CANTICO: Final report from CNRS/LMD
Laurent Li (li@lmd.jussieu.fr)
Laboratoire de Météorologie Dynamique du CNRS
Casier 99 – UPMC, 4 place Jussieu, 75252 Paris cedex 05
May 2011
Abstract: In the framework of CANTICO, LMD has accomplished two simulations with a regional
ocean-atmosphere coupled model. The first one was driven by the ERA40 reanalysis for the period
from 1958 to 2002. It was the most realistic run to simulate the Mediterranean Sea variation for that
period. The second one was a scenario run, driven by the outputs of the IPSL-CM4 global coupled
model. It covers the period from 1951 to 2050. The historical estimation of greenhouse gases in the
atmosphere is used before 2000 and the scenario IPCC-A1B is used after the year 2000. Main outputs
of the two simulations are stored in a DODS server in Paris and freely available to the whole scientific
community.
Description of the regional coupled model
The regional coupled model consists of an atmospheric component, LMDZ-med, and an oceanic
component, NEMO-Med8. The two independent models were running together through a coupler
OASIS. LMDZ-med provides heat fluxes and wind stress at the ocean-atmosphere interface to NEMOmed8. NEMO-med8 provides in return the sea-surface temperature (SST) to LMDZ-med. The coupling
frequency is set to each day, which implies a neglect of the diurnal cycle in the ocean-atmosphere
interaction. A schematic of the coupled system is given in Figure 1.
LMDZ-med is a regionally-oriented atmospheric general circulation model. It is identical to LMDZglobal, but with a variable grid. A local zoom is put over the Mediterranean basin to enhance the
resolution at regional scale to about 30 km. LMDZ-med is operated just as an ordinary regional
climate model when it is nested into observations or into outputs of global model simulations. In the
vertical, LMDZ-med has 19 layers. The model’s physics keeps the same as in LMDZ-global.
NEMO-MED8 is the oceanic general circulation model for the Mediterranean Sea. It is developed in
Meteo-France, on the NEMO platform. Its spatial resolution is 1/8°, about 12 km. The NEMO-MED8
model is the Mediterranean version of the NEMOv2 ocean platform developed at LOCEAN (France)
also called OPA9. The NEMO-MED8 model has been particularly designed with an implicit free
surface option, a buffer zone in the near Atlantic ocean, explicit river runoff fluxes, 43 Z-levels with
partial steps and a spatial resolution of 1/8° (actually from 9 to 12 km from the north to the south of
the basin). NEMO-MED8 does not cover the Black Sea. Its grid is tilted and stretched within the Strait
of Gibraltar to increase its resolution up to 6 km and to follow better the SW-NE orientation of the
strait. The grid definition, the surface forcing method as well as many physical options can be found
in Sevault et al. (2010).
Description of the simulations
The above-described regional coupled model was used to produce two different simulations. The
first one, referred to as ERA40 simulation, is driven by the ERA40 reanalysis at the atmospheric
model’s boundaries (which are, in our case, the whole earth except the zoomed domain). A
relaxation procedure is applied to make model’s temperature, circulation (horizontal wind) and
specific humidity close to those from the ERA40 reanalysis. The time scale used in the relaxation is
set to half an hour for temperature and circulation, but 2 hours for humidity. The ERA40 simulation
starts from 1958 and ends in 2002.
As for a global ocean-atmosphere coupled model, a spin-up simulation is also necessary for a
regional ocean-atmosphere coupled model, in order to obtain an adequate initial state for the ocean.
The general methodology to obtain an initial state for the global ocean is to run the oceanic model
only and then the coupled model in a perpetual mode for many years, typically a few hundreds of
years. For the regional coupled model, we can certainly use the same approach, but we have the risk
that the specific year used in driving the regional atmospheric model is a particular year: too warm,
too cold or having too strong wind stress. This can make the Mediterranean Sea into a particular
state which may not be appropriate for an initial state. We can overcome this problem by using a
sequence of observed years. But this needs a long sequence, not always available from observations.
We can also repeat the same short temporal sequence a few times, but we have the risk that an
artificial frequency (the length of the sequence) is introduced into the system.
Our solution is finally to use this short-term sequence randomly. That is, a long sequence is
constructed through a random choice of different years in the observed sequence. This random-year
approach has already been used by Bozec et al. (2006) for studying responses of the Mediterranean
Sea to interannual variability of the atmosphere. In our study, the observed sequence is the ERA40
data from 1958 to 1977, a total of 20 years. Each year is randomly used two times to ensure a total
length of 40 years in the spin-up simulation. We believe that his random-year approach is a good one
to initialize the ocean. The duration of 40 years is also a good duration for the upper layers of the
Mediterranean Sea to be in equilibrium. The deep sea, however, may need much more time to reach
its equilibrium. But for our purposes, it has small influences on the upper layers of the
Mediterranean Sea.
A few selected results
After the spin-up which lasts 40 years, the regional coupled model is driven by the ERA40 data from
1958 to 2001 (44 years). We need to emphasize that our coupled model has no flux adjustment for
the whole duration of the simulation, neither in heat flux, nor in fresh water flux.
Figure 2 shows the evolution of the Mediterranean thermal and saline contents for different layers,
from 1958 to 2002. Generally speaking the model is stable and has a good performance in
reproducing the observed evolution. For the surface temperature, the cooling from 1960s to 1980s is
well captured, so is the warming trend during the recent period. The warming can be probably
attributed to the global warming.
Figure 3 shows the mixed-layer depth for February and March respectively. They are the
climatological average for the whole ERA40 period. Main deep water formation sites are well
captured, namely, the Gulf of Lions, the Adriatic Sea and the Aegean Sea. The convection in the
Adriatic Sea is probably exaggerated, due to an excessive salinity bias.
The lower-right panel of Figure 4 shows the zonal overturning stream-function for the whole
Mediterranean, a longitude/depth diagram of the mass circulation. We can observe the Modified
Atlantic Water entering through the Strait of Gibraltar. We can also see the Levantine Intermediate
Water and the deep water from the Adriatic Sea. The other three panels of Figure 4 are the
meridional overturning stream-functions for the Western basin, the Adriatic Sea and the Aegean Sea
respectively. They show the deep water formation in these sub-basins, dominated by a meridional
overturning.
Figure 5 shows the deep-water formation rate (in Sverdrup) in the Gulf of Lions. There is a large
interannual variability for the period. The decade 1980s is remarkable for deep water formation in
the Gulf of Lions.
Figure 6 shows the temporal evolution of the Mediterranean thermal and saline contents for the
scenario run from 1951 to 2050 (historical emission of greenhouse gases before 2000 and A1B
scenario after 2000). The spin-up period for this run was not long enough, which implies the initial
drift in terms of temperatures. For salinity, there is a continuous drift, especially in the surface layer.
Figure 7 shows the zonal overturning function, for present-day climate and future climate. It is clear
that the Mediterranean overturning circulation is strongly reduced when the global climate warms
up. This is also confirmed in Figure 8 which shows the deep-water formation rate in the Gulf of Lions.
There is almost a complete stop of deep water formation. The consequence for the Mediterranean
functioning would be very important.
Climate change in the Gulf of Gabes
CANTICO focuses on the Gulf of Gabes as a demonstration site. We need to check a few results for
the Gulf of Gabes. The selected region, considered as the Gulf of Gabes, comprises a rectangular
from 10 to 11.2E and from 33.8 to 35.3N. A land-sea mask is applied when averaging over the area in
order to take into account only the sea points. Results are displayed in Figure 9 to show the time
evolution during the whole period from 1951 to 2050.
Panels (a, b, c, and d) give the four components of the surface heat flux: shortwave radiation,
longwave radiation, latent heat flux and sensible heat flux. There is no obvious trend for the
shortwave radiation and the sensible heat flux. The longwave radiation has a decreasing trend, due
to the increasing greenhouse gases in the atmosphere. The latent heat flux is increasing due to
enhanced evaporation when the climate is warmed. The two terms are roughly canceled each other
to keep the net surface energy balanced.
Panel (e) in Figure 9 gives the time evolution of rainfall rate from 1951 to 2050. We can observe a
weak decreasing trend, especially at the end of the considered period. Panel (f) is the surface wind
speed which presents also a decreasing trend. The last two panels (g and h) give the surface water
temperature and salinity. The global warming makes an increase of temperature by about 2°C at the
end of the period, compared to the beginning. The salinity shows a weak decreasing trend, although
evaporation increases and precipitation decreases in the same region. It is clear that the saline
budget of the Gulf of Gabes is under strong influence of horizontal advection, due to strong costal
currents.
Availability of the simulations
Results of the two simulations, in the format of NetCDF, are available from a DODS server in Paris.
Please check the web page http://www.lmd.jussieu.fr/~li/cantico to download the data. For the time
being, the database is open to the general public through the web site without any restriction. A few
results for the Gulf of Gabes were used by other partners and also available on their web site with a
more convenient graphic interface.
Figure 1: Schematic of the regional ocean-atmosphere coupled model developed and used in LMD.
The atmospheric component LMDZ-med (about 30 km as horizontal resolution) and oceanic
component NEMO-MED8 (horizontal resolution at 1/8°) are coupled together through the coupler
OASIS. The time interval of exchange is daily.
Figure 2: Thermal (left) and saline (right) contents in function of time, integrated for the whole
Mediterranean Sea and for different layers (top to 150 m; 150-600 m; 600 m to bottom; and total).
Units are °C for temperature and PSU for salinity.
Figure 3: Mixed-layer depth (m) of the Mediterranean for February (top panel) and March (bottom
panel) respectively.
Figure 4: (the larger panel at lower-right) The zonal overturning stream-function for the whole
Mediterranean. (the three smaller panels) The meridional overturning stream-function integrated for
the whole Western basin, Adriatic Sea and Aegean Sea. Units are Sv.
Figure 5: Deep-water formation rate in the Gulf of Lions (Sv).
Figure 6: Thermal and saline contents in function of time, as in Figure 2 but for the scenario A1B
simulation.
Figure 7: The zonal overturning stream-function (Sv) simulated in the scenario A1B simulation. The
upper panel is for the average from 1961 to 1990, and the lower panel from 2021 to 2050.
Figure 8: Deep-water formation rate in the Gulf of Lions as simulation in the A1B scenario.
Figure 9: Evolution of different surface variables averaged over the Gulf of Gabes, simulated in the
A1B scenario. a) shortwave radiative flux (W/m2); b) longwave radiative flux (W/m2); c) latent heat
flux (W/m2); d) sensible heat flux (W/m2); e) rainfall rate (mm/day); f) wind speed (m/s); g) water
temperature (°C); h) water salinity (psu).
a
b
c
d
e
f
g
h