A PRELIMINARY STUDY ON THE CHARACTERISTICS AND EVOLUTION OF MEAN MONTHLY CIRCULATION, Ⅰ.ZONAL INDEX

This paper is the first report of a research project concerning.the characteristics and evolution of the general circulation in recent ten years. The results of the analysis of the monthly tonal index in the temperate latitude belt (35°-55°N) derived from the mean monthly 500 mb charts show that the tonal circulation is characterized by: 1. The mean tonal index in the period from 1951 to 1960 is about 5-10% higher than that in the period from 1941 to 1950, but slightly lower than that in the period. from 1900 to 1939. In western hemisphere (180°W-10°W) the westerlis,is strenthened mainly in the summer half year (May-October), whereas in the eastern hemisphere (0°-170°E) this trend is more pronounced in the winter half year (November-April). 2. The half yearly.mean tonal index shows a clear periodic fluctuation, the length of most prominent periods are 10,years, 5.5 years and 26 months. It is highly probable that the first two periods are related to the 11-years sunsport cycle. 3. In the summer half year there exists a high positive correlation between the half yearly mean index of eastern hemisphere. and that of western hemisphere, the correlation coefficient is about 0.77. But in the winter half year the correlation becomes negative and its value is about -0.54. Other analysis suggests that this may be O due to the asymmetry of circumpolar current. 4. The summer half yearly mean index has a negative (positive) correlation coefficient with that of following winter half year in eastern (western) hemisphere. But from winter to summer there is a reverse trend in the change of zonal index. 5. This association of tonal index from one half year to another, indeed, induces a binnial oscillation. This is not only concerned in eastern and western hemisphere separately, but also in northern hemisphere as a whole, becouse the oscillations of the tonal index in eastern and western hemisphere are 900 out of phase. 6. The monthly mean tonal index has a tendency to keep the same characteristic through-out the winter or summer half year period. Moreover in eastern hemisphere the high tonal index pattern shows a greater persistence them the low tonal index pattern, while in western hemisphere, to opposite of this, the low tonal index pattern usually persists from month to month.