1 Marginal Thermobaric Stability in the Weddell Sea Miles McPhee McPhee Research Company

2 Thermobaric Instability Following Loyning and Weber, JGR, 102, p z z = 0 ambient 2-layer system, upper layer colder and less saline Linearized equation of state: Thermal expansion coefficient increases with depth

3 Greenland Sea Weddell Sea

4 Marginal stability line Strength of thermobaric tendency must exceed the background stratification

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10 ANZFLUX Ship CTD station 50, linearized

hour average of turbulence measurements centered at time (Warm Regime drift). Circles are averages; lines are twice the std dev of the 15-min samples.

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13 Increase S ml Two-layer (Type II) stability diagram following Akitomo (1999) for idealized Ship Station 50

14 The “thermobaric barrier” calculation: (1)Calculate the actual density (pressure included), subtract density of a water column with mixed layer properties. Determine the level (z max ) of the maximum difference:  max. (2) Determine the sensible heat that must be vented to reduce water temperature above z max to  ml. (3) Add the latent heat loss required to increase salinity (by freezing) enough to eliminate  at z ml (4)H tot is the total heat loss.

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16 Pentagrams indicate H to t < 100 MJ/m 2

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22 McPhee, Kottmeier and Morison, JPO, W m -2

23 Ice temperatures from the AWI Buoy thermistor string

24 Mean values almost identical: 30 W m -2

25 Friction velocity prescribed from buoy results. Heat flux also from buoy ice measurements but scaled by the calculated ice thickness Ocean heat flux calculated prognostically, ice thickness determined by enthalpy balance at the interface. Dynamic mixed layer depth based on buoyancy frequency (pot density). Scalar based on difference from near surface value. The model neglects thermobaric effects but calculates thermobaric barrier parameters at each time step.

26 The 1-D model forced with buoy data and initialized with YU075 No thermobaricity effect considered.

27 The 1-D model forced with buoy data and initialized with YU075 Eddy viscosity set to 2000 cm 2 /s across vertical domain after

28 Horizontally Homogeneous Model Results Initialize model with every profile with H tot < 100 MJ m -2 forced by buoy time series (38) 27 1-D profiles became unstable by the end of August

29 Is there a simple way of getting a handle on eddy viscosity and scalar diffusivity when thermobaric mixing is occurring? 1.Parameterize entrainment process in terms of conversion of PE to TKE 2.Base the mixing length on a fraction (  of the entrained layer depth 3.Then

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33 Summary Thermobaric instability was not observed directly during the ANZFLUX 94 project but there is a strong inference that it occurred shortly after. About ¼ of the profiles observed with the yoyo CTD system during the Maud Rise drift went thermobarically unstable by the end of winter in a simple 1-D model forced with drifting buoy data. In the model, preconditioning of the initial density profile to include distinct step-like structure in the upper pyncnocline was necessary for instability. Steps were found mostly in the “halo” region surrounding Maud Rise ( m isobaths)

34 Summary (cont) There may be a good chance of encountering episodes of Type II convection near Maud Rise in late winter. Measuring turbulent dissipation rates and turbulent fluxes directly during a Type II episode is feasible based on ANZFLUX experience. Such data would be of great value in evaluating and guiding numerical model development. Even in the absence of direct Type II convection, studying processes that maintain the step structure and “pycnocline weather” in the Weddell would add significantly to our understanding of the system.