Mixing, 3D mapping, and Lagrangian evolution of a thermohaline intrusion

Observations of the three-dimensional structure and evolution of a thermohaline intrusion in a wide, deep fjord are presented. In an intensive two-ship study centered on an acoustically tracked neutrally buoyant float, a cold, fresh, low-oxygen tongue of water moving southward at about 0.03 in s(-1) out of Possession Sound, Washington, was observed. The feature lay across isopycrial surfaces in a 50-80-m depth range. The large-scale structures of temperature, salinity, velocity, dissolved oxygen, and chlorophyll were mapped with a towed, depth-cycling instrument from one ship while the other ship measured turbulence close to the float with loosely tethered microstructure profilers. Observations from both ships were expressed in a float-relative (Lagrangian) reference frame, minimizing advection effects. A float deployed at the tongue's leading edge warmed 0.2 degrees C in 24 h, which the authors argue resulted from mixing. Diapycnal heat fluxes inferred from microstructure were 1-2 orders of magnitude too small to account for the observed warming. Instead, lateral stirring along isopycnals appears responsible, implying isopycnal diffusivities O(1 m(2) s(-1)). These are consistent with estimates, using measured temperature microstructure, from an extension of the Osborn-Cox model that allows for lateral gradients. Horizontal structures with scales 0(100 in) are seen in time series and spatial maps, supporting this interpretation.