This dissertation discusses the predictability of the atmosphere-ocean climate system on interannual and decadal timescales. We investigate the extent to which the atmospheric internal variability (weather noise) can cause climate prediction to lose skill; and we also look for the oceanic processes that contribute to the climate predictability via interaction with the atmosphere. First, we develop a framework for assessing the predictability of a linear stochastic system. Based on the information of deterministic dynamics and noise forcing, various predictability measures are defined and new predictability-analysis tools are introduced. For the sake of computational efficiency, we also discuss the formulation of a low-order model within the context of four reduction methods: modal, EOF, most predictable pattern, and balanced truncation. Subsequently, predictabilities of two specific physical systems are investigated within such framework. The first is a mixed layer model of SST with focus on the effect of oceanic advection.Analytical solution of a one-dimensional model shows that even though advection can give rise to a pair of low-frequency normal modes, no enhancement in the predictability is found in terms of domain averaged error variance. However, a Predictable Component Analysis (PrCA) shows that advection can play a role in redistributing the predictable variance. This analytical result is further tested in a more realistic two-dimensional North Atlantic model with observed mean currents. The second is a linear coupled model of tropical Atlantic atmosphere-ocean system. Eigen-analysis reveals that the system has two types of coupled modes: a decadal meridional mode and an interannual equatorial mode. The meridional mode, which manifests itself as a dipole pattern in SST, is controlled by thermodynamic feedback between wind, latent heat flux, and SST, and modified by ocean heat transport. The equatorial mode, which manifests itself as an SST anomaly in the eastern equatorial basin, is dominated by dynamic feedback between wind, thermocline, upwelling, and SST. The relative strength of thermodynamic vs dynamic feedbacks determines the behavior of the coupled system, and enables the tropical Atlantic variability to be more predictable than the passive-ocean scenario.

La variabilité du contenu de chaleur hivernal de l'océan supérieur et ses mécanismes de causalité ont été étudiés en utilisant des observations et des produits de réanalyse dans le Pacifique Nord-Ouest. La relation entre la mousson d'hiver dans l'Asie de l'Est (EAWM) et l'Oscillation du Pacifique Nord (NPO) et leurs impacts sur la température de surface de la mer (SST) sont non stationnaires, avec un changement soudain en 1987/1988. L'EAWM et la NPO, qui étaient bien corrélées en 1973-1987, ne sont pratiquement plus corrélées en 1988-2002. Cette relation non stationnaire est liée au fort affaiblissement décennal de la haute pression Sibérienne après le changement de régime en 1988, ainsi qu'au changement concomitant du dipôle de la NPO positive. L'influence de l'EAWM et de la NPO sur la SST hivernale dans la région d'étude a significativement diminué après 1990. Le bilan de chaleur dans les 400 premiers mètres a été analysé à l'aide des sorties d'un modèle de la circulation océanique générale à haute résolution. Le taux de stockage de chaleur hivernal des échelles interannuelles à décennales est principalement déterminé par l'advection océanique plutôt que par le flux net de chaleur air-mer. Le rôle de l'advection de chaleur devient particulièrement important après le changement de régime en 1990, en association avec la réduction de la variabilité du flux de chaleur en surface causée par une faible variabilité de la SST. Le flux net de chaleur air-mer freine les anomalies thermiques créées par la dynamique océanique associée avec le déplacement méridien du front de l'Extension de l'Oyashio, qui est fortement corrélé avec les modes de téléconnexion de WP et PNA. / Winter upper-ocean heat content variability and its causal mechanisms are investigated using observational and reanalysis products in the western North Pacific. The relationship between the East Asian winter monsoon (EAWM) and the North Pacific Oscillation (NPO) and their impact on the sea surface temperature (SST) are nonstationary, with a sudden change at 1987/1988. During the 1973-87, the EAWM and NPO were significantly correlated to each other, but their correlation practically vanishes during the 1988-2002. This nonstationary relationship is related to the pronounced decadal weakening of the Siberian high after the 1988 regime shift as well as the concomitant positive NPO-like dipole change. The influence of EAWM and NPO to the winter SST in the study region is significantly decreased after the sudden change near-1990. The upper 400 m heat budget in the western North Pacific is analyzed using outputs from a high resolution ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with the West Pacific (WP) and Pacific-North America (PNA) teleconnection patterns.