Influences of the East Asian Jet Stream on Winter Climate in China

The European Center for Medium Range Weather Forecasts reanalysis data sets and the observed data for precipitations and temperatures in China are used to depict the relationships between the eastern China climate and the East Asian jet stream (EAJS). With an emphasis on the northern winter seasons and on the interannual timescales, the variations of the intensity and the meridional displacement of EAJS and their relationships with the precipitations and temperatures are analyzed, and the relative connections of the EAJS and the winter monsoon to the anomalies of atmospheric circulation in the eastern China region are assessed. The intensity index(normalized mean u₂₀₀ averaged over 30°—35°N, 127.5°—155°E)and the shear index of EAJS (normalized u₂₀₀ difference between two areas over 15°—25°N, 100°—115°E and 30°—40°N, 100°—115°E) are first defined, which can reasonably reflect the interannual variation of the intensity and meridional displacement of EAJS, then relative connections of intensity index and shear index with winter temperatures and precipitations in China for period 1957—2001 are investigated respectively. The correlation coefficient between the intensity index and the shear index is -0.48 (exceeding 99% confidence limit), which means that when EAJS intensifies the meridional displacement of EAJS is abnormally southward. It is found that the intensity of EAJS is associated with the extensive atmospheric circulation in Asia and west Pacific, while the meridional displacement of EAJS does strongly link to the atmospheric circulation in the tropical central eastern Pacific and the Indian Ocean. The possible way of winter EAJS affecting the precipitation and temperature of China can be described as follows: At the left side of the entrance region of EAJS the pressure of low level atmosphere increases due to the convergent flows, meanwhile at the left side of exit region of EAJS the pressure of low level atmosphere decreases because of the divergent flows. So in the mid lower level of the troposphere the pressure over Mongolia and Siberia increases and that over the north Pacific decreases, which results in both the difference between them increasing and the East Asian trough deepening. The increased difference of geopotential height between land and sea and the intensified East Asian trough may make winter monsoon intensify, resulting in the drop of temperature in the east part of China, meanwhile there are descending flows over the 30°—40°N that make the precipitation decrease over the northern China, the middle China and the mid lower reaches of the Yangtze River. When EAJS weakens abnormally and the displacement of EAJS is abnormally southward, the south wind occurs in the whole East Asia indicating the weak winter monsoon, while there are ascending flows over the east part of 25°N. At that time the precipitation in the southern China, Inner Mongolia and the northern China increases, and the temperature of Inner Mongolia is high abnormally.