The general circulation of the atmosphere: an updated diagnostic.

The atmospheric general circulation redistributes energy, momentum, and moisture between low and high latitudes, thereby maintaining the global climate system. Classical diagnostics of this circulation were formalized in the framework of Peixoto and Oort (1992). At the time, however, global observations were limited, which constrained the evaluation of their diagnostics on long time scales. In the present thesis, the Peixoto-Oort diagnostic framework is revisited using ERA5 reanalysis data for the period 1940–2022. Zonal-mean fields and meridional transports of temperature, momentum, and moisture are recomputed using Reynolds decomposition into mean flow, stationary eddies, and transient eddies. Atmospheric variability is examined using empirical orthogonal function analysis. The canonical three-cell structure of the circulation and the eddy-driven nature of midlatitude transport remain clearly identifiable throughout the record. The variability is dominated by two leading modes: an ENSO-like mode representing internally generated interannual variability and a spatially coherent trend-like mode associated with tropospheric warming. A further step was taken toward a multi-decadal assessment. In this thesis, the 83-year ERA5 record is divided into four 20-year periods for further diagnostics. The results show that the large-scale circulation remains structurally stable throughout the record. At the same time, variability evolves from predominantly ENSO-driven Winter dynamics toward increasing influence of a warming-related mode after the 1980s. Overall, the present work confirms that the diagnostic framework developed in the previous century remains fully applicable to modern observational datasets. The results suggest that ongoing warming does not fundamentally restructure the atmospheric circulation, but instead modulates its variability and transport intensity.