VOL. 39, No. 3, MARCH, 1958 149 On the Behavior Patterns of Cyclones and Anticyclones as Related to Zonal Index DOROTHY L. BRADBURY University of Chicago 1 (Manuscript received 19 February 1957) ABSTRACT A statistical study of the variations in the frequency patterns of cyclone and anticyclone centers with respect to the zonal index is described. The results indicate significant variations, especially over the oceans, in the ratio of the frequency of cyclone and anticyclone centers during high-index months to the frequency during low-index months. 1. Introduction five-degree squares was obtained from the Data HE geographical distribution of the fre- Control Division of the Air Weather Service. quency of cyclones and anticyclones has Since the investigation was limited to only the Tbeen investigated by Petterssen (1950). It winter (December, January, February) and sum- was shown that there are certain preferred regions mer (June, July, August) seasons, a total of in the northern hemisphere where the percentage 120 months for each season was considered. The frequency of these centers has pronounced mean monthly sea-level zonal index between 35 maxima and minima. These occur, most gener- and 55°N was computed for each of these months. ally, in connection with certain geographical fea- Then, from each of the two seasons, thirty months tures, such as mountain ranges, bays, inland bodies for which the mean monthly index was nearest of water and coastal configurations. Willett the forty-year seasonal normal were eliminated. (1940) compared the variations in the zonal index The remaining ninety months were then classified with the behavior of some of the principal centers as either high- or low-index according to whether of action, such as the Aleutian Low, Icelandic Low they were above or below the seasonal normal. and Asiatic High. On weekly mean charts it was The frequency of cyclones and anticyclones within shown that there were distinct displacements of five-degree squares was tabulated separately for these centers with changes in the zonal index. It each of the two seasons and for both high- and was thought that it might be of value to routine low-index conditions. Since the number of days forecasting to determine the influence, if any, of during high-index months did not equal that dur- the zonal index on the location and frequency of ing low-index months the frequency values were cyclone and anticyclone centers. Thus, the pres- converted to percentage frequency. Next, the ent study was undertaken as an extension of the ratio of the frequencies during high-index months previous work with the purpose of relating the to the frequencies during low-index months was patterns of frequency of cyclones and anticyclones computed. In order to smooth out the irregulari- to zonal index. ties in the patterns of frequency distribution over- lapping means for ten-degree squares were used. 2. Statistical material and methods used Ratios in excess of unity (5/4, 3/2, 7/4, 2/1) indicate abundance during high-index circulations, The basic data used in this study were obtained and ratios less than unity (4/5, 2/3) indicate from the Historical Weather Maps published by abundance during low-index circulations. the U. S. Weather Bureau and covered the forty- year period 1899-1939. The tabulation of the There are large areas of the northern hemi- frequency of cyclone and anticyclone centers in sphere where the frequency of cyclone and anti- cyclone centers within a 5-deg square was so small that the ratio of percent frequency between 1 The research reported in this article has been made possible through support and sponsorship extended by high- and low-index circulations had little mean- the Geophysics Research Directorate, Air Force Cam- ing. Thus a minimum frequency of 50 centers per bridge Research Center, under Contract AF 19(604)- 5-deg square was chosen as the boundary of the 1293. Unauthenticated | Downloaded 09/30/21 04:48 AM UTC 150 BULLETIN AMERICAN METEOROLOGICAL SOCIETY ern plains states (North Dakota, South Dakota and Nebraska), and 3. over the New England states and southeast Canada. Fig. 2 shows the distribution of cyclone cen- ters during the summer months. During this season the pattern over the oceans is similar to that in winter but not as pronounced. Since the mean zonal index during the summer months is much less than that during the winter months, the distribution patterns are not nearly as well defined. However, the general tendency is for a greater frequency of cyclone centers at higher latitudes during high-index months and at lower latitudes during low-index months. 4. Anticyclone frequency distribution The pattern of the ratio of frequency of anti- cyclones between high- and low-index circulations during the winter season is shown in fig. 3. Over the Pacific Ocean between 20° and 35°N the tendency is for an abundance of anticyclones at higher latitudes during high-index and at lower latitudes during low-index months. In the region between 135° and 175°W there is a definite abun- dance of anticyclones during high-index circula- FIG. 1. Ratio of frequency of cyclone centers during tions. Over the North American continent the high-index months to frequency during low-index months trend is reversed with the greater number of anti- for the winter season. Solid line encloses area with 50 cyclones at lower latitudes during high-index con- or more centers per 5-deg square. areas to be considered in this study. Since the frequency drops off quite rapidly outside this boundary the choice of this limit was considered satisfactory for our purpose. 3. Cyclone frequency distribution Fig. 1 represents the distribution of the ratio of percentage between high- and low-index months during the winter season. Over the Pacific Ocean there is a definite pattern with an abundance of cyclone centers north of about 50 °N during high- index circulations. The frequency distribution in the Atlantic Ocean shows an abundance of cy- clone centers off the east coast of North America between 40° and 70°N during low-index months and east of 45°W between 45° and 70°N during high-index conditions. This pronounced abun- dance of cyclones during high-index months in the eastern Atlantic Ocean is the most outstanding feature of fig. 1. In the Mediterranean region the cyclone frequency is greater during low-index months. Over the North American continent be- tween 30° and 55°N those regions where cyclone frequency is greater during low-index months are: 1. the area west of 110°W, 2. over the north- FIG. 2. Same as fig. 1 for summer season. Unauthenticated | Downloaded 09/30/21 04:48 AM UTC VOL. 39, No. 3, MARCH, 1958 151 FIG. 3. Ratio of frequency of anticyclone centers dur- FIG. 4. Same as fig. 3 for summer season. ing high-index months to frequency during low-index months for the winter season. Solid line encloses area with 50 or more centers per 5-deg square. and coastal configurations exert the same influence upon the frequency of cyclones and anticyclones ditions and at higher latitudes during low-index regardless of zonal index. In those areas where months. This latter distribution also exists over a pronounced maxima of frequency of cyclone or the eastern half of the Atlantic Ocean and western anticyclone centers is due to some geographical Europe, but it is much more pronounced in this feature, the ratio of frequency between high- and region than over the North American continent. low-index months was approximately unity. The ratio reaches the value of 2:1 in that part of The author is aware of the fact that the data the Atlantic Ocean off the coast of Morocco and used are not infallible since the use of mean southern Spain. monthly indexes tends to smooth out significant During the summer months the frequency dis- variations, particularly during the shorter index tribution of anticyclones was similar to that of the cycles. winter season but much less pronounced, as shown However, since the months with an index near- by fig. 4. The area with the most pronounced est the seasonal normal were eliminated and only abundance of anticyclones during high-index con- those months with definite high- or low-index ditions is that over southeast Europe between the values are included in the final tabulations, the Black and Caspian Seas. There is no correspond- patterns shown in figs. 1-4 must be representative ing region for low-index months. of the actual distribution. If the frequency values for individual index cycles had been available, the 5. Conclusion patterns would have been more pronounced. Figs. 1-4 show that there is a definite relation- REFERENCES ship between the zonal index and the geographical Petterssen, S., 1950: Some aspects of the general circu- distribution of cyclone and anticyclone centers. It lation of the atmosphere. Cent. proc. r. meteor. is especially pronounced over the ocean areas. Soc., 120-155. Over continents, certain geographical features Willett, H. C., et al, 1940: Report on an experiment in five-day weather forecasting. Papers in Physical such as mountain ranges, inland bodies of water, Oceanography and Meteorology, 8, No. 3, 94 pp. Unauthenticated | Downloaded 09/30/21 04:48 AM UTC.