Theoretical work and general circulation model (GCM) experiments suggest that the midlatitude jet stream's interaction with large-scale topography can drive intraseasonal oscillations in large-scale atmospheric circulation patterns. In support of this theory, we present new observational evidence that mountain-induced torques play a key role in 15--30-day oscillations of the Northern Hemisphere circulation's dominant patterns. The affected patterns include the Arctic Oscillation (AO) and the Pacific-North-American (PNA) pattern. Positive torques both accelerate and anticipate the midlatitude westerly winds at these periodicities. Moreover, torque anomalies anticipate the onsets of weather regimes over the Pacific, as well as the break-ups of hemispheric-scale regimes. ¿ 2001 American Geophysical Union