Casassa A Riveraf and M Schwikowski

*Centro de Estudios Científicos, Valdivia, Chile fDepartment of Geography, University of Chile, Santiago, Chile ffiaul Scherrer Institut, Labor für Radio- und Umweltchemie, Switzerland

Southern South America (SSA), i.e. south of 30°S, comprises an estimated glacier area of ca. 27,500km2 (Fig. 47.1), which represents ca. 89% of all Andean glaciers. The glaciers in the region show a generalized retreat and thinning (Naruse, this volume, Chapter 46). In spite of their importance in water resources within a region affected by climate changes, glacier mass balance is to a large extent largely unknown. This summary aims at reviewing the few glacier mass-balance studies available for SSA, obtained by means of firn coring and by the traditional stake method.

Data for 15 glacier sites with annual mass-balance observations are presented in Table 47.1. Data obtained from modelling studies, climatological extrapolations or seasonal (and intrasea-sonal) stake observations are not included in this work. Firn core data include seven sites, covering periods from 1yr to 68 yr; multistake mass-balance studies which cover complete glacier basins exist for five sites and three sites show mass-balance data from single or multiple stakes covering limited portions of glaciers.

The annual values of accumulation, ablation and net balance have been plotted for the above glacier sites in Fig. 47.2. Data representation is limited due to the restricted spatial coverage of the mass-balance data. In general both accumulation and ablation values increase southward, consistent with the meridional gradient in precipitation driven by the southern position of the storm tracks controlled by the westerlies, as opposed to the location of the stable Pacific anticyclone to the north which results in very dry conditions. This north-south gradient is clearly seen in the data, with net balance data in the upper accumulation areas ranging from 30cm water equivalent (w. eq.) at Cerro Tapado, largely controlled by sublimation (Ginot et al., 2002), to a record value of 1540cm w. eq. at Glaciar Tyndall in Patagonia (Shiraiwa et al., 2002; Kohshima et al., accepted for publication).

A clear west-east gradient is observed in the mass balance south of 33°S, particularly in Patagonia, which is affected by an enhanced westerly circulation with precipitation occurring largely on the windward side of the Andes. For example, on the upper accumulation area of the Southern Patagonia Icefield a minimum net balance of 31cm w. eq. has been recorded at 2300m a.s.l. on Cerro Gorra Blanca (Schwikowski et al., 2003), located on the eastern margin, in contrast to the record value of 1540cm w. eq. obtained near the ice divide at Glaciar Tyndall.

The low values of net accumulation measured on the leeward-facing glaciers of Patagonia are not regarded as representative for the accumulation area, and are much smaller than expected from hydrological models (e.g. Escobar et al., 1992), precipitation estimates from neighbouring meteorological stations (Carrasco et al., 2002), in situ precipitation observations (Fujiyoshi et al., 1987; Casassa et al., 2002a), and glacier dynamics models (e.g. Naruse et al., 1995), which estimate that an annual accumulation of 510 m w. eq. is needed in the upper accumulation areas for maintaining a steady-state at SPI, in order to balance the large annual ablation values that occur at lower elevations, such as those measured at Glaciar Moreno (Stuefer, 1999) and Glaciar Lengua (C. Schneider, personal communication, 2004).

Strong interannual variations are found within the massbalance records of Glaciar Piloto Este and Glaciar Echaurren (ca. 33°S), closely related to ENSO events, with dry La Niña years and wet El Niño years. At Glaciar La Ollada, Cerro Mercedario, inter-annual variations in snow accumulation related to ENSO events are not obvious in the record. After 1988 both mass-balance series of Glaciares Piloto Este and Echaurren Norte show a clear negative trend, consistent with the retreating trend of glaciers in the region, which probably is largely due to a regional warming (Rosenblüth et al., 1997). A negative trend is also observed in the short mass-balance record of Glaciar Martial Este.

Table 47.1 Glacier mass-balance data. The mass-balance data represent mean annual values for the indicated period. In the case where mass balance is monitored with a stake array, minimum and maximum represent the altitudinal range of the glacier. For firn core and single stake sites, minimum and maximum represent the altitude of the site. The altitude of the firn core site at Glaciar Moreno is taken from Godoi et al. (2001). At Glaciar Tyndall the mass-balance data is an average value from multiproxy analysis based on stable isotope data and microalgae concentrations

Table 47.1 Glacier mass-balance data. The mass-balance data represent mean annual values for the indicated period. In the case where mass balance is monitored with a stake array, minimum and maximum represent the altitudinal range of the glacier. For firn core and single stake sites, minimum and maximum represent the altitude of the site. The altitude of the firn core site at Glaciar Moreno is taken from Godoi et al. (2001). At Glaciar Tyndall the mass-balance data is an average value from multiproxy analysis based on stable isotope data and microalgae concentrations

(°W)

Location*

Altitude (m

a.s.l.)

Ablation (cm w. eq.)

Accumulation (cm w.eq.)

Balance (cm w. eq.)

Period

(year)

References

Minimum

Maximum

Cerro Tapadot

30.13

69.92

E

5550

5550

24

54

30

1920-1998

Ginot et al., 2002

La Olladat

31.97

70.12

E

6100

6100

45

1986-2002

Bolius et al., 2004

Piloto Este$

32.50

70.15

E

4185

4740

110

77

-33

1979/80-1996/97

Leiva, 1999

Echaurren Norteé

33.55

70.13

W

3650

3880

289

269

-20

1975/76-2003/04

Escobar et al., 1995; F. Escobar, personal communication, 2004

Mocho $

39.92

72.03

W

1603

2422

346

258

-88

2003/04

Rivera et al., 2005

San Rafaelt

46.73

73.53

W

1296

1296

345

1984

Yamada, 1987

Neft

46.93

73.32

E

1500

1500

220

1996

Matsuoka & Naruse, 1999

Cerro Gorra Blancat

49.13

73.05

E

2300

2300

31

1995-2001

Schwikowski et al., 2003

Chico§

49.18

73.18

E

1444

1444

57

1994/95-2001/2002

Rivera, 2004

De los Tres$

49.27

73.00

E

1120

1830

225

232

7

1995/96

Popovnin et al., 1999

Moreno§

50.50

73.00

E

365

365

1025

1998-1999^1

Stuefer, 1999

Morenot

50.63

73.25

E

2000

2000

120

1980/81-1985/86

Aristarain & Delmas, 1993

Tyndallt

50.98

73.52

W

1756

1756

1540

1998/99

Shiraiwa et al., 2002; Kohshima et al., accepted for publication

Lengua§

52.80

73.00

W

450

450

640

2001-2004**

C. Schneider, personal communication, 2004

Martial Este$

54.78

68.40

E

1000

1180

86

81

-5

2000-2002

Strelin & Iturraspe, accepted for publication

*Location of the glacier site, E(W) indicates that the glacier is located east(west) of the main Andean range. tFirn core data.

¿Net mass-balance data from stake array.

§Net balance data from one single stake.

(20 March 1998 to 7 March 1999, average for three stakes.

**Annual average for three stakes.

Figure 47.1 Map of southern South America showing the glacier sites mentioned in the text: GCT, Glaciar Cerro Tapado; GLO, Glaciar La Ollada; GPE, Glaciar Piloto Este; GEN, Glaciar Echaurren Norte; GMO, Glaciar Mocho; GSN, Glaciar San Rafael; GN, Glaciar Nef; GG, Glaciar Cerro Gorra Blanca; GC, Glaciar Chico; GTS, Glaciar de los Tres; GM3, Glaciar Moreno, ablation stakes site; GM1, Glaciar Moreno, firn core site; GT, Glaciar Tyndall; GL, Glaciar Lengua; GME, Glaciar Martial Este.

Figure 47.1 Map of southern South America showing the glacier sites mentioned in the text: GCT, Glaciar Cerro Tapado; GLO, Glaciar La Ollada; GPE, Glaciar Piloto Este; GEN, Glaciar Echaurren Norte; GMO, Glaciar Mocho; GSN, Glaciar San Rafael; GN, Glaciar Nef; GG, Glaciar Cerro Gorra Blanca; GC, Glaciar Chico; GTS, Glaciar de los Tres; GM3, Glaciar Moreno, ablation stakes site; GM1, Glaciar Moreno, firn core site; GT, Glaciar Tyndall; GL, Glaciar Lengua; GME, Glaciar Martial Este.

Figure 47.2 North-south transect along the Andes showing annual ablation and accumulation values, as quoted in the text. Solid squares and solid triangles represent accumulation values for western Andean glaciers and eastern Andean glaciers respectively. Hollow squares (triangles) represent ablation values for western (eastern) glaciers. Values increase southward to a maximum at ca. 50-51°S.

Figure 47.2 North-south transect along the Andes showing annual ablation and accumulation values, as quoted in the text. Solid squares and solid triangles represent accumulation values for western Andean glaciers and eastern Andean glaciers respectively. Hollow squares (triangles) represent ablation values for western (eastern) glaciers. Values increase southward to a maximum at ca. 50-51°S.

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