The Persistent Anomaly Of Summertime Circulation Over Eurasian Mid-high Latitude

Based on the NCEP/NCAR reanalysis data and so on, the persistent anomaly of summertime circulation over mid-high latitude of Eurasia is investigated. We define an index'I'representing the primary character of the summertime circulation over Eurasian mid-high latitude, and identify the persistent anomaly of summertime circulation over mid-high latitude of Eurasia. With method of composite, correlation, EOF, wavelet, and so on, the climatic feature of these typical persistent anomalies of circulation are discussed, their annual and decadal variation are analyzed, and the character and key area of the influencing factors are explored. The maintenance mechanism of these persistent anomalies is addressed in terms of a wave-activity flux and the westerly waveguide structure. Based on the diagnoses, we verify the connection existing between these persistent anomalies and the factor influencing them, seek the precursory signal for these anomalies, and reveal the evolution and the predictability at monthly time-scale of these circulations. The main result are as follows:1. An index'I'to describe Eurasian mid-high latitudes circulation in summer is proposed, with it two kinds of typical persistent circulation patterns, i.e. type E and C, can be well identified. The index proves representative, rational and bears clear synoptic significance.2. The deviation of the observed frequence of'I'from the normal distribution of probability shows a bimodal feature, i.e., the prior is smaller than the latter about the mean value and larger at the big absolute value which features the relative high frequency of occurrence for type E and C.3. Based on the daily circulation in summer, index averaged for 1959-2000 presents an obvious intraseasonal variation in stages in resemblance to the northward advance of the subtropical high over the West Pacific and summertime rain belt in China. Evidence shows that there is close relation between these anomalies of circulations and summertime rainfall anomalies in China.4. There is clear contrast between the SSTA of type E and C, especially over the North Atlantic and the North Pacific. The distribution of heat field corresponding to type E and C is similar to SSTA over above oceans. So, the influence on type E(C) of the two oceans can not be ignored. The Arctic sea ice in winter prior to type E(C) have obvious contrast too.5. For these typical anomalies of circulation, the Rossby wave dispersion is primary maintainance mechnism. The wave activity flux usually point from a centre of action to the next. The development and maiteance of these centres are linked by Rossby wave energy dispersion. The characteristics of Rossby wave propagation, however, is significantly different between type E and C and in the different periods of summertime.6. Numerical simulation confirm the physical meaning of the distribution of SSTA in spring corresponding to type E and C. The North Atlantic is a key area where SSTA plays a primary role for the formation and maintenance of type E and C, while the influence of North Pacific SSTA has no direct positive contribution to type E and C. For the simulation and forecast in the time scale of two weeks-month, the role of initial states are important too.7. Investigation of potential predictability in terms of the monthly average circulation index shows only a small portion of the cases studied are highly predictable, which reflects the prediction being subject to more uncertainties for mid-high latitudes circulation. However, the greater portion of emsemble forecast remain with some indicative meaning. For those cases where SSTA can give the significant signal, such as the typical SSTA over the North Atlantic corresponding to type E(C), there would be higher potential predictability.8. From 1959 to 2000, the summertime circulation over Eurasian mid-high latitude experienced obvious interdecadal variation that there presented a shift of the average index around 1980. The NAO index(in winter) and the circulation pattern index(in summer) show a similar trend for interannual and interdecadal variation. Highly statistic significant positive correlativity exist between the two index in interdecadal time-scale.