1. A. Bozec 1, M. Kageyama 1, G. Ramstein 1, M. Crépon 2, L. Li 3 and P. Bouruet-Aubertot 2 1 - Laboratoire des Sciences du Climat et de l’Environnement/IPSL, Gif-sur-Yvette, France 2 - Laboratoire d’Océanographie et de Climatologie -Expérimentations et Approches Numériques-, ex- LODYC/IPSL, Paris, France 3- Laboratoire de Météorologie Dynamique, Paris, France The Mediterranean thermohaline circulation during the Last Glacial Maximum inferred from a 1/8° resolution oceanic model: impact of the sea-level change IMPACT OF THE SEA-LEVEL DECREASE PRESENT TIME MEDITERRANEAN THERMOHALINE CIRCULATION Contact : alexandra.bozec@cea.fr LAST GLACIAL MAXIMUM REFERENCES Goudie, A., 1992: Environmental Change. Clarendon, Ed., 329. Hayes, A., Kucera, M., Kallel, N., Sbaffi, L., Rohling, E.J., 2005: Glacial Mediterranean sea surface temperatures based on planktonic foraminiferal assemblages. Quaternary Science Reviews, 24, 999- 1016. Kucera, M., Rosell-Melé, A., Schneider, R., Waelbroeck, C., Weinelt, M., 2005: Multiproxy approach for the reconstruction of the glacial ocean surface (MARGO). Quaternary Science Reviews, 24, 813- 819. Lascaratos, A., Roether, W., Nittis, K., Klein, B., 1999: Recent changes in deep water formation and spreading in the eastern Mediterranean Sea: a review. Progress in Oceanography, 44, 5-36. Madec, G., Delecluse, P., Imbard, M., Lévy, C., 1999: OPA8.1, Ocean General Circulation Model, Reference manual. Myers, P. G., Haines, K., Rohling, E., 1998: Modeling the paleocirculation of the Mediterranean: The last glacial Maximum and the Holocene with emphasis on the formation of sapropel S1. Paleoceanography, 13, 586-606. During the Last Glacial Maximum (LGM), 21,000 years ago, ice sheets covered an important part of Canada and northwestern Eurasia (Fig. 1). The averaged temperature of Earth was colder than the present one by around 4°C, but the cooling was unequally distributed: intense in the North Atlantic ocean and over the ice-sheets, it was weaker in the tropical regions. Thus, an important quantity of water was kept by ice-sheets leading to a global decrease of the sea-level of 120m (Fig. 2) and then to a redefinition of the coastal areas. For example, the English Channel and the continental shelf of Brittany were exposed during the LGM. In the Mediterranean Sea, this sea- level decrease also changed the coasts of the basin (Fig. 5 b). Associated with strong atmospheric forcing, this change of sea-level may have strongly influenced the Mediterranean thermohaline circulation (THC). Observations in this region give evidence of a decrease of precipitation and temperature (Goudie, 1992). Myers et al. (1998) attempted to describe the LGM Mediterranean THC using a 1/4° oceanic model. Their LGM initial state was built using T and S anomaly profiles. As results, they found no reversal of the circulation at Gibraltar and Sicily straits. Deep convections appeared in the Western and in the Levantine basin but not in the Adriatic. The purpose of this study is first to quantify the impact of the sea-level change and then to try to reproduce the LGM conditions in a 1/8° regional OGCM, MED8, based on the OPA oceanic model (Madec et al., 1998) as in Myers et al (1998). In a second step, the initial condition of the LGM simulation will be provided by the IPSL coupled model and by the MARGO SST data (Fig. 3) from the MARGO project (Kucera et al., 2005; Hayes et al., 2005). Figure 1: Ice-sheet extention a) during the Last glacial Maximum and b) in present day Figure 2: Evolution of the sea-level anomaly during the last million years. Figure 4 : Schematic of the Thermohaline circulation in the Mediterranean Sea. (Lascaratos et al, 1999) The Mediterranean Sea is an evaporative basin (~1m/y, compensated by the Atlantic water imports). Its thermohaline circulation is driven by the density gradient between the Atlantic waters and the Mediterranean waters. The main circulation cell can be described as follow (Fig4): Atlantic water enters the Gibraltar strait and flows along the Algerian coast. It then passes the Sicily strait and flows in the Eastern basin along the Middle East coast and then further through the Levantine basin. During this progression temperature and salinity properties of the Atlantic water are modified due to surface fluxes and mixing processes, leading to an increase of its density. These changes eventually lead to the formation of an intermediate water mass, the Levantine Intermediate water (LIW) at a depth of 400-500m. This salty LIW crosses back the Sicily strait in depth and flows through the Western basin and eventually through the Gibraltar strait. Beside this main circulation cell, there are two main sites of deep water formation : the Gulf of Lyons in the Western basin for the Mediterranean Deep Water (WMDW) and the Adriatic Sea for the Eastern Mediterranean Deep Water (EMDW). Deep convection can reach 2500m in the western basin, whereas the EMDW formed in the shallow Adriatic basin (800-1000m) then crosses the Otranto strait and sinks to the bottom of the Eastern basin. The Mediterranean thermohaline circulation (THC) is very sensitive to climatic conditions. Two main controlling factors explain this variability, namely the west-east density gradient and the sea level that strongly influences the exchanges through the Gibraltar and the Sicily straits. We investigate the impact of these two factors during the Last Glacial Maximum (LGM, 21 000 years ago). Indeed during that period in addition to cold climatic conditions, the sea level was decreased by 120m compared to the present one. A 1/8° resolution ocean numerical model based on the OPA model is used. The control simulation (MED8-P) reproduces present climate and Mediterranean Sea conditions. In a second simulation (MED8-T) the sea level was decreased to its LGM level, while the atmospheric forcing is the present one. The third simulation (MED8-G) will reproduce the oceanic LGM configuration (LGM sea-level and forcing). As first results, we present here the impact of the sea-level change on the Mediterranean circulation. PERSPECTIVES : PREPARATION OF THE MEDITERRANEAN LGM CONFIGURATION SIMULATION WITH LGM Sea-level (-120m). INITIAL CONDITIONS: MEDATLAS climatology + anomalies T/S of the IPSL coupled model (LGM configuration). ATMOSPHERIC CONDITIONS: high-resolution ECMWF fluxes and wind + anomalies of the IPSL coupled model (LGM configuration). Figure 6: a) Gibraltar and b) Sicily straits transport for MED8-P (dashed line) and MED8-T (full line). MED8-P Figure 5: Winter surface circulation (arrows) for a) MED8-P and b) MED8-T on the salinity (colour scale). The surface circulation is similar in MED8-T and in MED8-P. No reversal of the transport is observed at the principal straits of the basin. However, the intensity of the circulation is reduced (Fig 5). At the Gibraltar and Sicily straits, we find a decrease of about a 2/3 of the present transport only due to this change of sea-level (Fig. 6). This decrease of the transport has an impact on the evolution of the thermohaline properties of the basin. Figure 7 presents the temperature and salinity gradient between the Eastern and the Western basin and as a first result, we observe a decrease of the temperature and salinity gradient averaged over the water column. However, if we divide the basin into 3 distinct layers (surface, intermediate and bottom layer), we can see a increase of these gradients in the surface layer. This can be explained by the less important import of warm and fresh water from the Atlantic ocean increasing the gap between the two sub-basins. This result can be compared with the strong SST-gradient found with the MARGO-SST reconstruction (see Fig. 3). Nevertheless, the MED8-T gradient increase is less important than the MARGO-SST gradient (6°C in winter, 8°C in summer). Figure 7: Temperature (a, b, c and d) and salinity (e, f, g and h) gradient for the whole water column (a and e), between 0-250m (b and f), between 250 and 1400m ( c and g) and below 1400m (d and h). MED8-P in black and MED8-T in blue. MED8-T a) b) a)b) MED8-T MED8-P GIBRLATAR SICILY Figure 3: Reconstruction of the MARGO-SST in the Mediterranean Sea interpolated on a 1/4° grid for a) Winter and b) Summer. 0.7 ± 0.05 Sv 0.23 ± 0.03 Sv 1.12 ± 0.14 Sv 0.41 ± 0.08 Sv QUESTION: Was the Mediterranean LGM Thermohaline Circulation similar to the present one ? METHOD: 1- Impact of the sea-level Change under present climate condition. 2- 100 year simulation of a Mediterranean Sea LGM configuration forced by LGM atmospheric conditions. CONCLUSIONS : IMPACT OF SEA-LEVEL CHANGE Decrease of the intensity of the circulation of about 2/3. No reversal of the circulation at the principal straits. Slight increase of the east-west gradient of surface temperature and salinity due to the slowing down of the circulation. VARIATIONS OF THE SEA-LEVEL a) b) MED8-P: MEDATLAS climatology forced with high resolution (60km) ECMWF fluxes and wind, present sea-level. MED8-T: MEDATLAS climatology forced with high resolution (60km) ECMWF fluxes and wind, LGM sea-level (-120m).