北半球におけるZonal index の季節別経年変動について

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In order to research the cause of recent unusual weather, this study investigated the variation of westerlies by zonal index, using NCEP-NCAR reanalysis dataset after the Climate shift. We analysed zonal index (from 40°N to 60°N) over planetary wave fields (Europe, East Asia, East Pacific and North America ) at 500hPa geopotential height fields in four seasons in the Northern Hemisphere. The results are summarized as follows. (1) There were apparent differences in the distribution of subtropical high pressure belts at between 200hPa and 500hPa geopotential height field in summer. Subtropical high pressure belts at 500hPa geopotential height field was influenced by subtropical jet stream flowing north of south Asian subtropical high centered at the Tibetan Plateau at 200hPa geopotential height field. (2) Therefore, we attempted to research the feature of pressure fields at 500hPa mean geopotential height field in high and low index years respectively, by standard deviation for 21 years mean 500hPa geopotential height field. In high index year, the deviation in mid-latitude showed positive anomaly field, and negative anomaly field in high-latitude. On the contrary, positive anomaly appeared in high-latitude, and negative anomaly in mid-latitude in low index year. These anomalous fields were corresponded to planetary wave fields where zonal index was high. (3) In planetary wave fields, zonal index in East Asia area and East Pacific area were high in autumn, although North America area indicated high zonal index in winter and low in spring. In Europe area, zonal index was high in autumn and winter, low in spring and summer.

北半球におけるZonal index の季節別経年変動について

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