ITCZ/Monsoon Model Intercomparison Project

Aiko Voigt, Michela Biasutti and Jack Scheff, August 4, 2015

4 August 2015: Clarified whether fields should be saved as means over the

output period or as snapshots.

14 July 2015: The land setup is revised and has changed with respect to

earlier versions. We also now include plots for the ECHAM6.1 AquaControl

and LandControl simulations for reference.

This document describes the simulation setup and the requested simulations for the model-

intercomparison project organized within the 2015 WCRP Grand Challenge workshop on

ITCZ and monsoons. Five simulations in idealized aquaplanet and idealized continent setup

are performed to study the dynamics and model robustness of how carbon dioxide, land and

the orbital configuration affect the ITCZ and the monsoons as well as the interaction between

the two. We note that this MIP is not part of CMIP6, but we are very much interested in

discussions and exchanges with CMIP6 activities related to monsoons and the ITCZ.

1 Model setup

The models are to be coupled to a slab ocean with time-mean zonal-mean ocean heat trans-

port (“q-flux”) given in section 4. The boundary conditions largely follow the CMIP5 aqua-

planet protocol except for interactive sea-surface temperatures and the inclusion of a seasonal

cycle in insolation. The following parameters should be specified:

• CO2: 348 ppmv

• CH4: 1650 ppbv

• N2O: 306 ppbv

• No Halocarbons (CFCs)

• Ozone following values for the AquaPlanet Experiment (see http://www.met.

reading.ac.uk/~mike/APE/ape_ozone.html)

• Total solar irradiance: 1365 Wm−2

• No radiative effects of aerosols

• Diurnal cycle in insolation

1

• Calendar: a 360 days calendar (each month has 30 days) is desirable, but if such a

calendar is not available then a 365 days calendar without leap years should be used

(second preference) or a 365 days calendar with leap years (third preference)

• Orbit: zero eccentricity, 23.5◦obliquity, NH spring equinox on March 21; modelling

groups should check the timing of the NH spring equinox by looking at daily TOA

shortwave data because a correct timing is essential to meaningfully compare different

models (a deviation from March 21 by 1-2 days is considered acceptable)

• Slab ocean depth: 30 m

• Sea-ice formation should be inhibited, and ocean temperatures should be allowed to

cool below the freezing temperature of sea water (see, e.g., Kang et al., 2008; Voigt

et al., 2014)

For the land simulations, a rectangular continent extending from 30◦ S to 30◦ N and 0◦ E to

45◦ E should be introduced. The q-flux should be zeroed out over this region, which requires

a small uniform correction of the q-flux over ocean areas of -0.59 Wm−2 (see Sect. 4). Land

should be represented as a very shallow ocean of depth 0.1 m with increased surface albedo

and with decreased evaporation.

Regarding the land surface albedo: the land albedo should be the ocean albedo plus 0.07.

So if the ocean has an albedo of 0.06, the land should have an albedo of 0.13. For some

models the ocean albedo is not a constant number but depends on the zenith angle and the

ratio of diffuse vs. direct radiation. In this case, the land albedo should be the ocean albedo

calculated for the specific conditions plus 0.07.

Regarding decreased evaporation over land: over land we aim at a reduction of evaporation

compared to ocean. This is achieved by rescaling the moisture transfer coefficient, Cq, which

affects the surface evaporation via

E = Cq| ~V1|(q1 − qs), (1)

with subscripts 1 and s denoting values at the lowest model level and at the surface. Over

land, the transfer coefficient Cq should be halved, i.e.,

Cq → Cq ·1

2, (2)

which assuming no changes in surface wind speed and boundary-layer humidity will reduce

the evaporation by a factor of 2. Over ocean the transfer coefficient must not be decreased.

For the LandOrbit simulations, the orbital parameters should be set to

• eccentricity: 0.02

• obliquity: 23.5◦

2

• NH spring equinox on March 21

• longitude of perihelion=270◦, implying that NH solstice occurs during aphelion.

This orbit roughly corresponds to Earth’s comtemporary orbit (Joussaume and Braconnot,

1997). The solar constant and greenhouse gases should be as in the other simulations.

Table 1 – List of requested simulations.

Simulation years specificationsAquaControl 15+30 aquaplanet control simulation (includes 15 years of spin up)Aqua4xCO2 40 as AquaControl but with quadrupled CO2,

to be restarted from end of AquaControl (Dec 30 of year 45)LandControl 40 as AquaControl but with idealized continent,

to be restarted from end of AquaControl (Dec 30 of year 45)Land4xCO2 40 as LandControl but with quadrupled CO2,

to be restarted from end of LandControl (Dec 30 of year 40)LandOrbit 40 as LandControl but with changed orbital parameters,

to be restarted from end of LandControl (Dec 30 of year 40)

2 Requested simulations

Table 1 lists the requested simulations. To quantify the transient response and the adjusted

radiative forcing, all of the simulations except AquaControl should be restarted from another

simulation as described in the table.

3 Requested Output

We request output following the standard output of CMIP5 (see http://cmip-pcmdi.llnl.

gov/cmip5/docs/standard_output.pdf). To facilitate the analysis of the simulations, the

data should be “cmorized”. By cmorizing we here mean that the variables are named ac-

cording to the CMIP5 names and have the same units as in CMIP5, and that 3d-data is

interpolated to the 17 CMIP5 pressure levels (1000, 925, 850, 700, 600, 500, 400, 300, 250,

200, 150, 100, 70, 50, 30, 20, 10 hPa)

• Monthly-mean output: all fields of the CMOR Table Amon except fields related to

carbon mass flux and mole fractions of ozone etc., saved for all years and simulations

except the 15 years of initial spinup for AquaControl

• Daily output: same fields as for monthly-mean output, saved for the last 10 years of

each simulation

3

• 3-hourly output: same fields as for monthly-mean output, saved for the last 3 years of

each simulation

As for whether fields should be saved as means or snapshots, we adapt the CMIP5 conven-

tions. For the monthly and daily output streams, all fields should be means over the output

period. For the 3-hourly output stream, T (including surface and atmosphere), u, v, ω, q, z

(geopotential height), should be snapshots whereas all other fields should be means.

4 Specification of the ocean q-flux

The slab ocean allows interactive sea-surface temperatures without the large computational

burden from coupling to a dynamic three-dimensional ocean model. The q-flux is derived

from present-day observations, with details given below. It is provided as a netcdf file that

includes the q-flux for the aquaplanet simulations and a corrected q-flux for the simulations

with land. The q-flux can also be implemented in the models by using the polynomial

representation that is given in section 4.1. Fig. 1 shows the q-flux and the corresponding

ocean heat transport, and compares the two with the present-day values. The q-flux is

constant in time. For the aquaplanet simulations, the q-flux is zonally-symmetric. For the

land simulations, the q-flux is zonally-symmetric over ocean regions and zero over land.

4.1 Derivation of the q-flux

For the q-flux we make use of present-day observations of the annual-mean TOA radiation

budget and reanalysis estimates of the atmospheric energy transport (ATM). From the

observed time-mean estimate of the total ocean heat transport,

qobs(λ, ϕ) = TOA(λ, ϕ)− ATM(λ, ϕ), (3)

we calculate the zonal-mean time-mean q-flux

qobs(ϕ) =1

2π

∫ 2π

0qobsdλ. (4)

For the zonal average, land points are included with their q-flux being set to zero. I.e., the

zonal average is not weighted by the sea-land mask. Then, we fit qobs by a hemispherically-

dependent polynomial of degree four to smooth out smaller-scale wiggles in the mid- and

high-latitudes. This is done to avoid that these wiggles aggravate model differences in the jet

position, and to avoid an imprint on these observed wiggles on the extratropical atmospheric

circulation in the idealized simulations pursued here. Specifically, the q-flux is given by

q(ϕ) = p0 + p1 · ϕ+ p2 · ϕ2 + p3 · ϕ3 + p4 · ϕ4. (5)

4

60S 30S Eq 30N 60N60

50

40

30

20

10

0

10

20

30qflux a

t oce

an g

rid c

ells

(W

m-2

)

observed q-flux

idealized q-flux on aquaplanet

idealized q-flux with continent

60S 30S Eq 30N 60Nlatitude

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

oce

an h

eat

transp

ort

(PW

)

Figure 1 – Q-flux and ocean heat transport from observations (blue) and the fit used in theMIP (green). The q-flux for simulations with land is given in red; note that in the case of landthe q-flux shown is not the zonal-mean q-flux but the q-flux over ocean points only.

5

For the aquaplanet simulations, the polynomial coefficients are in the Northern hemisphere

(ϕ > 0)

pNH0 = −50.1685

pNH1 = 4.9755

pNH2 = −1.4162 · 10−1

pNH3 = 1.6743 · 10−3

pNH4 = −6.8650 · 10−6,

where ϕ has units of deg latitude. In the Southern hemisphere (ϕ < 0),

pSH0 = −56.0193

pSH1 = −6.4824

pSH2 = −2.3494 · 10−1

pSH3 = −3.4685 · 10−3

pSH4 = −1.7732 · 10−5

As described above, for the land simulation the q-flux is set to zero over land. This requires

a small uniform correction of -0.59 Wm−2 to guarantee that the global-mean q-flux is zero.

The uniform correction is applied to all ocean points (but not to the land points) by setting

pNH0 → pNH0 − 0.59 and pSH0 → pSH0 − 0.59.

6

5 ECHAM6.1 reference plots

This section provides plots for the ECHAM6.1 AquaControl and LandControl simulations

as a reference for other modelling groups. This does not mean that other models must show

similar or the exact same results, but the plots might provide guidance to check whether

the setup was implemented correctly. We particularly encourage modelling groups to check

the top-of-atmosphere solar insolation, the land surface albedo diagnosed from the surface

shortwave fluxes, and the q-flux over ocean, which can be diagnosed as the sum of shortwave

and longwave radiative fluxes and the latent and sensible heat fluxes.

5.1 TOA shortwave downward insolation

50 0 50latitude

2

4

6

8

10

12

month

TOA incoming solar (Wm-2)

0.0

68.8

137.5

206.2

275.0

343.8

412.5

481.2

550.0

100 50 0 50 100latitude

150

200

250

300

350

400

450

TO

A inco

min

g s

ola

r (W

m-2

)

Figure 2 – Top-of-atmosphere downward shortwave irradiance.

5.2 AquaControl

100 50 0 50 100latitude

60

50

40

30

20

10

0

10

20

30

qflux

diagnosed from sfc balance

protocol

100 50 0 50 100latitude

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

oce

an h

eat

transp

ort

(PW

)

100 50 0 50 100latitude

6

4

2

0

2

4

6

atm

. heat

transp

ort

(PW

)

ECHAM6.1 AquaControl (itczmip001)

Figure 3 – ECHAM6.1 AquaControl: ocean q-flux (right) and meridional energy transportof the ocean (middle) and the atmosphere (right).

7

20 15 10 5 0 5 10 15 20latitude

2

4

6

8

10

12

month

Tropical Precipitation (mm/day)

0.00

3.25

6.50

9.75

13.00

16.25

19.50

22.75

26.00

100 50 0 50 100latitude

1

2

3

4

5

6

7

8

9

10

Pre

cipit

ati

on (

mm

/day)

global mean: 4.05 mm/day

50 0 50latitude

2

4

6

8

10

12

month

Surface temperature (K)

264.00

270.29

276.57

282.86

289.14

295.43

301.71

308.00

100 50 0 50 100latitude

275

280

285

290

295

300

305

Surf

ace

tem

pera

ture

(K

)

global mean: 295.4 K

ECHAM6.1 AquaControl (itczmip001)

Figure 4 – ECHAM6.1 AquaControl: seasonal cycle of tropical precipitation (top left) andsurface temperature (bottom left). Right: time-mean zonal-mean precipitation and surfacetemperature; global-mean values are given on top of the plots.

8

5.3 LandControl

100 50 0 50 100latitude

60

50

40

30

20

10

0

10

20

qflux o

ver

oce

an p

oin

ts (

W/m

2)

diagnosed from sfc balance

protocol

100 50 0 50 100latitude

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

oce

an h

eat

transp

ort

(PW

)

100 50 0 50 100latitude

6

4

2

0

2

4

6

atm

. heat

transp

ort

(PW

)

ECHAM6.1 LandControl (itczmip003)

Figure 5 – ECHAM6.1 LandControl: ocean q-flux over ocean points (right), and meridionalenergy transport of the ocean (middle) and the atmosphere (right).

9

150 100 50 0 50 100 150longitude

50

0

50

lati

tude

Surface albedo (%)

4

6

8

10

12

14

ECHAM6.1 LandControl (itczmip003)

Figure 6 – ECHAM6.1 LandControl: time-mean surface albedo calculated by the surfacedownward and upward shortwave radiative flux.

10

20 15 10 5 0 5 10 15 20latitude

2

4

6

8

10

12

month

Tropical Precipitation (mm/day)

0.00

3.25

6.50

9.75

13.00

16.25

19.50

22.75

26.00

100 50 0 50 100latitude

1

2

3

4

5

6

7

8

Pre

cipit

ati

on (

mm

/day)

global mean: 3.94 mm/day

50 0 50latitude

2

4

6

8

10

12

month

Surface temperature (K)

264.00

270.57

277.14

283.71

290.29

296.86

303.43

310.00

100 50 0 50 100latitude

275

280

285

290

295

300

305

Surf

ace

tem

pera

ture

(K

)

global mean: 294.8 K

ECHAM6.1 LandControl land+ocean (itczmip003)

Figure 7 – ECHAM6.1 LandControl: seasonal cycle of tropical precipitation (left) and surfacetemperature (bottom left). Right: time-mean zonal-mean precipitation and surface tempera-ture; global-mean values are given on top of the plots.

11

20 15 10 5 0 5 10 15 20latitude

2

4

6

8

10

12

month

Tropical ocean precipitation (mm/day)

0.00

3.25

6.50

9.75

13.00

16.25

19.50

22.75

26.00

100 50 0 50 100latitude

1

2

3

4

5

6

7

8

9

Oce

an p

reci

pit

ati

on (

mm

/day)

50 0 50latitude

2

4

6

8

10

12

month

Ocean surface temperature (K)

264.00

270.57

277.14

283.71

290.29

296.86

303.43

310.00

100 50 0 50 100latitude

275

280

285

290

295

300

305

Oce

an s

urf

ace

tem

pera

ture

(K

)

ECHAM6.1 LandControl ocean only (itczmip003)

Figure 8 – ECHAM6.1 LandControl: seasonal cycle of tropical precipitation (left) and surfacetemperature (bottom left) only over ocean points. Right: time-mean zonal-mean precipitationand surface temperature only taking into account ocean points.

12

20 15 10 5 0 5 10 15 20latitude

2

4

6

8

10

12

month

Tropical land precipitation (mm/day)

0.00

3.25

6.50

9.75

13.00

16.25

19.50

22.75

26.00

30 20 10 0 10 20 30latitude

1

2

3

4

5

6

7

Land p

reci

pit

ati

on (

mm

/day)

50 0 50latitude

2

4

6

8

10

12

month

Land surface temperature (K)

264.00

270.57

277.14

283.71

290.29

296.86

303.43

310.00

30 20 10 0 10 20 30latitude

297

298

299

300

301

302

303

304

305

306

Land s

urf

ace

tem

pera

ture

(K

)

ECHAM6.1 LandControl land only (itczmip003)

Figure 9 – ECHAM6.1 LandControl: seasonal cycle of tropical precipitation (left) and surfacetemperature (bottom left) only over land points. Right: time-mean zonal-mean precipitationand surface temperature only taking into account land points.

13

-120 0 120-90

-60

-30

0

30

60

90Surface temperature (K)

275.00

279.12

283.25

287.38

291.50

295.62

299.75

303.88

308.00

-120 0 120-90

-60

-30

0

30

60

90Precipitation (mm/day)

0.00

1.11

2.22

3.33

4.44

5.56

6.67

7.78

8.89

10.00

-120 0 120-90

-60

-30

0

30

60

90Latent heat flux (W/m2)

0

20

40

60

80

100

120

140

160

180

-120 0 120-90

-60

-30

0

30

60

90Sensible heat flux (W/m2)

0.00

4.44

8.89

13.33

17.78

22.22

26.67

31.11

35.56

40.00

ECHAM6.1: LandControl (itczmip003)

Figure 10 – ECHAM6.1 LandControl: time-mean surface temperature, precipitation andsurface latent and sensible heat fluxes.

14

References

Joussaume, S. and P. Braconnot, 1997: Sensitivity of paleoclimate simulation results to

season definitions. Journal of Geophysical Research: Atmospheres, 102 (D2), 1943–1956,

doi:10.1029/96JD01989.

Kang, S. M., I. M. Held, D. M. W. Frierson, and M. Zhao, 2008: The Response of the ITCZ

to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with a GCM. J.

Climate, 21, 3521–3532, doi:10.1175/2007JCLI2146.1.

Voigt, A., B. Stevens, J. Bader, and T. Mauritsen, 2014: Compensation of hemispheric

albedo asymmetries by shifts of the ITCZ and tropical clouds. J. Climate, 27, 1029–1045,

doi:10.1175/JCLI-D-13-00205.1.

15