The total angular momentum variability of the atmosphere (below 75 mbar) for the 120-month period May 1963 to April 1973 is determined (using a data set provided by A. Oort, Geophysical Fluid Dynamics Laboratory) and compared with the observed length of day (l.o.d.) variations during this time. With the mean, annual, and semi-annual cycle removed, the angular momentum shows an irregular oscillation with periods of 3 to 4 years during this decade. The corresponding relative change that this would produce in the length of day is approximately ¿0.2¿10-8 which agrees with the observed interannual l.o.d. variations 1963--1973. This oscillation is evidently related to the well-known oscillations in tropical air (and sea surface) temperatures which is part of the Southern Oscillation and lags behind the temperature oscillation by a few months. A simple thermal wind argument indicates that the observed amplitude (1¿C) of the oscillation in tropical temperatures can produce a relative change in the rotation of the earth on the order of 0.2¿10-8. Variations in the length of the day and a Southern Oscillation Index are shown to be very similar during the 20-year period 1955--1974.