I. Objectives and Methodology

1. DETERMINATION OF CIRCULATION IN NORTH ATLANTIC BY INVERSION OF ARGO FLOAT DATA Carole GRIT, Herlé Mercier I. Objectives and Methodology II. Reference Circulation: 1995 -1998 The ARGO data set provides unique observations of the large-scale and low-frequency variability of the ocean thermal and haline contents and circulation. That signal is aliased by the mesoscale motions that are only partly resolved by the observing system. A challenge is thus to estimate the large-scale and low-frequency signals of interest from the ARGO data set. We present here a methodology developed in the frame work of the EC funded GYROSCOPE project that aims at evaluating our ability to estimate the North Atlantic circulation and heat budget variabilities at seasonal time scale using the ARGO data set. The methodology is based on the finite difference inverse model developed at LPO (Mercier et al. J. Phys. Oceanogr. 1993). The model looks for a best estimate of the geostrophic circulation that minimizes the weighted sum of the squared distance to the temperature and salinity profiles and the squared residuals of dynamical constraints (mass, heat and potential vorticity conservations). The weights are error covariance matrices. In this first step the dynamics is stationary. A first model run based on high quality CTD data from 1995-1998 (Fig.1) provides a reference to which the circulation estimated using the 2002 ARGO data set (Fig.2) will be compared. When observations of temperature and salinity are not available, the model is restored to the Reynaud et al. (1998) climatology. This is in particular the case for depths greater than 2000 m when using ARGO data. Both models were forced using wind stresses observed from space and air-sea heat fluxes estimated by numerical weather prevision models (see Table). Fig.3. Barotropic Stream Function. Transport in Sv. Fig. 4. Meridional overtuning circulation. Transport in Sv. Fig. 5. Accumulated volume transport across 49ºN. The integration is carried from west to east. Transport is in Sv. The model does not conserve mass exactly to account for errors which results in a small net northward transport through the section (black curve). The circulation patterns revealed by the model agree with our knowledge of the North Atlantic general circulation. The barotropic circulation (Fig.3) is similar to the circulation that could be obtained using a coarse resolution numerical model. The Deep Western Boundary Current (DWBC) is well resolved by the inverse model. At 49°, we estimate 18 Sv for the DWBC transport (Fig. 5, blue curve). As a consequence, the meridional overturning circulation is realistic (Fig. 4). Fig. 1. CTD stations collected during 1995-98 Fig. 2 . ARGO profiles collected in 2002 III. Circulation for 2002 Using ARGO Data IV. Perspectives: Seasonal Inversions To resolve the seasonal variability of the circulation and heat budget, the stationary dynamics of the model has been modified to include a time evolution term in the heat budget which reads: /t + U.  = Qnet Where  is the heat content, U the 3D advection and Qnet the net air-sea heat flux. The time evolution of heat content is estimated using centered finite difference scheme based on seasonal averaged fields. It is expected that at seasonal time scale the balance will be between the air-sea flux and the time evolution of the heat content, but the model will allow us to identify regions where the heat advection modifies this balance. In addition, the sea surface elevation of the model is constrained by the merged Topex, Poseidon and ERS sea level anomaly provided by CLS and averaged for the given season. As the satellite product is a deviation from the mean, we used as an estimate of the mean sea surface that from the 95-98 inverse. Thus the constraint reads: _model = _mean_95_98 + sla_seasonal_average Fig. 7. Meridional heat transport before (--) and after (*) inversion, in 95-98 and 2002. Heat transport in Pw. Fig. 6. Barotropic Stream Function. Transport in Sv. The comparison of the barotropic stream function estimated for 1995-98 (Fig. 3) with that for 2002 (Fig.6) reveals apparent changes in the circulation. In particular, a weakening of the transport of the eastern limb of the subpolar gyre and a strengthening of the subtropical gyre transport are observed between 95-98 and 2002 as well as modifications of the circulation patterns in the tropics. Further investigations are needed to understand the reasons for the observed changes and to quantify the respective shares of variabilities in the forcings and the stratification. Fig. 9. Barotropic Stream Function for fall 2002. Transport in Sv. The circulation in the tropics is questionable due to an under estimation of the errors in t air-sea heat fluxes in this experiment. Fig. 8 . ARGO profiles collected in fall 2002 Laboratoire de Physique des Océans IFREMER - Centre de Brest B.P.70 29280 Plouzané http://www.ifremer.fr/lpo