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

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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.