Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation

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
  (Hudsons Bay) global
• Viscous parameters
• Had not been set cleanly in SMBH, coefs had been
  changed twice during integration
• Here we concentrate mostly on sensitivity to
  viscous params, 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 coefs

• As standard practice we scale horizontal mixing
  coefs 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)
• C1 for standard set of simulations

15
Basin Mean Kinetic Energy
50
C1/4
40
C1/2
SMBH
C1
0.2º, C1/8
g(cm/s)2
same dissipative parameters
C8
same dissipative parameters
0.2º, C1
10
0.4º, C1
0
2000
1996
1992
1988

• 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

16

• Munk width of 0.1º case at C1 is just under 2
  grid lengths.

17
C1, 0.4º

• Boundary eddy in non-separating case
• Northern recirc gyre extends to Hatteras in C1,
  0.1º case

C1, 0.2º
C1/8, 0.2º
C1, 0.1º
C8, 0.1º
18
C1, 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
  (C1/4) case?

C1/2, 0.1º
C1/4, 0.1º
19
Gulf Stream paths from intersection of 12C
isotherm and 400m, 1998-2000
C1, 0.1º

• By this point in time (years 13-15) the more
  viscous (C1) case has a more realistic
  separation (obs in green, model mean, 1 ? and
  extreme envelope in blue)

C1/2, 0.1º
C1/4, 0.1º
20
1990-1992
1998-2000
C1
C1/4
Separation sets up earlier in less viscous case.
21
SSH, C1/2 case
22
C1
TOPEX
C1/2
C1/4

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

23
C1

• 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.

C1/4
24
Eddy Kinetic Energies, 55º W
C1

• 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 dont see relation between low eddy activity
  and separation at Hatteras
• but maybe high eddy activity for reattachment at
  Grand Banks

C1/4
25
Obs from Pickart and Smethie. Model with C1/2.
2000
1998
1999
26

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

27
C1/4
-20.0
32.5

• 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.

28

• DWBC looked good off Abaco in SMBH
• Now were 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
Legg, Hallberg and 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

30
Conclusions

• With biharmonic horizontal dissipation in POP at
  0.1º, best compromise between GS separation and
  NW Corner, Azores Current is C1/2 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