Tropospheric warming and changes in weather variability over the Northern Hemisphere during the period 1967–1991

Summary The relationship between global circulation, temperature distribution and weather variability as a mechanism of meridional heat exchange over the northern hemisphere has been examined for the period 1967–1991 using analysis data from the German Weather Service. From geopotential heights five parameters, relating to different climatic features, were computed. The data were filtered using a Principal Component Analysis (PCA) to omit the random noise. The time series of amplitudes for the Principal Patterns were investigated with respect to their linear trends and extreme events.The period 1967–1991 is characterised by a transition from a relatively cool period-beginning in the early 1960s-to a warmer one. The strongest warming took place at different latitudes in the Atlantic and the Pacific section, respectively. Due to the warming the meridional temperature gradient and the geostrophic zonal wind intensified. Strengthened baroclinic conditions reinforced the activities on the synoptic scale and the meridional eddy heat flux. The results corroborate the hypothesis that the enlarged synoptic activity is responsible for the accumulated occurrences of extreme midlatitude storms over the Atlantic and Europe within the last few years and, therefore, that the observation of more frequent deep cyclones is neither random nor due to improved observation techniques.With 6 Figures