Fluctuations in Earth rotation over time scales of 2 years or less are dominated by atmospheric effects; spectral analyses of length-of-day (LOD) and atmospheric angular momentum (AAM) data show significantly increased variability in the 40--50 day band. LOD and AAM fluctuations on the 40--50 day time scale have previously been linked to tropical, convectively driven waves of the type first described by Madden and Julian (1971) (referred to as MJ hereinafter). A significant spectral peak centered at 42 days has also been found, however, in the AAM of a 3-year (1120-day) perpetual-January simulation of the global atmosphere, performaned using a version of the University of California at Los Angles (UCLA) general circulation model (GCM) which does not give rise to MJ oscillations in the tropics. In the present work the 40--50 day oscillation is studied using the 12-year overlap between two records: (1) AAM data, compiled from the National Meteorological Center (NMC): and (2) LOD variation from the Jet Propulsion Laboratory Kalman-filtered Earth rotation series. We analyze the NMC records by latitude belts, in lighth of the UCLA GCM results, in order to identify possibly distinct sources of the AASM oscillation in the mid-latitudes and the tropics. Results suggest that two intraseasonal oscillations exist in the Earth-atmosphere system: a tropical, 50-day oscillation associated with the convectively driven MJ wave and a mid-latitude, 40-day oscillation associated with the interaction of nonzonal flow with topography. ¿American Geophysical Union 1991