Evaluation of two global HYCOM 1/12º hindcasts in the Mediterranean Sea Cedric Sommen 1, Alexandra Bozec 2, Eric P. Chassignet 2 Experiments Transport (1Sv=10 6 m 3 /s) Salinity In/out (psu) Temperature In/Out (C) Observations~ / /13. Gibraltar H10.63/ / /13.8 Gibraltar H20.77/ / /13.4 Observations1.10+/ / / Sicily H11.60/ / /18.1 Sicily H21.0/ / / Special military school of Saint-Cyr, French Army, Guer, France 2 Center for Ocean-Atmospheric Predictions Studies, Florida State University, Tallahassee, FL, USA At the Sicily Strait, Hindcast 1 transport is on the high side of the observed estimation range, as we expected from the strength of the circulation. Hindcast 2 stays closer to the average estimate. In surface, the eastward flow is warmer and saltier for both simulation. At depth, despite the deep convection events in the eastern basin, the westward flow is slightly fresher (-0.1psu) and warmer (+2ºC) for Hindcast 1 and fresher (-0.3psu) and warmer (+1ºC) for Hindcast 2. At the Gibraltar strait, both transport are close to the observations but the properties of the outflow are warmer ( ºC) and fresher ( psu). In order to see the performance of the two assimilation methods, we compare the results of Hindcast 1 and Hindcast 2 with two assimilated ARGO trajectories (Figure 4 and 5). First we observe a mild negative bias between the GDEM3 climatology and the ARGO data in the western and eastern basin. In Western Basin: Hindcast 1 shows salinity differences of more than 0.5 psu and more than 3ºC over the water column, Hindcast 2 shows differences of less than 0.3psu and less than 1ºC (Figure 4). In the Eastern basin: Hindcast 1 shows a better agreement with the ARGO profiles in the eastern basin compared with the western basin. Hindcast 2 results are still better than Hindcast 1 in the eastern basin (Figure 5). The assimilation method using MODAS synthetics gives better results in the Mediterranean Sea than the Cooper-Haines assimilation method in terms of: large and small scale T/S properties large-scale Circulation pattern transport at the strait ABSTRACT The Mediterranean Sea is a mid-latitude marginal sea connected to the Atlantic Ocean by the Gibraltar Strait. It has the particularity to produce dense (warm and salty) water that spreads into the Atlantic Ocean at a rate of about 0.7 Sv and thus can potentially have an impact on the global thermohaline circulation. In this study, we evaluate the ability of two hindcast simulations of the global 1/12º HYCOM on the period June 2007 to September 2008 to reproduce the main features of the Mediterranean Sea. Results show a salty and warm bias in the salinity and temperature of the whole basin for both simulations when compared with the GDEM3 climatology. Transports at the Gibraltar strait are in agreement with the observations but the outflow waters are warmer and fresher than observed for both simulations. In the first hindcast, deep convections persist in the eastern basin along the year, possibly due to a destabilization of the water column by the assimilation scheme. The second hindcast presents a stable thermohaline circulation and is able to maintain most of the observed water mass. The comparison with ARGO profiles shows a better agreement with the second hindcast results compared with first hindcast results, especially in the western basin.. EXPERIMENTAL SET-UP In this study a global configuration of the HYbrid Coordinate Ocean Model (HYCOM 2.2) combined with the Navy Coupled Ocean Data Assimilation (NCODA) system for data assimilation is used. The initial condition in temperature and salinity are given by the GDEM3 climatology and the surface forcing is provided by the Navy Operational Global Atmospheric Prediction System (NOGAPS) ( Hindcast 1: Spin-up with climatological ERA Hindcast with 3-hourly 0.5º NOGAPS from Nov 2003-present Vertical projection of observed SSH using (Cooper and Haines, 1996). Hindcast 2: Spin-up with climatological ERA winds scaled to Quickscat windspeed Hindcast with with 3-hourly 0.5º NOGAPS + winds scaled to Quickscat windspeed from June present. Using synthetics temperature profiles from the Modular Ocean Data Assimilation System (MODAS) database. Figure 4: trajectory of float between Jun1007 and Sep Anomalies of temperature and salinity between GDEM3 (top), Hindcast 1 (middle),Hindcast 2 (bottom) and the ARGO profiles. The green dot is the starting point and red dot is the ending point of the trajectory. Figure 5: idem figure 4 for float SUMMARY Figure 3: Surface circulation in winter for Hindcast 1 (top) and Hindcast 2 (bottom). The main features of the western Mediterranean circulation are reproduced in both simulation (Millot et al. 1999). However, in the eastern basin, Hindcast 1 presents a cyclonic gyre settled between North Africa and the Cretan Island that is in disagreement with the observations. We can also notice that the intensity of the circulation is more important in Hindcast 1.CIRCULATIONTRANSPORT Figure 1: Time series of Temperature and Salinity for GDEM3 (Black), Hindcast 1 (blue) and Hindcast 2 (green) for the whole basin (top), the western basin (middle) and the eastern basin (bottom). Hindcast 1 shows a bias of 0.4ºC in the western basin as well as a salty bias of 0.04psu in the eastern basin. We can notice that Hindcast 2 is closer to the GDEM3 climatology than Hindcast1. Figure 2: Vertical section of salinity at 34ºN for GDEM3, Hindcast 1 and Hindcast 2. Hindcast 1 shows a deep convection that actually persist during the year. That causes a salinity bias in this area. Hindcast 2 shows the same behavior however only during winter. We can notice that Hindcast 2 is closer to the GDEM3 climatology than Hindcast1. Moreover we can observe the presence of Eastern Mediterranean Deep Water (38.7psu) at 2000 meters at 17°E that is not present in Hindcast 1. TEMPERATURE and SALINITY COMPARISON WITH ARGO PROFILES