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Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation

Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation. Matthew Hecht, Los Alamos Frank Bryan, NCAR Rick Smith, Los Alamos. SMBH 2000. In 2000 paper SMBH, saw pretty good sub-tropical gyre and Gulf Stream system.

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Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation

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  1. Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation Matthew Hecht, Los Alamos Frank Bryan, NCAR Rick Smith, Los Alamos

  2. SMBH 2000 • In 2000 paper SMBH, saw pretty good sub-tropical gyre and Gulf Stream system

  3. Gulf Stream separated from the coast at Cape Hatteras • if somewhat too zonal

  4. Azores Current matched obs well • far better than in earlier 0.28º global POP simulation 0.1º NA 0.28º global At 32º W

  5. 0.28º global 0.1º NA North Atlantic Current turned North around the Grand Banks, into the Northwest Corner (sea surface height shown).

  6. Resolution was a big part of the story • grid spacing, varying with latitude, shown, with first Rossby radius

  7. Resolution is not the whole story Maltrud and McClean, 2004 (MM): Most major jets quite realistic in 0.1ºglobal version of POP, but Gulf Stream did not separate at Hatteras. Also see experience of Boning et al, 1/12º.

  8. Do Gulf Stream/North Atlantic Current biases matter? SST North- West Atlantic Corner / Labrador Sea SST Bias Sea Surface Temperaturesand ErrorRelative to Observations, from Community Climate System Model

  9. Working towards high res climate simulation, with eddy-resolving ocean • With NCAR, CRIEPI (Japan), NPS • Appears possible to greatly reduce NA SST biases, with a combination of informed choices and luck, so… • Would like to understand how best to configure ocean model

  10. So, SMBH had good GS system, MM not as satisfactory. Likely factors: • Source waters (NA overflow, Ant. Bottom Water, etc.) • Grid (mercator NA regional -vs- “displaced pole” (Hudson’s Bay) global • Viscous parameters • Had not been set cleanly in SMBH, coef’s had been changed twice during integration • Here we concentrate mostly on sensitivity to viscous param’s, w/ some experimentation w/ horizontal grid resolution as well.

  11. Model • POP free surface z-coord hydrostatic primitive equation code • “Full cell” topography (partial cell option also exists) • Mercator grids (x= y, so y also scales w/ cos(y)) • Mostly 0.1º res, but also 0.2 º, 0.4 º • 40 Levels • 10 m in upper ocn, 250 m in deep, 5500 m max • 20º S to 73º N • Including Gulf of Mexico, Western Mediterranean

  12. Forcing • Barnier monthly heat flux climatology • with penetrative solar • E/P from 30 day (over 10 m) restoring to Levitus monthly climatology • Restore SST to -2º under diagnosed ice • Daily ECMWF winds, 1986--2000 • Restoring at lateral boundaries • North of NA Sill, S Atlantic, Sicily Channel

  13. Mixing • Pacanowski and Philander Ri-number-based vertical mixing • Have used KPP in other 0.1º simulations • Biharmonic horizontal mixing of tracers and momentum • Have used anisotropic viscosity, GM, anisotropic GM in other simulations

  14. Scaling of biharmonic coef’s • As standard practice we scale horizontal mixing coef’s with (area)3/2 so as to maintain constant grid-Reynolds (-Peclet) number: • Grid-Re = U (area/area0)3/2/, for biharmonic form • Scaled within simulation, as grid cell area varies, and between simulation, by default • In these experiments we also include an adjustable factor C so that •  = C 0 (area/area0)3/2 • 0=-2.7x1010m4/s, • area0=(11.2 km)2 (equatorial grid cell area) • C=1 for “standard” set of simulations

  15. Basin Mean Kinetic Energy 50 C=1/4 40 C=1/2 SMBH C=1 • 15 year integration gives reasonable equilibration of KE • Time series of three 0.1º cases shown • Averages over last 3 years shown for other cases 0.2º, C=1/8 g(cm/s)^2 same dissipative parameters C=8 same dissipative parameters 0.2º, C=1 10 0.4º, C=1 0 1988 1992 1996 2000

  16. Munk width of 0.1º case at C=1 is just under 2 grid lengths.

  17. C=1, 0.4º • Boundary eddy in non-separating case • Northern recirc gyre extends to ~Hatteras in C=1, 0.1º case C=1, 0.2º C=1/8, 0.2º C=1, 0.1º C=8, 0.1º

  18. C=1, 0.1º • Recirculation gyres really spin up with decreasing viscosity • Does northern recirculation gyre cause Stream to go too zonal off Hatteras in low viscosity (C=1/4) case? C=1/2, 0.1º C=1/4, 0.1º

  19. Gulf Stream paths from intersection of 12°C isotherm and 400m, 1998-2000 C=1, 0.1º • By this point in time (years 13-15) the more viscous (C=1) case has a more realistic separation (obs in green, model mean, 1  and extreme envelope in blue) C=1/2, 0.1º C=1/4, 0.1º

  20. 1990-1992 1998-2000 C=1 C=1/4 Separation sets up earlier in less viscous case.

  21. SSH, C=1/2 case

  22. C=1 TOPEX • Northwest Corner is better with lower viscosity, in SSH var. C=1/2 C=1/4

  23. C=1 • How different the density (thermal) fronts are! Yet: • Many of the features of the flow are similar here, and deeper -- the E/W portions of the flow as it winds northward. • More viscous case tends to lose much of the NAC out eastern boundary relatively early. C=1/4

  24. Eddy Kinetic Energies, 55º W C=1 • Deeper penetration of Current in less viscous case • Ozgokmen (97) found jet needed to be highly inertial with low eddy activity to separate and cross f/H, in process study. • We don’t see relation between low eddy activity and separation at Hatteras • but maybe high eddy activity for reattachment at Grand Banks C=1/4

  25. Obs from Pickart and Smethie. Model with C=1/2. 2000 1998 1999

  26. Magnitude of NAC somewhat weak • Structure is reasonable • WBC too strong?

  27. C=1/4 -20.0 • Peak velocities in NAC match pretty well • but see how 10 cm/s isotach is at 1600 m in model, 3500 m in obs • Particularly weak in recirculated “Lab Sea”, “upper-Lab Sea” waters • Too much transport of South/Westward-flowing boundary water in this least-viscous case. +32.5

  28. DWBC looked good off Abaco in SMBH • Now we’re looking in more detail upstream of Hatteras, where stream and bathymetry are both complex, comparison less satisfactory

  29. Unexplored tangent: low mixing in idealized overflow at ~0.1º • NW Corner not as realistic as Gulf Stream, Azores Current. May be worth looking more at: • model source waters, mixing in overflow, inclusion of passive tracers? • Nevertheless, SST biases reduced over low res models Legg, Hallberg and Girton, OMOD, in press.

  30. Conclusions • With biharmonic horizontal dissipation in POP at 0.1º, best compromise between GS separation and NW Corner, Azores Current is C=1/2 case. • Some evidence for convergence at undesirably high level of dissipation. • As expected, not converged at more desirable level of dissipation • Has led to investigations of anisotropic horizontal viscosity, GM, partial bottom cells, smoothing of topography

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