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