Sensitivity and Resolution Convergence Studies of

North Atlantic Model Circulation

Matthew Hecht, Los Alamos

Frank Bryan, NCAR

Rick Smith, Los Alamos

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SMBH 2000

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

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• Gulf Stream separated from the coast at Cape Hatteras– if somewhat

too zonal

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• Azores Current matched obs well– far better than in earlier

0.28º global POP simulation

0.1º NA

0.28º global

At 32º W

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North Atlantic Current turned North around the Grand Banks, into the Northwest Corner (sea surface height shown).

0.1º NA 0.28º global

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• Resolution was a big part of the story– grid spacing,

varying with latitude, shown, with first Rossby radius

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Resolution is not the whole storyMaltrud and McClean, 2004 (MM): Most majorjets quite realistic in 0.1ºglobal versionof POP,but Gulf Streamdid not separateat Hatteras.

Also see experience of Boning et al, 1/12º.

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Do Gulf Stream/North

Atlantic Current biases

matter?

SST

SSTBias

North-West AtlanticCorner /LabradorSea

Sea Surface Temperaturesand ErrorRelative to Observations, from Community Climate System Model

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• 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

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• 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.

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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

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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

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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

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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

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• 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

C=1/4

C=1/2

C=1SMBH

0.2º, C=1/8

C=80.2º, C=1

0.4º, C=1

same dissipative parameters

same dissipative parameters

Basin Mean Kinetic Energyg(

cm/s

)^2

5040

100

2000199619921988

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

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C=1, 0.4º

C=1, 0.2º

C=1, 0.1º C=8, 0.1º

C=1/8, 0.2º

• Boundary eddy in non-separating case

• Northern recirc gyre extends to ~Hatteras in C=1, 0.1º case

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• 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=1/4, 0.1º

C=1/2, 0.1º

C=1, 0.1º

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Gulf Stream pathsfrom intersection of 12°C isotherm and 400m, 1998-2000

• 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, 0.1º

C=1/2, 0.1º

C=1/4, 0.1º

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1990-1992 1998-2000

C=1

C=1/4

Separation sets up earlier in less viscous case.

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SSH, C=1/2 case

QuickTime™ and aPhoto decompressor

are needed to see this picture.

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• Northwest Corner is better with lower viscosity, in SSH var.

C=1

C=1/2

C=1/4

TOPEX

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• 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

C=1/4

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• 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

Eddy Kinetic Energies, 55º W

C=1

C=1/4

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1998 1999

2000

Obs from Pickart and Smethie. Model with C=1/2.

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• Magnitude of NAC somewhat weak– Structure is reasonable

– WBC too strong?

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• 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.

C=1/4

+32.5-20.0

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• 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

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Unexplored tangent: low mixing in idealized overflow at ~0.1º

Legg, Hallbergand Girton,OMOD, in press.

• 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

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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