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

• Published in: Journal of physical oceanography Vol. 35; no. 9; pp. 1689 - 1711
• Main Authors: ALFORD, Matthew H, GREGG, Michael C, D'ASARO, Eric A
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.09.2005
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Lagrange multiplier; Ocean temperature; Oceanography; Physics of the oceans; Thermohaline structure and circulation. Turbulence and diffusion; Three dimensional imaging; United States; Washington; Puget Sound; Washington (state); INE, USA, Washington; ocean currents; Ship observation; salinity; Acoustic measurement; Three dimensional structure; Mixing process; Thermohaline convection; salt-water intrusion; cartography; North America; ocean circulation; fjords
• ISSN: 0022-3670; 1520-0485
• DOI: 10.1175/JPO2780.1

Abstract 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 m s−1 out of Possession Sound, Washington, was observed. The feature lay across isopycnal 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°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 m2 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 O(100 m) are seen in time series and spatial maps, supporting this interpretation.