The mechanisms and the predictability of the Arctic oscillation and the North Atlantic oscillation /

The Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) are the most pronounced modes of extratropical atmospheric wintertime variability in the Northern Hemisphere. This thesis investigates different aspects of the AO and NAO on the in traseasonal and seasonal time scales. First, the question of how the differences between the AO and NAO are influenced by the choice of the definitions of the NAO and to what extent the AO and NAO differ from each other is investigated using the daily NCEP/NCAR reanalysis data spanning 51 boreal winters. One AO index and four different NAO indices are used in this study. It is found that the AO and NAO are quite similar to each other when both are defined using pattern-based indices, while some notable differences are observed between them when the NAO is defined using a station/gridpoint-based index. Then the predictability of the AO and NAO is examined using a simple general circulation model (SGCM). Numerical experiments are performed to determine the sensitivity of the setup processes of the AO and NAO to the details of the initial conditions. The predictive skills for the AO and NAO are compared to each other. Finally, the potential role of tropical Pacific forcing in driving the seasonal variability of the AO is explored using both observations and the SGCM. The results indicate that a negative thermal forcing over the western tropical Pacific and a positive forcing north of the equatorial mid-Pacific play important roles in producing an AO-like atmospheric response.