Seasonal Variation of the Polar Jet and Its Relationship to the Asian Monsoon

The features of the polar 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 polar jet stream is prominent and active at 300hPa in winter over the region from 45N-60N and west of 120E. The polar jet stream is represented by a ridge area of high wind speed and dense stream lines in the monthly or seasonal mean wind field, but it corresponds to an area frequented by a large number of jet cores in the daily wind field and exhibits a distinct boundary that separates itself with the subtropical jet. A comparison of the meridional wind component of the polar jet stream with that of the subtropical jet shows that the northerly wind in the polar jet stream is stronger than the southerly component of the subtropical jet, which plays an important role in the polar jet stream formation and seasonal evolution, thus the intensity change of the meridional wind component can be used to represent the polar jet stream’s seasonal variation. Analysis of the temperature gradient in the upper troposphere indicates that the polar 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 polar jet. In addition, there exists a coordinated synchronous change between the movement of the polar 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 Huai River Valleys, and it correlates well with summer and winter rainfall variations in East China. The polar jet intensity change is earlier than that of the Asian summer monsoon onset and Meiyu, so it may be used as a precursor for the prediction of the Asian summer monsoon onset and the beginning of Meiyu. Keywords: Polar jet stream, Subtropical jet, Asian summer monsoon, Seasonal evolution.