NCEP/NCAR reanalysis data is used widely in the atmospheric sciences. Although these data are generated using homogenous techniques, the effect of changes in the atmospheric observing system is unavoidable. One prominent impact is the introduction of satellite data in 1978, particularly over the southern hemisphere where conventional observations are sparse. This paper attempts to quantify the extent to which the introduction of satellite data impacts both daily and inter-annual scales of variability, using a Self-Organizing Map (SOM) analysis technique. It is clear that daily circulation statistics are quite different before and after 1979, and are generally more typical of model climatology before 1979. Inter-annual variability also appears to be reduced before 1979 in the mid-latitudes of the southern hemisphere. These caveats need to be borne in mind when performing studies over the southern hemisphere pre-1979.