242 BULLETIN AMERICAN METEOROLOGICAL SOCIETY
Effect of the Andes Mountains on the General Circulation Over the Southern Part of South America *
LT. JORGE ALBERTO BOFFI, Argentine Navy
ABSTRACT The upper-air circulation over the southern part of South America is determined with use of pilot-balloon observations and cyclone tracks. The resulting flow patterns are compared with those to be expected in a westerly current crossing a large mountain barrier, and they are found to be in agreement. Comparison with cyclone tracks of the United States permits qualitative evaluation of the order of magnitude of the mountain effect to that of the local solenoid field at east coasts.
1. INTRODUCTION tions in the study of the general circulation is open to two well-known objections: in the first N the past, the general circulation over the place, in spite of the increase in observational ma- Southern hemisphere has usually been con- terial in recent years, the data available present sidered to have a very simple structure. It I still a bare minimum to establish the upper flow was thought that there is a ring of strong westerly pattern in view of the large size of the Continent; winds blowing all around the hemisphere in mid- in the second place, the pilot-balloon observations dle latitudes and that these westerly winds are very do not reach the higher levels on days with much persistent and subject only to relatively small cloudiness, and they are also frequently lost at low aperiodic changes. levels when the winds aloft are very strong. Thus Meteorologists have been aware of the presence a certain amount of selectivity is introduced in the of the large mountain range extending all the way calculations which may be thought to render them from the equatorial zone to Cape Horn along the to some degree unrepresentative. western part of South America, but its presence A stronger argument to support the calculations has found but little consideration in the studies would be obtained if another source of data with and models of general circulation in the southern independent evidence that confirms the wind calcu- hemisphere. It will be the object of this report lations could be obtained. Such independent evi- to determine as well as possible the general circu- dence is furnished by statistics on cyclone tracks lation over the southern part of South America over South America. As is well known in all and to attempt to find an explanation for the ob- parts of the world, and as has been noted by many served circulation. Previously the general circu- authors and incorporated into the daily forecast lation over South America has been studied by routine of many weather services, cyclone tracks Serra [3] and by Serra and Ratisbona [4]. in general tend to parallel rather closely the flow of the middle troposphere. This has been referred 2. MATERIAL AVAILABLE FOR STUDY to as steering of the cyclones by the upper-air cir- During the 1930's, the Argentine Republic and culation. It may be stated now that there is very other countries in South America began to take good agreement between the upper-air circulation pilot-balloon observations. These observations at as obtained from statistical calculations and cy- first were very scattered, but in recent years the clone tracks. It may, therefore, be claimed that the number of stations taking them grew to such an computed flow patterns aloft must correspond very extent that upper-wind data were taken at eight- closely to the true ones. een stations in Argentina alone since 1940. These upper-air winds as printed in the Carta 3. BRIEF DESCRIPTION OF SOUTH AMERICAN del Tiempo, published by the Argentine weather TOPOGRAPHY service, were utilized to obtain the average flow The Argentine Republic extends from Latitude pattern aloft from one to eight kilometers for the 21° south to 55° south, but has a relatively small four seasons of the year. The use of these observa- east-west extent. The Andes Mountain range covers the western part of the country, while all * Condensed version of Master's thesis completed at University of Chicago, 1947, the territory east of the mountains has the char-
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acter of a plain that is rising gradually from sea vorticity becomes smaller. The increase in abso- level at the Atlantic coast toward the foothills of lute vorticity, therefore, must be accomplished by the Andes. an overcompensating increase in the relative vor- To the north of Latitude 32° the Andes extend ticity. The anticyclonic flow curvature must de- solidly above four kilometers. Near Latitude 40°, crease rapidly and give way to cyclonic-flow curva- the average height of the mountain chain drops ture, turning the current back toward east and below two kilometers, maintaining, however, con- eventually to southeast. The trough east of the siderable elevations to the southern tip of the mountains must be more sharply defined than the Continent. Isolated mountain peaks extend to ridge over the mountains, at least in its western much greater heights; notable especially is the portions. Aconcagua which attains an elevation of more It is evident that the higher the mountain range than 7,000 meters. which the broadscale air current must cross, the Thus in spite of very small east-west extent, the more pronounced will be both the resulting ridge Ancles, situated normal to the prevailing flow, con- of high pressure over the mountains and the stitute the largest obstacle to the circumpolar trough to their east. It follows also that the larger westerlies found anywhere in the world. the obstacle over which the air has to flow the more permanent a feature of the general circulation 4. DYNAMIC EFFECT OF THE ANDES must be the deformation in the general flow pat- tern produced by the mountains, and the smaller The broadscale air currents arriving from the the aperiodic variations from average conditions Pacific Ocean evidently cannot flow around an that should be found. enormous barrier of the kind just indicated, but must cross it. Therefore the vertical extent of the Thus the ridge over the Andes and the trough to air columns as they ascend the mountain range the east must be the most pronounced and most must shrink and on the lee side it must again in- stable features of the general circulation over crease. Thus there is horizontal divergence west South America, if not in the entire world. They of the mountains and convergence to their east. serve as anchor points for the pattern of long The flow pattern that results in a broadscale air waves in the westerlies in the Southern hemisphere current that is subjected to alternating divergence and the position of the upper troughs and ridges and convergence as described has been analyzed over the oceans east and west of the South Ameri- previously with the aid of the vorticity theorem can continent. (see for instance [2] ), which may be expressed in the form: 5. CYCLONE TRACKS The first part of the evidence to be put forward /—jj +— ? = const. to establish the general circulation over the south- ern part of South America will consist of statistics where f is the Coriolis parameter, £ the relative obtained from the cyclonic centers observed over vorticity about the vertical axis, and D the pres- South America; the second part will present the sure difference between top and bottom of the column considered. upper wind observations. As the air currents approach the mountains The cyclone statistics were prepared from data from the west, their vertical depth decreases, and for the years 1938 to 1945 inclusive, as obtained therefore their absolute vorticity must also de- from Carta del Tiempo. FIGURES la and 2a give crease. In a broadscale current such as is con- the cyclone tracks for summer and winter. The sidered here, extending over a wide band of number at the head of each vector denotes the latitude, the decrease of absolute vorticity is ac- total number of cyclones observed. The upper complished by turning of the air columns toward number at the central part of the vector gives the lower latitudes, associated with the appearance of length of the vector in kilometers and the lower anticyclonic-flow curvature over the mountains. figure indicates the number of cyclones used to This decrease of the relative vorticity and the obtain the length of the vector. The FIGURES transport of air towards lower latitudes both con- thus give two pieces of information: 1. They in- tribute to the decrease in the absolute vorticity. dicate the number of cyclones which moved along East of the Continental Divide the absolute vor- the different trajectories indicated; 2. They give ticity again increases. Since the upper-air cur- average speed of motion (km/day) of the cyclones rent there is flowing from the southwest, it is still for all cases in which identification of the cyclones moving toward regions where the earth's own over several days was possible.
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FIG. 1. General circulation over South America in summer; (a") cyclone tracks, (b) resultant winds at 1, 3, and 6 km.
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FIG. 2. General circulation over South America in winter; (a) cyclone tracks, (b) resultant winds at 1, 3, and 6 km.
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At all times of the year the trajectories were re- markably uniform. North of Latitude 50° most cyclones move from southwest to northeast until they begin to reach the subtropics near Latitude 30°. The Figures, therefore, bear out the broad- scale flow pattern deduced in the preceding sec- tion. South of the Continent most cyclones were de- flected southeastward from the Pacific to the At- lantic as may be expected. North of Latitude 50° no cyclones actually cross the Andes Mountains. Over the northern part of Argentina and Uruguay the variability of storm tracks increases as the width of the Continent also increases. North of the Rio cle La Plata the east coast of South Amer- ica evidently is located much closer to the mean FIG. 4. Change of intensity of cyclones moving (a) upper trough than the southern part of the coast. toward northeast, (b) toward southeast. The variability of the storm tracks also in- creases from summer to winter. This may be ex- Comparison of the storm tracks along the east plained in accordance with what is known concern- coasts of North and South America shows that in ing the general circulation in other parts of the both hemispheres cyclones move in exactly the world. In summer the oceanic high-pressure same direction, from southwest to northeast, to- centers are most pronounced, most stable, while ward higher latitudes in North America and to- ward lower latitudes in South America. in the winter their influence decreases and gives way frequently to dominance of intense cyclonic Over the United States, comparison of 10,000- foot isobars and storm tracks indicates that the systems. It is reasonable to expect that because mean position of the 10,000-foot trough is some- of the greater strength of the secondary circula- what to the east of the longitude at which storm tions during the winter, such systems are most apt tracks revert from a southeasterly to a north- to produce temporary dislocations in the mean flow easterly direction. The same may be expected to pattern at that time of year. hold true over South America. It is of interest to compare cyclone tracks ob- Thus there is an essential difference between the served over South America with those noted situation at the east coast of North America and over North America. The average storm tracks the east coast of South America. The mean over North America for January are presented in trough aloft is situated west of the coast line over FIGURE 3 as obtained from the mean monthly North America but east of the coast line over charts published by Bowie and Weightman [1]. South America. The storm tracks observed at the South American coast correspond to those observed over the western plains of the United States. The mean trough aloft roughly appears to be located at a similar distance both from the Rocky Mountains and from the Andes. It is evident that the position of the mean ridges and troughs aloft is primarily a function of the dy- namic effect that the mountains produce on the broadscale air currents crossing the mountains, irrespective of the distance of the eastern coast line from the mountains. Therefore it is deduced that the local solenoid field produced at east coasts by the temperature contrast between land and water is an effect of an entirely subordinate order of magnitude as compared with the influence of the large mountain barriers. These mountain bar- FIG. 3. Average January isobars at 10,000 feet for the riers, moreover, must be considered as the primary United States (solid lines) and storm tracks after Bowie anchor of the long wave pattern in the westerlies. and Weightman (1).
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6. DEEPENING AND FILLING OF CYCLONES 7. AVERAGE FLOW PATTERN ALOFT It is of interest, also, to consider the changes of To obtain the average flow patterns aloft and to intensity experienced by cyclones on their trajec- study the relations between the upper-air circula- tories northeastward and southeastward. These tion obtained from upper wind statistics and cy- two trajectories are the most important ones ob- clone tracks, the wind data of eleven stations were served. The pertinent statistics are presented in summarized. Stations cover the entire area under FIGURE 4 which indicates the percentage of cy- consideration and the data are based on pilot bal- clones that deepened, filled, or moved without loon observations during the years 1943 to 1945, change of intensity for each season and for the en- obtained from the Cartas Diarias del Tiempo at tire year. Change of intensity is defined here as 8:00 a.m. change in central sea-level pressure. It is readily From the observations of wind direction and apparent that the great majority of cyclones mov- velocity for the levels from 1 to 8 km the following ing southeastward increased in intensity and that values were derived: meridional and zonal com- most cyclones moving northeastward either filled ponents, mean monthly values, and finally mean or did not change their central pressure. Such seasonal values. Based upon these components changes of intensity correspond to what has also the average vector wind direction and wind velocity been noted in other parts of the world. Cyclones were calculated and charts were drawn for the dif- moving toward lower latitudes to the rear of ferent levels and seasons. In this report, stream- major upper-air troughs in general tend to de- line charts for summer and winter only are re- crease in intensity, while most intensifying systems produced. (FIGS. 1,2.) are situated east of major upper-air troughs and In general, anticyclonic circulation with north- moving toward higher latitudes. While no en- erly components prevails up to the 3 km level tirely satisfactory dynamical explanation has as over the entire country north of Latitude 35°S, yet been presented in the literature concerning the and with southerly and westerly components south reasons for these relationships, their existence of this latitude. Above the 3 km level the northerly nevertheless sheds important light on the fact that component disappears completely and winds from cyclones of great intensity are rarely observed over south and west predominate, the southerly com- Argentina. ponent becoming more conspicuous with increas- In the past this fact has frequently been attrib- ing elevation. The fact that the wind velocity val- uted to the narrow width of the Continent at its ues at higher levels are much smaller than is to be southern end which prevents the formation of large expected can be attributed to the lack of pilot bal- masses of polar continental air. This argument loon observations with strong winds aloft. does not seem very convincing, even at first sight, because the most intense cyclones generally form 8. ACKNOWLEDGMENT over the oceans in maritime polar air. This holds The author acknowledges with gratitude sug- true for both hemispheres and the cyclonic develop- gestions and help extended to him by Dr. C.-G. ments over the oceans of the southern hemisphere Rossby, Dr. Erwin R. Biel, and Dr. Herbert Riehl, lag in no respect behind those of the northern hem- who was dissertation supervisor. isphere. It now appears that a more satisfactory explanation of the absence of strong cyclonic sys- 9. REFERENCES tems over Argentina may be given on the basis of [1] Bowie, E. H., and Weightman, R. H., "Types of the preceding analysis of the general circulation. Storms of the United States and Their Average The upper winds are mainly from the southwest Movement." Mo. Wea. Rev. Supplement No. 1, 1914. and the major upper air trough is situated east of [2] Holmboe, J., Forsythe, G. E., and Gustin, W., Dy- the Continent over the ocean. Thus cyclones mov- namic Meteorology. New York: John Wiley and ing northeastzvard toward lower latitudes to the Sons, Inc., 1945. [3] Serra, A. B., La Circulation Generate de L'Amerique rear of major upper air troughs lose rather than du Sud. Rio de Janeiro, Servicio Nacional de gain intensity while situated in this field of motion Meteorologia, 1939. which is known to be dynamically unfavorable for [4] Serra, A. B., and Ratisbona, L., As Massas de Ar da America do Sul. Rio de Janeiro, Servicio de cyclone formation. Meteorologia, 1942.
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