中国东北冬季极端低温的年代际变化及其成因

利用JRA-55再分析资料和中国824站日最低气温数据,分析了1961—2013年冬季中国东北地区极端低温的年代际变化及其动力成因。结果表明:东北冬季极端低温1980s中期前频发,1980s后期到2000s初少发,2000s后恢复多发。这与贝加尔湖阻塞高压频次偏多—偏少—偏多的同步年代际变化直接相关,与北极涛动(Arctic Oscillation,AO)和准定常行星波活动的年代际振荡相联系。相比于1980s中期前,1988—1999年期间行星波沿极地波导向平流层的传播减弱,引起高纬度平流层下层E-P通量辐合变弱;而行星波沿低纬波导往副热带对流层上层的传播增强,造成副热带对流层中上层E-P通量辐合增强。行星波E-P通量散度的异常由于波—流相互作用,分别导致了极夜急流加强、副热带急流减弱,因而AO增强,不利于贝湖阻高建立和环流经向型发展,极端低温减少。2000—2013年,行星波活动相反,导致AO减弱,贝湖阻高多发,极端低温恢复增多。

Characteristics and causes of the interdecadal variability of winter low temperature extremes in Northeast China

On the basis of the daily minimum surface air temperature data at 824 stations in China and reanalysis data of the JRA-55 from 1961 to 2013, the characteristics and internal dynamical causes of the interdecadal variability of winter low temperature extremes in Northeast China were analyzed. Results show that an apparent interdecadal variability of winter low temperature extremes with the high-frequency period before the mid-1980s, the low-frequency period from the late 1980s to the early 2000s, and the frequently-occurring recovery after the early 2000s. It is found that the interdecadal variability was synchronized with the frequency of Baikal Blocking High changing from high to low, and then to high on the interdecadal timescale, and might be modulated by the interdecadal variation in the Arctic Oscillation (AO) and the quasi-stationary planetary wave activity. Compared with the period before the mid-1980s, for the period of 1988-1999 the upward propagation of quasi-stationary planetary waves into the stratosphere over high latitudes along the polar wave-guide was reduced, decreasing the convergence of Eliassen-Palm (E-P) fluxes in the lower stratosphere over high latitudes of the Northern Hemisphere; whereas the horizontal propagation into the troposphere over the subtropics along the low-latitude wave-guide was enhanced, increasing the convergence of E-P fluxes in the middle and upper troposphere over the subtropics. These resulted in an enhanced polar front jet and a weakened subtropical jet, respectively, through planetary wave-mean flow interaction. The wintertime AO was then enhanced, against the formation of Baikal Blocking High and the development of meridional circulation, which led to a low occurrence of extreme low temperatures. After entering the period 2000-2013, the opposite process caused to the weakening of the AO, which in turn led to the frequent occurrences of Baikal blocking high and extreme low temperatures.