Glaciers of the Wet Andes

By Louis Lliboutry¹

Abstract

In the southern part of South America in the Wet Andes, the mean air temperature at sea level decreases progressively from 13.7 degrees Celsius at latitude 37°23' South to 6.5 degrees Celsius at latitude 53°10' South, where west winds become almost permanent and very strong. Precipitation reaches 4.0 to 4.7 meters per year on the west, windward side of the mountains, and 6 to 7.5 meters per year on the Patagonian ice fields, but it remains very low on the east, leeward side. In Patagonia, precipitation is evenly distributed throughout the year, but in summer, it is frequently rainy even on the ice fields.

Between latitude 35° and 45°30' South (extended Lakes Region), 37 volcanoes have about 300 square kilometers of glaciers, most of them on the west side of the ice divide. South of latitude 41° South, cirque glaciers are found as well. South of latitude 45°30' South in the Patagonian Andes, a very large number of glaciers are present in addition to three large ice fields: the Northern Patagonian Ice Field (4,200 square kilometers, with 30 outlet glaciers [Editor's note: Masamu Aniya inventoried 28 outlet glaciers in 1988 from this field]), the Southern Patagonian Ice Field (13,000 square kilometers, with 48 outlet glaciers of more than 20 square kilometers in area), and the ice field of Cordillera Darwin in the southwest part of Tierra del Fuego (2,300 square kilometers).

In the 1990's in Patagonia, the time of geographical exploration and of the conquest of virgin summits is almost over, but glaciological investigations have replaced them. Some glacier-velocity, mass-balance, and even energy-balance measurements have been made on 4 outlet glaciers of the Northern Patagonian Ice Field and 11 outlet glaciers of the Southern Patagonian Ice Field. Subglacier topography has been determined along a single east-west profile across the Northern Patagonian Ice Field; the elevation of the glacier bed ranges there between +596 meters and -223 meters. Two glaciers flowing westward, Glaciar San Rafael (Northern Patagonian Ice Field) and Glaciar Brüggen (or Pío XI) (Southern Patagonian Ice Field) have flow velocities near their calving fronts of more than 17 meters per day and 15.2-36.8 meters per day, respectively. Three bands of tephra ejected by Cerro (Volcán) Lautaro are visible on a large part of the Southern Patagonian Ice Field. They are the outcrops of three layers of tephra within the ice.

Patagonian ice fields are temperate. The mean mass balance at 1,296 meters on Glaciar San Rafael, about 250 meters above the equilibrium line altitude, was found to be 3.45 meters per year (water equivalent). The main climatic factor providing glacier fluctuation is the elevation of the limit between rain and snowfall during every precipitation event.

Glacier fluctuations in the Wet Andes have been monitored since 1945 by aerial photographic surveys and satellite imagery. A general recession has taken place, but different patterns emerge from one glacier to another. The largest recessions are those of Glaciar O'Higgins (12.4 kilometers), which calves into Lago San Martín/O'Higgins, and of Glaciar Upsala, which calves into Lago Argentino. An abnormal behavior is the large advance of Glaciar Brüggen (Pío XI) into Fiordo Eyre. It is suggested that its former recession was due to the volcanic activity of Cerro (Volcán) Lautaro.

Many glaciations (maybe 40) have taken place in Patagonia during the last 7 million years, but only one at 1.2-1.0 Ma was more extensive than the last glaciation (by 80 kilometers at the latitude of Lago Argentino). The last glaciation left two morainic systems, the inner one resulting from at least five glacier advances between 70 ka and 11 ka. The elevation of the ice divide on the northern Southern Patagonian Ice Field was probably 2,100+-200 meters at that time, 300 to 700 meters higher than today. Thus, all the relief was not covered by a convex ice cap, as assumed by others. To explain the scouring and overdeepening of north-trending Patagonian channels, it is suggested that local ice fields often formed on the Pacific islands. Four "Little Ice Ages," dated at 3.6 ka, 2.3 ka, 1.4 ka, and 250 years before present, have been recognized in Patagonia. The last one followed a time that had a milder climate than the one today, and had winds from the northeast, as documented by old logbooks.

Mapping, Aerial Photography, and Satellite Imagery

In 1954, the orography of the Andes Mountains south of lat 35°S. was more or less well known as far south as lat 42°S. on the Chilean side and as far south as lat 43°S. on the Argentine side. This was because mountaineers from Club Andino Bariloche (C.A.B.) had explored the area around Lago Puelo (lat 42°10'S., long 71°38'W.). In the same year, the U.S. Army Air Force Preliminary Charts (Carta Prelíminar, CP) became available. At a scale of 1:250,000, these were compiled from 1945 Trimetrogon aerial surveys. After reduction, they became the 1:1,000,000-scale U.S. Air Force (USAF) Operational Navigation Charts (ONC) R-23, S-21, and T-18 that cover the Wet Andes area. On the CP's, contour lines were drawn at 500 feet, 1,000 feet, and then at 1,000-foot contour intervals. On a large part of the ONC's, no contour lines are shown at all.

North of Puerto Aisén (lat 45°25'S., long 72°42'W.), the Trimetrogon aerial photographic survey was done too early in the season, when extensive snowpack covered the terrain. Therefore, these charts, CP's at a scale of 1:250,000 and ONC's at a scale of 1:1,000,000, cannot be used as a basis for a glacier inventory. In particular, the extensive glaciers shown on the northern part of ONC S-21 on the Argentine side of the popular Lakes Region simply do not exist.

South of Puerto Aisén, most of the Andes lie in Chile. The west side is almost always hidden in the rain and fog. At Grupo Evangelistas (lat 52°20'S., long 75°05'W.), there are 360 rainy days a year! Rain forest, swamps, fjords, and ice fields that have tidal outlet glaciers make ground exploration exceptionally difficult and commonly nearly impossible. In spite of considerable effort by mariners since the 16th century, the fantastic labyrinth of channels and fjords along the west coast of Chile south of Puerto Aisén was very poorly known before publication of the CP. The Trimetrogon aerial surveys were carried out in this area from December 1944 to March 1945 on the very rare cloudless days. Publication of the chart in 1953-54 allowed the biggest map revision in the Earth's geography to be made in modern times. Only mysterious Isla Santa Inés at lat 53°46'S., long 72°40'W., remained incompletely surveyed. This highly dissected island has several fjords, one of which hid the German battleship Dresden in 1914 after the battle of the Falkland Islands (Islas Malvinas).

In spite of the fact that the aerial surveys of the southern Wet Andes were carried out under optimum conditions, the ice fields, outlet glaciers, and other glaciers are very poorly defined on the CP and on the three ONC's (R-23, S-21, and T-18). In addition, geographic place-names are few and far between, and many are incorrect. For this reason, my sketch maps, published in Lliboutry (1956), are reproduced here and have some new geographic names added (see figs. 27, 29, 30, and 37).

Because the Trimetrogon aerial survey of January and February 1945 remains an essential source of data for the Northern and Southern Patagonian Ice Fields, the following information is provided on Sorties (flights) and photographic frame numbers in order to complete the ones given by Mercer (1967, p. 133-145). The survey mission designation for all the U.S. Army Air Force aerial survey flights over southern Patagonia is 91-PC-5M-4028, except for Sortie 406, which is 91-PC-4M-4028 (Masumu Aniya, written commun., 1997).

Northern Patagonian Ice Field:

Sortie 406, Frames 85-124: East side from Glaciar Circo (Glaciar Grosse) to Glaciar Pared Sur

Sortie 558, Frames 10-41: West side from Glaciar Steffen to Golfo Elefantes

Southern Patagonian Ice Field (northern part):

Sortie 556 (2 January 1945),

Frames 16-49: East side from Río Pascua to Glaciar Viedma

Frames 53-85: West side from Meseta del Comandante (Caupolicán) to the north limit of the ice field

Frames 100-110: West side, 30 km farther west overflying Glaciar Occidental

Frames 115-149: Center of ice field from Glaciar Brüggen to the vicinity of Fiordo Calén (Cerro (Volcán) Lautaro on 556-V-124)

Southern Patagonian Ice Field (southern part):

Sortie 410 (23 January 1945),

Frames 115-223: From the south end (Cordillera Sarmiento) to Glaciar O'Higgins (valley from Fiordo Peel to Fiordo Mayo) on 410-V-168 (Nunatak del Viedma on 410-V-207)

Sortie 411, Frames 1-25: From Lago Argentino to the Paine group (terminus of Glaciar Moreno on 411-V-9)

[In Lliboutry (1956), I wrote 1946 instead of 1945 for the date of the photography, as was told to me at the Instituto Geográfico Militar of Chile (IGMC). However, Prof. Aniya has brought to my attention that the months and years are printed on all of the CP maps, and they are always from December 1944 to March 1945.]

The era without accurate maps is now over. Aerial surveys by the Chilean Air Force and by the USAF started in May 1966 and used Doppler positioning to measure and locate surveyed peaks accurately. The surveys allowed the progressive publication from north to south in the 1970's and 1980's by the IGMC of maps at a scale of 1:50,000. The map of the Northern Patagonian Ice Field, based on aerial photographs of 1974, was published in 1982. However, the elevations and contour lines that are essential for glaciological work remain questionable on the large ice fields of southern Patagonia, where the ground is uniformly white and stereoscopic observation of photographs is impossible. As for the highest summit, Monte San Valentín, an elevation of 3,876 m was based on terrestrial triangulation by Nordenskjöld in 1921. Later the elevation was thought to be 4,058 m. The 1:50,000-scale map shows 3,910 m. A French group that climbed the peak in 1993 included two surveyors, who calculated an elevation of 4,080+-20 m by using a Global Positioning System (GPS).

On the Argentine side, the IGMA compiled maps at a scale of 1:100,000 that cover the east side of the ice field from Monte FitzRoy/Cerro Chaltél to Lago Frias. Geodetic ground control was provided through triangulation, traversing, and some Doppler (Transit system) satellite determinations. Although these maps have been available for sale to the public since their publication in the late 1980's, my sketch maps of 1956 are still used by mountaineers visiting this region. Argentina also made aerial surveys of the area between Monte FitzRoy/Cerro Chaltél and Lago San Martin/O'Higgins in 1966 and 1981. The IGMA compiled maps at a scale of 1:50,000 from the coverage of 1966, as required by the Argentine-Chilean Commission in charge of establishing the international boundary in this region. From this map and the 1981 aerial photographs, González and Veiga (1992) drew a map of the FitzRoy group.

A comparison of the 1945 aerial surveys and more recent data (Landsat images, aerial photographs, and maps, for example) allows a comparative time-lapse study of glacier variation in Patagonia. Unfortunately, good satellite images without cloud cover are scarce. The most useful Landsat 1, 2, and 3 multispectral scanner (MSS) images of glaciers of the Wet Andes are listed in table 8. Naruse and Aniya (1992) published a Landsat 5 Thematic Mapper (TM) false-color mosaic of the Southern Patagonian Ice Field [see fig. 32B] using three images (table 1) acquired on 14 January 1986 under very rare, almost cloudless conditions.

Table 8.--Most useful Landsat 1, 2, and 3 images of the glaciers of the Wet Andes

[Table 1 lists all the optimum Landsat 1, 2, and 3 images of the glaciers of Chile and Argentina]

In spite of extremely adverse weather conditions, the mountains and ice fields of southern Patagonia have been the goal of many expeditions (Naruse and Aniya, 1992, 1995). The results of these expeditions allow confirmation of interpretations made from aerial photographs and satellite images.

This section of the "Satellite Image Atlas of Glaciers of the World" ("Glaciers of South America" volume) is an assessment of existing knowledge in 1997. An extremely important development of Patagonian glaciology is foreseeable in the near future with the use of spaceborne imaging radar, which can survey the Earth's surface through cloud cover. Moreover, interferometric observations from sequential radar images will allow daily measurements of the velocities of fast outlet glaciers (Rignot and others, 1996b).

Climatic Setting

As one travels to the south, the mean annual temperatures decrease progressively. Near sea level, the mean annual temperatures at the following meteorological stations are as follows:

At the same time, the wind (always from the west or northwest) becomes stronger and stronger, and the climatological differences between the west and the east sides of the Andes Mountains become more pronounced.

Very few meteorological stations exist in the Andes and in Chilean Patagonia. Therefore, the type of vegetation present is a very useful indicator of the climate and for preparing climatic maps (Quintanilla, 1974).

At lat 35°S., the annual precipitation in the Andes is about 1,500 mm a^-1. It is 2,471 mm a^-1 at the Albanico hydroelectric plant (lat 37°20'S., elevation 850 m) and 3,083 mm a^-1 at the Las Raíces tunnel on the Lonquimay railroad (lat 38°30'S., elevation 1,200 m). In this region at moderate elevations, the characteristic flora (which includes Peumus boldus and Quillaja saponaria) of the Tinguiririca and Cachapoal valleys is replaced by a roble forest (Nothofagus obliqua). At higher elevations, the forest is mainly the spectacular pehuén (Araucaria araucana).

At lat 39°30'S., the already high annual precipitation shows a major increase, and precipitation becomes distributed throughout the year. Whereas the annual precipitation is 2,489 mm a^-1 at Valdivia on the Pacific coast (lat 39°50'S.), in the Andes at the same latitude, 4,970 mm a^-1 has been measured at Puerto Fuy on Lago Pirehueico (lat 39°52'S., elevation 750 m). At Petrohué on Lago Todos Los Santos (lat 41°08'S., elevation 700 m), the figure is 4,000 mm a^-1, although this site is in the lee of Volcán Osorno. Under this extremely wet climate at moderate elevations, the roble forest is replaced by the Bosque valdiviano along with Aextoxicum punctatum (olivillo), Eucryphia cordifolia (ulmo), and Drimys winteri (canelo). At higher elevations, the Araucaria forest is replaced by an impenetrable rain forest that has evergreen leaves of Nothofagus dombeyi (coigüe) and other species.

South of lat 42°S., the Bosque valdiviano disappears, and Nothofagus dombeyi is progressively replaced by Nothofagus betuloides (guindo). On the drier Argentine side, forests of Fitzroya cupressoides appear (alerce, which has given its name to an Argentine national park), including individuals as old and as large as those in the sequoia forests of North America.

Near sea level, no further increase in precipitation exists. On most west coasts, it has only been measured at lighthouses, the only inhabited places. At Valdivia (lat 39°50'S.), the precipitation was measured at 2,489 mm a^-1. At Melinca (lat 43°54'S.), the measurement was 3,174 mm a^-1; at Cabo Raper (lat 46°50'S.), it was 2,000 mm a^-1; and at Islas Evangelistas (lat 52°20'S.), it was 2,900 mm a^-1. In the interior of fjords and channels, precipitation is higher, similar to that in the Lakes District (Región de los Lagos) of Chile. At the meteorological station of Laguna San Rafael (lat 46°37'S.), the mean annual precipitation during the years 1981-85 was 4,440 mm a^-1; at the entrance from the Pacific Ocean to the Strait of Magellan (lighthouse of Bahía Félix, lat 52°58'S.), it was 4,700 mm a^-1. Precipitation increases with elevation and exceeds 6,000 mm a^-1 of water equivalent on the Patagonian ice fields (Inoue and others, 1987; Peña and Gutiérrez, 1992). From the discharge of rivers, Escobar and others (1992) infer 7,000 mm a^-1 of water equivalent on the western part of the Northern Patagonian Ice Field, 6,000 mm a^-1 on its eastern part, and 6,000 to 7,500 mm a^-1 on the Southern Patagonian Ice Field.

In the lee of the Andes, precipitation decreases sharply. At Estancia Madsen (12 km east-southeast of FitzRoy, at lat 49°10'S.), 850 mm a^-1 was measured in the 1940's. Farther east, the Patagonian pampa is a steppe that has 200-300 mm a^-1 of annual precipitation. This steppe extends south to the east side of Torres del Paine, where a small endorheic salty lake, Laguna Amarga, is found.

To the south, changes in vegetation are the result of colder temperatures. Evergreen species are replaced by deciduous species, such as Nothofagus pumilio (lenga) and Nothofagus antarctica (ñirre), and the forest becomes penetrable wherever no bogs are found. The highland forest extends upward in elevation to bare rock, perennial snow, or glaciers. No intervening highland zone of grasses exists, as in the European Alps.

¹ 3, Avenue de la Foy, 38700 Corenc, France.