On Northern Hemisphere climate change and variability during the last half of the 20th century

The Northern Hemisphere climate underwent significant change during the last half of the 20th century. This thesis focuses on the changes over the Euro-North Atlantic sector characterized by an upward trend in the North Atlantic Oscillation (NAO) index and an eastward shift in the spatial pattern of interannual NAO variability.; Correlation analysis unravels a special relationship between the NAO and the first mode of North Atlantic storm activity and the emergence of a spatially coherent variability regime associated with the shifted NAO after the 1970's. Model experiments using a simplified AGCM (Hall model) driven by forcing diagnosed from NCAR/NCEP reanalysis data confirm the importance of the storm activity in the NAO shift and reveal the nonlinear dependence of the spatial pattern of the NAO on the NAO index, the pattern being shifted to the east (west) for high (low) NAO index, with confirmation being found in the observations. Therefore, it is suggested that the eastward shift of the NAO is a consequence of the relatively high NAO index during 1978--97 versus the relatively low index during 1958--77. The upward trend of the NAO is part of a hemispheric trend associated with a deepening of both the Icelandic and Aleutian lows. The trend is shown to strongly resemble the Cold Ocean Warm Land pattern and to be associated with a wave train emanating from the Indo-Pacific region of the tropical Pacific. The North Atlantic storm track is shown to feed back positively on the wave signal and give rise to a resonant NAO-like dipole. Non-stationary impacts of El Nino-Southern Oscillation on Euro-Atlantic interannual climate variability are also investigated in the context of the 1970's climate regime change.