Seasonal Variation of the Meridional Wind in the Temperate Jet Stream and Its Relationship to the Asian Monsoon

The features of the temperate jet stream including its location, intensity, structure, seasonal evolution and the relationship with the Asian monsoon are examined by using NCEP/NCAR reanalysis data. It is indicated that the temperate jet stream is prominent and active at 300 hPa in winter over the region from 45 60 N and west of 120 E. The temperate jet stream is represented by a ridge area of high wind speed and dense stream lines in the monthly or seasonal mean wind eld, but it corresponds to an area frequented by a large number of jet cores in the daily wind eld and exhibits a distinct boundary that separates itself with the subtropical jet. A comparison of the meridional wind component of the temperate jet stream with that of the subtropical jet shows that the northerly wind in the temperate jet stream is stronger than the southerly component of the subtropical jet, which plays an important role in the temperate jet stream formation and seasonal evolution, and thus the intensity change of the meridional wind component can be used to represent the temperate jet stream's seasonal variation. Analysis of the temperature gradient in the upper troposphere indicates that the temperate jet stream is accompanied by a maximum zonal temperature gradient and a large meridional temperature gradient, leading to a unique jet stream structure and particular seasonal evolution features, which are different from the subtropical jet. The zonal temperature gradient related to the land-sea thermal contrast along the East China coastal lines is responsible for the seasonal evolution of the temperate jet. In addition, there exists a coordinated synchronous change between the movement of the temperate jet and that of the subtropical jet. The seasonal evolution of the meridional wind intensity is closely related to the seasonal shift of the atmospheric circulation in East Asia, the onset of the Asian summer monsoon and the start of Meiyu in the Yangtze and Huaihe River Valleys, and it correlates well with summer and winter rainfall variations in East China. The temperate jet intensity change is earlier than that of the Asian summer monsoon onset and Meiyu, and thus it may be used as a precursor for the prediction of the Asian summer monsoon onset and the beginning of Meiyu.