| Summary: | . Broad-peak intraseasonal oscillations in large-scale Northern Hemisphere atmospheric circulation patterns provide a basis for extended-range weather prediction, given an understanding of the underlying mechanisms. Theoretical work and general circulation model (GCM) experiments suggest that the midlatitude jet stream's interaction with large-scale topography can drive such oscillations through a topographic instability. We present new observational evidence that mountain-induced torques play a key role in 15-30-day oscillations of the circulation's Northern Hemisphere 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. Oscillatory topographic instability provides the most p.
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