Seasonal variation of the northern annular mode and its inter-seasonal correlation

北极涛动或北环模态是北半球冬季最主要的模态。本研究项目主要研究了NAM的季节变化及其季节联系。夏季不结盟运动的经向尺度小于冬季不结盟运动，且局限于对流层。冬季不结盟运动与随后的夏季不结盟运动呈正相关。我们表明，这种季节性联系部分是由北部大陆的积雪造成的。积极的冬季不运动使欧亚大陆的降雪量减少，积雪减少持续到晚春。早融雪增强了欧亚大陆和北冰洋在初夏的热对比，产生了夏季NAM的正相。这是不结盟运动季节性联系的一个可能机制。另一个可能的机制是与臭氧有关的过程。在非运动正年冬季，平流层极涡较强，平流层下层从热带向温带的臭氧输送减少。由于臭氧在平流层下层的光化学寿命较短，臭氧的减少将持续到初夏。初夏，臭氧的减少使高纬度地区降温，并加强了平流层低层的纬向风。因此，涡旋强迫改变了对流层中的波传播，使对流层的不平动电位变为正的。通过数据分析，我们认为这一机制也对季节性联系起作用。2003年的夏天，欧洲异常炎热，而日本却异常凉爽。从夏季不结盟运动的角度分析了这一异常天气。7月中旬至8月初，夏季不结盟运动值较大，对应于欧洲和东西伯利亚上空的阻塞。这些阻塞高压导致欧洲和日本出现异常天气。

The Arctic Oscillation or the northern annular mode (NAM) is the most dominant mode in the winter Northern Hemisphere. The seasonal variation of the NAM and its seasonal linkage are studied in this research project. The summer NAM has a smaller meridional scale than the winter NAM and confined in the troposphere. The winter NAM has a positive correlation with the following summer NAM. We showed that this seasonal linkage is partly caused by snow over the northern continents. The positive winter NAM produces less snow fall over Eurasia and the reduced snow cover persist until late spring. Early snowmelt enhances heat contrast between Eurasia and the Arctic Ocean in early summer, which produce a positive phase of NAM in summer. This is one possible mechanism for seasonal linkage of the NAM. The other possible mechanism is the ozone-related process. In the winter of positive NAM years, the stratospheric polar vortex is strong and hence the ozone transport from the tropics to the extratropics in the lower stratosphere is reduced. Since the photochemical life time of ozone is ling in the lower stratosphere, the reduced ozone persist until early summer. The reduced ozone cools the high-latitude and strengthen the zonal wind in the lower stratosphere in early summer. Thus the wave propagation in the troposphere is changed and the tropospheric NAM become positive by eddy forcing. By data analysis, we suggested this mechanism is also working for the seasonal linkage.The summer of 2003 was abnormally hot in Europe and cool in Japan. This abnormal weather was analyzed from a viewpoint of summer NAM. From mid-July to early August, the summer NAM showed quite large value, which corresponds to blockings over Europe and east Siberia. These blocking highs caused the abnormal weather in Europe and Japan.