Mountain torques and atmospheric oscillations

Theoretical work and general circulation model (GCM) experiments suggest th at the midlatitude jet stream's interaction with large-scale topography can drive intraseasonal oscillations in large-scale atmospheric circulation pa tterns. In support of this theory, we present new observational evidence th at mountain-induced torques play a key role in 15-30-day oscillations of th e Northern Hemisphere circulation's dominant patterns. The affected pattern s include the Arctic Oscillation (AO) and the Pacific-North-American (PNA) pattern. Positive torques both accelerate and anticipate the midlatitude we sterly 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.