirical scalings both within the mixed layer and in the entrainment zone. Below the mixed layer near the GS, diapycnal diffusivities in the thermocline averaged about O(10–4) m2 s–1, and are approximately 10 times levels previously observed in the GS during other seasons. Horizontally coherent shear structures, with shoaling phase and clockwise rotation, indicate that downward-propagating near-inertial waves are responsible for much of this enhanced subsurface mixing.

Document Type: Research Article


Publication date: September 1, 2010

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  • The Journal of Marine Research publishes peer-reviewed research articles covering a broad array of topics in physical, biological and chemical oceanography. Articles that deal with processes, as well as those that report significant observations, are welcome. In the area of biology, studies involving coupling between ecological and physical processes are preferred over those that report systematics. Authors benefit from thorough reviews of their manuscripts, where an attempt is made to maximize clarity. The time between submission and publication is kept to a minimum; there is no page charge.
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