Interannual atmospheric angular momentum variability 1963–1973 and the Southern Oscillation

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 cycles 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 oscillation 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 day and a Southern Oscillation Index are shown to be very similar during the 20‐year period 1955–1974.