Abstract
The lower-frequency variations in the winter monsoon system have been examined using a l0-day low-pass filter. Results indicate that the lower tropospheric north-south thermal gradient over eastern Asia, the Siberian high pressure system, and the eastern Asia subtropical jet stream varied coherently with a time scale of 10-20 days. Composite study reveals that the 200-mb streamfunction variations on the 10-20 day time scale are longsynoptic-scale disturbances propagating slowly eastward. The cyclonic disturbances weaken as they move eastward south of the Tibetan Plateau and begin to strengthen when the lower tropospheric thermal troughs from Siberia move southward. The irrotational circulations on the 10-20 day time scale are closely related to the streamfunction variation with a quarter-wave phase shift. The tropical-midlatitude interactions in the winter monsoon system are seen to be the interactions between these long-synoptic-scale waves and the local monsoon circulations based on the phase relationships among various elements studied. The divergent circulations of the propagating waves trigger the cold air outbreak over eastern Asia when the wave trough is situated south of the Tibetan Plateau. The propagating waves are seen to intensify over the ocean suggesting a transfer of energy from the monsoon system to the waves.
