Characterization Study On Extratropical Cyclone Activity In North America-Atlantic Region

Based on the improved cyclone detecting and tracking algorithm, this article conducts statistical analysis on extratropical cyclones in North America-Atlantic Regions (20°N～80°N and 130°W～0°W) respectively by utilizing ERA-interim and FNL reanalysis data from 2000 to 2013. It could be found that the above two statistical results are the same approximately, only have some differences about the frequency of extratropical cyclogenesis, which is caused by different numerical models and assimilated data. Then, this article obtains the extratropical cyclone dataset of the North America-Atlantic Region from 1979 to 2013 by utilizing ERA-interim reanalysis data. The dominating conclusions, getting from statistical analysis, are as below:(1) the source region of extratropical cyclone are mainly concentrated upon the leeward slopes of Rocky Mountains and Appalachian Mountains, Northwest Atlantic Ocean and the southeast sea areas of Greenland. However, the source region of extratropical cyclone in different seasons are large difference in the leeward slopes of Rocky Mountains and Appalachian Mountains nearby 40N°. (2) The frequency (interannual) of the extratropical cyclogenesis tends to reduction on the whole, but the change of the extratropical cyclones have the cycles of three time scales, namely 2 to 3 years,4 to 8 years and 9 to 25 years, of which the cycles of the time scales of 2 to 3 years,4 to 5 years and 6 to 8 years are the most significant. In the significant cycle, the time scales of 2.3 years,4.1 years and 6.2 years are used as the main cycle. (3) The extratropical cyclones mainly have 6 migration paths in the migration process in which the east of Rocky Mountains, North America to north of 50N°, North Atlantic Ocean and northwest of Europe are the areas where the extratropical cyclone activities are very frequent. On the whole, the frequency of the extratropical cyclone activities tends to reduction from the leeward slopes of Rocky Mountains through northeast to Baffin Island Region and high-latitude (60N°-80N°) North Atlantic Ocean. The frequency of the extratropical cyclone activities tends to increase in the north and southeast as well as coastal region of North Americas and middle-latitude (40N°-60N°) North Atlantic Ocean. (4) Majority of the extratropical cyclones pass through North Atlantic Ocean Region to the north of 50N°, and are developed to reach the strongest state, and the intensity of majority of extratropical cyclones concentrate upon 970hPa to 990hPa. Finally, this article also discovers that the average weft and warp directions of the source regions of extratropical cyclones in winter and the average warp direction of the activity scopes are significantly accossiated with NAO index in winter. Via further composite analysis, compared with the negative phase event, the positions of 700hPa strong baroclinicity regions during the NAO positive phase event are more northward and eastward, and thereby making the source regions of extratropical cyclones be northward and eastward integrally during the NAO positive phase event. Compared with the negative phase event, the position of 500hPa westerlies during the NAO positive phase event are northward, and thereby making the activity scope of the extratropical cyclones be northward in NAO positive phase events. As NAO positive (negative) phases in winter display reduction (increase) tendency, the source regions of the extratropical cyclones in winter in the study region are shifted southwards and westwards and the activity scope is shifted southwards.