Flux-anomaly-forced model intercomparison project (FAFMIP)
FAFMIP is an atmosphere-ocean general circulation model intercomparison project
of CMIP6. It
addresses aspects of the Earth system response to forcing, and is of particular
relevance to the WCRP Grand Challenge on sea level rise and regional
impacts. Its goal is to explain the model spread in AOGCM projections of ocean
climate change forced by CO2 increase.
Projections of regional sea level change by CMIP5 AOGCMs,
like earlier AOGCM generations, show a substantial spread due to the different
models' differing simulations of regional ocean density and circulation
changes, especially in high latitudes and the North Atlantic (Yin, 2012,
0.1029/2012GL052947; Bouttes et al., 2012, 10.1029/2012GL054207; IPCC AR5 WG1
chapter 13, Church et al., 2013; Slangen et al, 2014,
10.1007/s10584-014-1080-9). By applying flux perturbations from a range of
CMIP5 models to the same AOGCM, previous analyses have shown that a substantial
fraction, but not all, of the diversity of sea level projections arises from
the spread in AOGCM projections of changes in surface fluxes of momentum
(windstress), heat and freshwater (Bouttes et al., 2012; Bouttes et al., 2013,
10.1007/s00382-013-1973-8).
In the FAFMIP experiments, which could be carried out with AOGCMs or with
OGCMs, a prescribed set of surface flux perturbations will be applied to the
ocean water surface. These perturbations will be obtained from the
ensemble-mean changes simulated at time time of doubled CO2 by CMIP5 AOGCMs
under the 1pctCO2 scenario, so they are representative of projected
anthropogenic climate change. The aims of the experiments are:
- to
quantify the difference in the geographical patterns of sea level change due to
ocean density and circulation change simulated by the models, when given common
surface flux perturbations.
- to provide information about the efficiency and interior distribution of
ocean heat uptake in response to climate change; the AOGCM spread in these
phenomena contributes to their spread in transient climate response and global
mean sea level rise due to thermal expansion.
- to provide information about the sensitivity of the Atlantic meridional
overturning circulation (AMOC) to prescribed buoyancy forcing of the character
expected for CO2 forcing, rather than idealised freshwater forcing such as has
been used in previous AMOC intercomparisons; change in the AMOC is of relevance
to both regional and global sea level rise, as well as to regional climate
change.
The FAFMIP experiments are aimed at increased physical understanding. They
are not themselves policy-relevant scenarios, but obviously the uncertainties
in projection of global and regional sea level and AMOC change are of great
policy relevance.
The experimental design, rationale and progress with the project
are further described in these documents:
FAFMIP discussions
FAFMIP has an email list for
discussion among participants and other interested parties. Please apply to
subscribe if you are interested. So far the list has been quite quiet!
We held our first FAFMIP meeting at GFDL on Mon 17 and Tue 18 July 2017, in
the week following the related WCRP
sea-level conference in New York.
We held our second meeting at the University of Reading UK on
Wed 3 to Fri 5 April 2019.
FAFMIP experiments
All the experiments will add perturbations to the surface fluxes computed by
the AOGCM (like a flux adjustment). The perturbative fluxes depend on the time
within the year but are the same in every year. All the experiments are 70
years long, and should branch from the standard CMIP DECK pre-industrial
control (piControl). The best point to branch would be the same point as the 1%
CO2 experiment (1pctCO2), with which FAFMIP results will be compared. All the
FAFMIP experiments have pre-industrial atmospheric composition and all other
forcing agents as in piControl.
Input files
The surface flux perturbations are supplied as climatological monthly
means in the netCDF files below, conforming to the CF metadata convention. The data variables
in these files are dimensioned (longitude,latitude,time) in Fortran
order, (time,latitude,longitude) in CDL. The time dimension
has size 12, for months from January to December. The data can be regarded as
applying at the middle of the month and it is recommended to interpolate
linearly between them in time to obtain updates at the atmosphere-ocean
coupling interval, as for e.g. AMIP simulations. The spatial resolution is 1
degree. The longitude dimension has size 360, with points running
eastward starting from 0.5degE, and the latitude dimension has size
180, with points running northward starting from 89.5S.
The current versions of these files were made available in August 2015.
- Eastward windstress tauu and
northward windstress tauv in Pa,
with missing data in rows near the poles (because these lie outside the
domains of some models, set to _FillValue of 1e20),
plotted, for faf-stress.
- Surface heat flux hfds in W m-2,
with missing data over land (set to _FillValue of 1e20),
plotted, for faf-heat.
- Surface heat flux
for faf-heat-NA50pct
- Surface heat flux for faf-heat-NA0pct.
- Surface water flux wfo
in kg m-2 s-1,
with missing data over land (set to _FillValue of 1e20),
plotted, for faf-water.
These fields are the time-mean of the difference of 1pctCO2 in years 61-80 from
the corresponding time-mean of piControl in the ensemble mean of 13 CMIP5
AOGCMs viz. CNRM-CM5 CSIRO-Mk3-6-0 CanESM2 GFDL-ESM2G HadGEM2-ES MIROC-ESM
MIROC5 MPI-ESM-LR MPI-ESM-MR MPI-ESM-P MRI-CGCM3 NorESM1-ME NorESM1-M. This was
the set for which all the required diagnostics were available.
Ocean process tendency diagnostics
For the scientific analyses of FAFMIP experiments, it will be very valuable to
have 3D ocean diagnostics of temperature and salinity tendencies
(∂T/∂t and ∂S/∂t) due to the individual physical processes
which modify the state (advection, diffusion, etc.). These diagnostics were
recommended by the CLIVAR Ocean Model Development Panel, and are
described in Appendix L of the paper by Griffies et al.
(2016) doi:10.5194/gmd-9-3231-2016 in Geoscientific Model
Development, on the OMIP contribution to CMIP6. For FAFMIP they are
requested for the portion of the DECK piControl which is parallel to the FAFMIP
experiments, and for the DECK idealised climate change experiments abrupt4xCO2
and 1pctCO2, as well as the FAFMIP experiments themselves.
The request to CMIP6 is for annual means of the d/dt diagnostics as priority
1, because groups might not feel it was practical to save monthly
means. However, if monthly means are saved, there will very likely be
scientific interest in them, for instance in studying unforced
variability. Monthly means are therefore requested at priority 2.
Please note when setting up your CMIP6 experiments that the CMIP request
should always be included when working out the diagnostic list for a given
experiment. The FAFMIP request alone is for the diagnostics specific to FAFMIP
(mostly the tendency diagnostics), not the core diagnostics (most of the things
you expect to use). The core request is included by default in the output of
the drq
utility.
Ocean surface flux diagnostics
It will be useful to have the net heat flux hfds and net water flux
wfo into the ocean water (these are standard CMIP diagnostics) in all
experiments for FAFMIP. Although the FAF experiments do not have modified CO2,
the surface fluxes can be indirectly affected by climate change due to the
perturbations applied. The standard CMIP definitions for these diagnostics are
unfortunately inconsistent regarding "flux adjustment", in that wfo
should include the FAFMIP water flux perturbation, but hfds
should not include the FAFMIP heat flux perturbation.
Method for the tier-1 faf-stress experiment
Interpolate the perturbative stress components to your own ocean grid, and add
them to the momentum balance of the ocean water surface. They should not enter
the sea-ice momentum balance, although presumably the sea-ice velocity will be
indirectly affected. There should be no perturbation applied to any turbulent
mixing scheme that depends on the windstress; the idea is that the perturbation
is just to the momentum balance of the ocean.
Method for the tier-1 faf-heat experiment
The redistributed heat tracer is the passive tracer Tc of Bouttes et al. (2014,
10.1007/s00382-013-1973-8), and the added heat tracer is their passive tracer
Ta.
- Interpolate the prescribed heat flux perturbation fields to your ocean
grid, conserving the ocean area integral as far as possible. For example, the
CDO remapcon tool could do it. Note that the fields have missing
data over land. Exact consistency between us will probably not be possible,
and if we have differences of <5% in the area-integral of the fields applied,
that will probably be substantially less than the model spread of results that
we will want to analyse. For comparative analysis it will be useful if you
could save the fields you actually apply on your grid. For comparison, the
area-weighted average (over the non-missing area i.e. the ocean) of the annual
mean of the heat flux is 1.80338 W m-2. The fraction of non-missing area in the
world is 0.730988 and the world area-integral of the field is 6.72440e+14
W. From January to December, the monthly area-integrals are: 2.89768e+14,
4.83648e+14, 6.21397e+14, 8.28595e+14, 8.54048e+14, 8.62668e+14, 8.40562e+14,
9.44224e+14, 6.79492e+14, 6.15859e+14, 6.37997e+14, 4.11016e+14.
- Implement two passive tracers, called added heat and redistributed heat
(both in units of temperature, like the ocean T field).
The first is initialised to
zero, the second is initialised to the ocean T field in the starting state of
the run. These tracers are requested as diagnostics in the CMIP6 FAFMIP list.
- The perturbative heat flux is added to the surface layer in both the ocean
T field and the added heat tracer field. The purpose of the added heat tracer
is to see where the heat goes - it does not affect the evolution.
- The atmosphere-ocean heat flux computed within the AOGCM is added to the
ocean T field as usual and also to the redistributed heat field.
- The redistributed heat field does not receive the perturbative heat flux.
It is used instead of the ocean T field to supply the SST used by the AOGCM
to calculate the surface heat fluxes. It is also used by the sea-ice model to
calculate the sea-ice basal melting heat flux. Thus, the atmosphere and the
sea-ice are not directly affected by the applied heat flux perturbation (though
they are indirectly by changes in ocean heat transport).
The FAFMIP design paper (Gregory et al.,
2016) comments on the implementation of the faf-heat experiment:
Careful formulation is required to ensure that Q (the atmosphere-ocean
heat flux computed by the model, not including the heat flux perturbation) is
applied in the same way to T (denoted θ in the paper
i.e. the "real" ocean temperature) and TR (the
redistributed ocean temperature), and some differences may be unavoidable,
depending on model formulation. In particular, absorption of solar radiation
should occur with the same vertical profile for both (assuming that some of it
penetrates the top layer), and the same heat flux should be applied to both of
them for evaporation and precipitation (if the sensible heat content of these
water fluxes is considered in the model). If the same amount of heat is
extracted from both tracers for frazil sea-ice formation, T may
sometimes fall below freezing point, requiring special treatment of the
equation of state; on the other hand if T and TR
are separately kept above freezing, there will be a difference in the heat
fluxes implied. It may be useful to check the implementation of
TR in the model with an experiment in which F=0,
which should reproduce the piControl experiment.
Different choices have been made in the various models.
Steve Griffies, Mike Winton and Bill Hurlin have compiled notes on how the faf-heat experiment has been
implemented in GFDL-ESM2M.
In their model, they calculated sea-ice (frazil) formation separately for
T and TR, meaning that the net heat flux applied to the two
tracers is not exactly the same.
In CanESM2, the heat flux is the same for the two tracers;
T is allowed to fall below freezing point,
and the equation of state has been modified to deal with this.
In HadCM3, the heat flux is the same,
and there is an extra implicit vertical mixing scheme (already
included in the model) to prevent
freezing when sea-ice formation does not occur.
Method for the tier-1 faf-water experiment
Interpolate the prescribed water flux perturbation fields to your ocean
grid, conserving the ocean area integral as far as possible. For example, the
CDO remapcon tool could do it. Note that the fields have missing
data over land. For comparative analysis it will be useful if you could
save the fields you actually apply on your grid. The area-weighted average
(over the non-missing area i.e. the ocean) of the annual mean of the water flux
field is very small, only 7.19009e-08 kg m-2 s-1, which is two orders of
magnitude smaller than the spatial standard deviation. This is because water
does not accumulate outside the ocean, in general, and must indicate that water
is reasonably accurately conserved in the CMIP5 model mean. The fraction of
non-missing area in the world is 0.730988 and the world area-integral of the
field is 2.68102e+07 kg s-1.
Method for the tier-2 faf-passiveheat experiment
This experiment should be identical in evolution to the piControl, with the
addition of a passive tracer. Hence if the passive tracer can be implemented in
the piControl, there is no need to do this experiment separately.
- Use the same heat flux input fields as for faf-heat.
- Include the added heat tracer in the model, initialised to zero. The
redistributed heat tracer is not needed.
- Add the perturbative heat flux to the surface layer of the added heat
tracer.
- No changes in coupling are needed. The added tracer does not affect the
model evolution.
Method for the tier-2 faf-all experiment
In this experiment, the same changes to tracers as in faf-heat should be
implemented, and the perturbative fluxes of faf-stress, faf-heat and faf-water
should all be applied.
FAFMIP steering committee:
Jonathan Gregory,
Stephen Griffies, Johann Jungclaus, Oleg Saenko, Detlef Stammer
and Laure Zanna