HYCOM and the need for overflow/entrainment parameterizations

1. HYCOM and the need for overflow/entrainment parameterizations

2. Overflow Representation in Numerical Models • Strongly dependent on the choice of the vertical coordinate • In fixed coordinate models (z and σ), the numerically induced entrainment (i.e. mixing) is larger than observed => no need for parameterization, the focus is on reducing the numerically-induced mixing to below observations (DYNAMO, Griffies et al. (2000), …) • In density coordinate models, the densest fluid will sink to the bottom => need for an entrainment parameterization

3. Entrainment parameterization • The specific parameterization will depend on the resolution: large difference between coarse climate models (horizontal resolution of 1º or lower ) and truly eddy-resolving models (horizontal resolution of ~1/10º). Different viscosity => different modeled Ri/Fr numbers • How do we address this?

4. HYCOM • Investigation in HYCOM 2º global configuration for the NCAR CCSM and OMIP • Investigation in the 1/12º North Atlantic Ocean prediction system (U.S. GODAE) • What are the targets? One needs to define the metrics.

5. U.S. GODAE: Global Ocean Prediction with the Hybrid Coordinate Ocean Model (HYCOM) Representation of the Mediterranean Outflow and Dennmark Straits Overflow

6. Objectives and Goals • A broad partnership of institutions that will collaborate in developing and demonstrating the performance and application of eddy-resolving, real-time global and basin-scale ocean prediction systems using HYCOM • To be transitioned for operational use

7. Opportunities • Global high resolution (1/12º) model outputs available to the community at large • Strong participation of the coastal ocean modeling community in using and evaluating boundary conditions from the global and basin-scale ocean modeling prediction systems

8. HYCOM • HYCOM is the result of a very effective collaboration between the U. of Miami, NRL/Stennis, and the Los Alamos National Laboratory. • HYCOM has been configured globally (up to 3/4º ~60km mid-latitude resolution) and basin-scale (up to 1/12º ~7km mid-latitude resolution – North Atlantic and Pacific)

9. The hybrid coordinate is one that is isopycnalin the open, stratified ocean, but smoothly reverts to a terrain-following coordinate in shallow coastal regions, and to pressure coordinate in the mixed layer and/or unstratified seas.

10. 1/25° HYCOM East Asian Seas Model Nested inside 1/6° HYCOM Pacific Basin Model Boundary conditions via one-way nesting and 6 hrly ECMWF 10 m atmospheric forcing

11. 1/25° East Asian Seas HYCOM (nested inside 1/6° Pacific HYCOM) North-south velocity cross-section along 124.5°E, upper 400 m blue=westward flow red=eastward flow density front associated with sharp topographic feature (cannot be resolved with fixed coordinates) Isopycnals over shelf region Yellow Sea flow reversal with depth Snapshot on14 October East China Sea Yellow Sea z-levels and sigma-levels over shelf and in mixed layer Snapshot on12 April

12. Configuration of the Prediction Systems • Basin-scale (NRL/Miami and NOAA)

13. Configuration of the Prediction Systems • Global • Sea Ice Options • Energy loan • 4-layer thermodynamic (Russel et al., 2000) • Los Alamos CICE • Target • 1/12º for NAVOCEANO • 1/4º (~20 km) for FNMOC (ocean component of coupled ocean-atmosphere)

14. Denmark Straits Overflow Along 31°W Colder fresher water forms over the shelf in the Nordic Seas and spills over the Denmark Strait Temperature and entrains more saline Irminger Sea water Salinity Results from 1/12° Atlantic HYCOM

15. Results from 1/12° Atlantic HYCOM

16. Results from 1/12° Atlantic HYCOM More on Dennmark Straits plans in Xiaobiao Xu’s presentation

17. Representation of the Mediterranean Outflow in MICOM and HYCOM • Entrainment parameterization in MICOM => Papadakis et al. (2003) • Outflow representation in MICOM and HYCOM • Entrainment parameterization in HYCOM vs. MICOM • Discussion

18. Entrainment Parameterization in MICOM (Papadakis et al., 2003) • Based in the implicit diapycnal mixing scheme of Hallberg (2000) • Uses a Richardson number dependent entrainment parameterization of Turner (1986) based on laboratory experiments • Evaluated in a realistic MICOM regional model (1/12º grid) with imposed boundary conditions at the Strait of Gibraltar

19. Domain Configuration

20. Boundary Conditions at the Strait of Gibraltar Inflow/outflow = .8 Sv

21. Sensitivity to Entrainment Parameterization (Turner, 1986)

22. Outflow Representation

23. Outflow Representation

24. Sensitivity to Mixing Frequency

25. Implementation in 1/12º North Atlantic MICOM Layer 10 thickness 1979-1986 Salinity Layer 10 1979-1986 Meddies dissipate quickly, outflow too shallow

26. MICOM 1/12º Transport ~ .7 Sv

27. MICOM 1/12º

28. HYCOM 1/12º Transport ~ 1 Sv

29. HYCOM 1/12º

30. KPP vs. Turner (1986) • The K-Profile parameterization (Large et al., 1994) is widely used in ocean models • KPP is derived from observations while Turner (1986) is primarily derived from laboratory experiments • There are non-oceanic aspect aspects in Turner (1986), i.e. lack of rotation, … • KPP is however a broad representation of the processes and may not be very relevant to overflows

31. Present Strategy • Evaluation of KPP and Turner (1986) [possibly other parameterizations as they are developed => CPTs on overflows] • Idealized configurations • Comparison to 2D non-hydrostatic configurations (i.e. Ozgokmen and Chassignet, 2002) • Document the sensitivity to numerical choices such as stratification, mixing frequency,…..