To assess whether trends ill the troposphere's circulation have occurred th
is century in concert with changes at the surface, we consider the low-freq
uency behavior in the global angular momentum of the atmosphere. Values of
this index have been computed each month since 1870 from model experiments
conducted by the UK Hadley Centre for Climate Prediction and Research, in w
hich the model atmosphere is driven by prescribed boundary forcings, most e
specially sea surface temperature (SST) fields. The interannual variability
of the model's atmospheric angular momentum compares favorably with that d
erived from the NCEP/NCAR reanalysis for the period of overlap (the second
half of the twentieth century). Evidence is found for a notable increase in
interannual variability since around 1970, but this recent high level of v
ariability may not be significantly greater than that during 1900-1920. Thi
s latter result is strongly dependent on the character of the SST field use
d in an experiment, because low-frequency signals in atmospheric momentum a
re closely linked to those in the El Nino/Southern Oscillation. Large diffe
rences exist, however, among SST estimates in the tropical Pacific for the
early part of the century.