Variabilité basse fréquence endogène et exogène de la ciruclation thermohaline

One of the consequences of the global warming is the modification of the hydrological cycle and then of the freshwater flux get by the ocean which one of its forcing. The ocean circulation, and more accurately the thermohaline circulation, is able to produce some low frequency variability. We are going to study the impact of the freshwater flux on the thermohaline circulation and mainly on the decadal to millennial variability. In the ocean, as in all dynamical systems, two paradigms coexist for the explanation of the observed variability : it can be endogenous or exogenous. Some outstanding results appear. During the study of endogenous variability in a 2D latitude-depth model, the growth mechanism and the oscillation one of a centennial mode are analyzed and theirs characteristics are described. This mode, corresponding to a salinity dominated density anomaly advected around the circulation, is feed by the positive salinity feedback on the advection in the freshwater forcing zone. A millennial oscillation cycle which appears through an infinite-period bifurcation is characterized. The bifurcation is due to higher frequency mode presence during the millennial cycle. Thus, we point out that the centennial oscillation is a precursor of millennial oscillation. During the study of the exogenous variability, the optimal perturbations of the sea surface salinity influencing the ocean circulation are performed in three models of the thermohaline circulation : a latitude-depth model, a planetary-geostrophic model and a primitive equation in global realistic configuration. these three models induce finite time growths on different time scales (respectively 67 yr, 24 yr and 10.5 yr). The physical mechanisms inducing these transient growths have been analysed. For the two first models, wa have been able to verify that it is the less damped eigenmode which control the transient growth. The optimal stochastic perturbations reveal a peak at the frequencies of these less damped (respectively 150 yr and 35 yr) eigenmodes. Moreover, by this study of the optimal perturbations, we have fixed modification bound of the ocean circulation at 0.75 Sv and at 0.03 PW.