A Research About Secular Variation Of Winter Blocking Action In The Northern Hemisphere

The persistent anomaly in the atmospheric circulation-blocking activity, is an important low-frequency process in the westerly. As blocking events greatly affect regional weather, such as summer rainfalls, winter cold waves, freezing rains, and so on. In addition, in order to improve the accuracy of minddle and long term weather forecasts, to provide the real basis for long-term forecasting, the research about blocking activities is of great significance. In this paper, applying NCEP/NCAR reanalysis weather data, by LO index, TM index, Dole index, and DLL index, it has been discussed variance and development of winter blocking events'features in the Northern Hemisphere since 1948 and since the late 1950, influences of North Atlantic Oscillation on it. Through analyzing it carefully, the main conclusions are as follows:In the Northern Hemisphere, winter blocking events with the strongest amplitude mainly distribute over the eastern North Atlantic and Continental Europe sector (20°W-40°E), the East Asia Okhotsk Sea to North Pacific sector (130°E-150°W). Close to the Ural Mountains region (50°E-90°E) also owns blocking activities, but their intensity are weaker than them occuring two major blocking distribution above.Before the mid-1980, The variance about average intensity of the winter blocking activities of the North Atlantic and Continental Europe sector, and the North Pacific and North America region, is in accordance with it over entire Northern Hemisphere. After 1985, the North Atlantic European average intensity increases slightly, the North Pacific and North America's is weakened; But for years, on average, the North Pacific North and North America winter blocking intensity, occurence and the total numbers of days are lower than the average conditions of the North Atlantic Europe, and the North Atlantic European region in the four sectors, the maximum winter blocking events occurred.Lately in about 20 years, winter blocking intensity, total numbers of blocking days, and its cases in the North Atlantic and Continental Europe are close to or higher than the average value, which are close to or lower in the North Pacific and North America. The comparison between 11 years before and after shows, the North Atlantic European winter average blocking intensity (lasting 7-12 days) declines from higher than average, while in the North Pacific and North America region it (lasting less than 10 days) increases from lower than average. In two regions above, the number of winter blocking days and cases (lasting 5-14 days) decline which are higher than average condition, meanwhile the part below the average rise, and both are close to the many years average station.In the Ural Mountains and East Asia Okhotsk Sea region, the average intensity, cases and the total numbers of winter blocking days are smaller than the annual change in the North Atlantic and Continental Europe, and North Pacific and North America. whether up or down in the linear trend, the movements are not obvious; Winter blocking cases and the total numbers of days are not highly different in both areas, and the two curves basically have the same fluctuation. The linear trends of average intensity apporch each other, which of cases and the total numbers of winter blocking days are intersection.Since the past 20 years, the cases and total number of winter blocking days over Ural Mountains area have been below the average value, while in the East Asia Okhotsk Sea region it is higher than mean. Winter blocking intensity (lasting 5-8 days) in the Ural Mountains has risen up to the average level since 1998, but in the East Asia Okhotsk Sea area it has basically reduced. The cases and numbers of days in the two areas above have a similar anomaly, the differences between which are small, and on the whole, they stay in the average level of many years.Since the late 1950, the variation trendency of North Atlantic Oscillation has affected winter blocking activity in the Northern Hemisphere to a certain degree. The linear regression analysis shows that the North Atlantic Oscillation (NAO) mainly affects the blocking events of the Atlantic Ocean, Europe and the Ural Mountains. There are negative relevant relations between NAO index and the frequency of blocking activities in the Atlantic arid the Urals region, which has positive correlation with the European blocking activities. During the negative NAO phase, the blocking activities are more frequent in the Atlantic region, which are inhibited in the positive phase. During the positive NAO phase, the blocking events in Europe are relatively active, and at the same time the blocking activities of Ural Mountains also significantly reduce. The feature of positive NAO index displays firstly enhanced and then weakened. During the positive NAO index increasing phase, the blocking activities of Atlantic Ocean are significantly restraint; but in the positive NAO index weakening stage, the blocking activities of the East Atlantic Ocean increase, particularly in European region. The annual changes in the regional blocking events show that during the enhancing phase of the NAO index the blocking situations in the Atlantic and the Urals region tend a downward trend, which increase in Europe. In the weakening phase of the NAO index, the Atlantic blocking cases present a weak uptrend, which trend downwards in Europe and the Ural region. NAO index changes have also significant effect on the blocking intensity. The Atlantic Ocean blocking intensity weakens with the strengthening NAO index, and the European blocking intensity reinforces with the increasing NAO index.Over the Northern Hemisphere, two distinct blocking regions are both in the storm track outlet region. This expresses that winter blocking activities have a link with the eddies. Positive NAO phase is beneficial to the occurrence and enhancement of winter blocking activities in the Continental Europe, and negative phase is good for the North Atlantic, besides, the strength and position of mid-latitude eddies play an important role in the NAO phase shift. The storm zones which strengthen towards east favor that NAO tends to positive phase, but its further reinforcement weaken NAO positive phase.This paper discussed variance and development of winter blocking events' features in the Northern Hemisphere and influences of North Atlantic Oscillation on it. This is just a preliminary investigation, but there are many issues needed further study.