Abstract
Northern and Southern Hemisphere series of surface temperature anomalies are compared. The series studied are the University of East Anglia Climatic Research Unit (CRU) monthly series (1854β1986) and the UK Meteorological Office (MO) quarterly series (1856β1989). A major difference is the marked seasonal variation of the warming trend in the Northern Hemisphere, with maximum in winter and minimum in summer. Analysis of the annual cycle of the CRU set suggests a shift in phase of the warm peak in the annual cycle of temperature anomalies from July to January for the period. This shift is a characteristic of the annual cycle itself and does not have to be associated with a greater warming trend in winter than in summer, although it may be.
The series were converted into interannual differences from one month or quarter to the corresponding month or quarter in the following year. Globally, the sequential values of the positive and negative modes of the interannual differences suggest a slow relaxation towards zero with the absolute values of the interannual coolings decreasing faster than those of the interannual warmings. Spectral analyses of the firstβorder interannual differences of the monthly CRU and of the quarterly MO series suggest that the temperature field reacts to a QuasiβBiennial Oscillation pulse in the Northern Hemisphere and to a stronger ENSO pulse in the Southern Hemisphere.