A New East Asian Winter Monsoon Index and Associated Characteristics of the Winter Monsoon

Abstract

A new East Asian winter monsoon index, which reflects the 300-hPa meridional wind shear associated with the jet stream, was defined to describe the variability of the winter monsoon in midlatitude East Asia. This index represents very well the seasonal mean winter temperature over Korea, Japan, and eastern China. The National Centers for Environmental Prediction?National Center for Atmospheric Research reanalysis data from 1958 to 2001 were used to examine the composite structures of strong and weak winter monsoons based on this index. The composite strong winter monsoon is characterized by an enhanced upper-level jet stream south of Japan, a strengthened midtropospheric East Asian trough, a stronger than normal Aleutian low and Siberian high, and increased low-level northeasterlies along the Russian coast. This composite structure suggests that a cold winter in Korea and Japan depends critically on processes that control the pressure gradients between the Aleutian low and the Siberian high. Power spectral analysis of the index shows significant peaks occurring in 3?4, 6?8, and around 18 yr. The decadal peak is primarily due to a prominent cold period from 1980 to 1986 versus a warm period from 1987 to 1993. The regressed sea level pressure field for the interannual component resembles the composite strong winter monsoon pattern, whereas the sea level pressure pattern for the decadal component bears close similarity to that of the Arctic Oscillation. These conditions in the winter monsoon are associated with excess snowfall in October over the Siberian high, northeastern China, and far eastern Russia. The sensitivity experiments with the Seoul National University general circulation model suggest that the change in snow depth in autumn over the Siberian high and northeastern China may lead to the variability of the winter monsoon intensity. The teleconnection analysis confirms that development of the Siberian high and/or the Aleutian low is associated with an enhanced East Asian winter monsoon; the Arctic Oscillation is closely related to the winter monsoon intensity on the decadal time scale.