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- Unauthenticated | Downloaded 10/06/21 08:17 PM UTC VOL. 30, No. 7, SEPTEMBER, 1949 243 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. Unauthenticated | Downloaded 10/06/21 08:17 PM UTC 244 BULLETIN AMERICAN METEOROLOGICAL SOCIETY FIG. 1. General circulation over South America in summer; (a") cyclone tracks, (b) resultant winds at 1, 3, and 6 km. Unauthenticated | Downloaded 10/06/21 08:17 PM UTC VOL. 30, No. 7, SEPTEMBER, 1949 245 FIG. 2. General circulation over South America in winter; (a) cyclone tracks, (b) resultant winds at 1, 3, and 6 km. Unauthenticated | Downloaded 10/06/21 08:17 PM UTC 246 BULLETIN AMERICAN METEOROLOGICAL SOCIETY At all times of the year the trajectories were re- markably uniform.