Influence Of The Winter North Atlantic Oscillation On Spring Climate In China

Studies of the relations of the winter North Atlantic Oscillation(NAO) to the spring precipitation and temperature over China by using the data for the last 50 years show that the NAO significant influence on variations in Chinese spring rainfall and temperature. Associated with the positive NAO, the precipitation over South China and Southwest China will increase and the precipitation over North China will decrease. Meanwhile the temperature over North China will be warmer and the temperature over Southwest China will be cooler.The East Asian jet stream is significantly different between positive NAO years and negative NAO years. The main body of the Esat Asian jet stream in stong positive NAO years is larger than that in strong negative NAO years. The speed of the Esat Asian jet stream in stong positive NAO years is also stronger than that in strong negative NAO years. In strong positive NAO years, the East Asian subtropical westerly jet enhanced and the East Asia polar-front jet weakened; while in strong negative NAO years, the East Asian subtropical westerly jet weakened and the East Asia polar-front jet enhanced. Further analysis indicates that winter NAO can exert notable influence on the atmospheric circulation over East Asia by propagation of wave train.The atmospheric circulation of low troposphere over East Asia is significant abnormaly while the winter NAO is abnormal. In strong positive NAO years, abnormally northeastly wind prevail over East China in the low teoposphere, which is not conducive to the water vapor for the precipitation in North China. But in strong negative NAO years, abnormally southwestly wind prevail over East China in the low teoposphere this air flow provide the water vapor for the precipitation in North China. These changes give rise to a drier condition in the northern China and a wetter condition in the southern China. The atmospheric circulation of the upper troposphere in East Asia also have close contact with the winter NAO. When the winter NAO display a positive phase, the Siberian High ridge is strengthened, the East Asian Ttough is deepened, the cold advection over East Asia is strengthened, the temperature of the upper troposphere oeve East Asia is cooler. The positive NAO-related atmospheric circulation result in abnormalities of temperatures at 500-200 hPa, which cause corresponding variations in meridional gradients of temperature. Perhaps it is one of the reasons why NAO impacts on westerly jet stream over East Asia. The upper level cooling is associated with an anomalous meridional cell with descending motions in the latitude 30°-40°N, ascending motions in the latitude 30°-40°N and low level northerly winds over eastern China, casusing deficient rainfall over southern China and northern China. Such circulation changes are also main factors for the decadal decrease of the total amount over northern China and the increase of the surface air temperature in the northern China. So it is suggested that the variations of the air temperature in upper troposphere is an important factor linking the winter NAO and the spring climate changes in China.In strong positive NAO years, the ascending motions over southwest China enhanced, which favors the formation of precipitation and clouds.The increasing of total cloud amount is responsible for the surface cooling over southwest China. In strong positive NAO years, the precipitation and clouds decrease, temperature over southwest China will be warmer.