Studies of climate variability in a simple coupled model

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2007.Includes bibliographical references (p. 74-76).The mechanisms of variability of a coupled atmosphere-ocean model are investigated through the study of two coupled configurations: an aquaplanet in which gyres are absent, and an aquaplanet in which a ridge extending from pole to pole supports gyres. Empirical Orthogonal Functions (EOFs) are used to explore the main features of variability exhibited by extended integrations of both configurations. In the aquaplanet a decadal variability is observed in the atmosphere and the ocean. Stochastic driving of the annular modes in the atmosphere generates an anomalous Sea Surface Temperature (SST) dipole through latent heat fluxes and Ekman pumping. A feedback of this SST dipole on the atmosphere enables a damping slow enough for anomalies to persist over decadal time scales. This air-sea feedback combined with a slow advection of the anomalies by mean ocean currents result in the observed decadal oscillation. A simple stochastic model captures the essence of this mechanism. In the ridge decadal variability is absent but centennial variability is observed in the atmosphere and the ocean. Stochastic driving of the annular modes in the atmosphere generates a weak SST tripole due to latent heat fluxes. The weak amplitude of this tripole prevents the existence of any significant air-sea feedback, implies a stronger damping than in the aquaplanet, and ultimately results in the absence of oscillations. The classic stochastic model of Hasselmann [19] explains the evolution of the SST anomaly through time. Within a delay of one year stochastic atmospheric variability additionally generates a baroclinic streamfunction as well as baroclinic Rossby waves at the eastern boundary of the basin. The former is slowly advected by the mean flow while the latter propagates towards the western boundary, inducing a feedback on the atmosphere with a delay of sixty years. A simple model is found to capture the essence of this mechanism. The results of the aquaplanet and the ridge are used to interpret the Drake, a third configuration in which a band of land extends from the North Pole to the line of -45' of latitude. In the northern hemisphere of the Drake mean state and variability are similar to the ones observed in the ridge. The observed centennial oscillation would correspond to a decadal oscillation in the Atlantic. In the southern hemisphere of the Drake, mean state and variability have elements of both the ridge and the aquaplanet.by Claude Abiven.S.M