Austral summer of 1998: Observations on El Niio and the North Patagonian I cef iel d

V. Winchester’, S. Harrison’, R. Washington‘ and C. R. Warren3 ’University of Oxford *Coventry University ’St. Andrew’s University

In February 1998 we visited the Nef Glacier (47”07’S, 73’12’W) on the east side of the North Patagonian Icefield, in southern Chile, as part of a long-term project aimed at investi- gating the links between recent glacier fluctuat- ions around the icefield and climate change. Of particular interest for our study is the effect of strong El Niiio Southern Oscillation (ENSO) events on icefield and glacier ablation in the region. Our expedition took place during one of the strongest ENSO events of this century in a region where weather stations are scarce and many gather precipitation data only (Fig. 1).

Although satellite data have, to some extent, remedied the lack of year-round observations over southern Chile and the south-eastern Pacific, detailed ground observations are still scarce and teleconnections with El Niiio are only studied in outline with research mainly focused on the more populated regions of cen- tral Chile (e.g. Yarnal 1985; Aceituno 1987; Rutllant and Fuenzalida 199 1). However, gen- eralised outlines can be misleading as, for example, that published in the Climate Diagnos- tics Bulletin of January and February 1998: “Enhanced westerlies have produced increased storminess and above-normal precipitation across much of the eastern South Pacific and South America”. Although the weather in cen- tral Chile in the months preceding and during our expedition was unusually wet and cool, this was not true for latitudes between 40 and 50”s. The weather over the southern Andes during the austral summer of 1998, in the latitudes of the roaring forties, normally among the most stormy and wet in the world, was exceptionally stable, dry and SUMY - ideal expedition weather.

Background

During ENS0 events, anomalous latent-heat release in the tropics is thought to change the position and intensity of jet streams and the behaviour of extratropical storms. This has been particularly well established for the North American sector. However, in Chilean Patago- nia south of 40”s the effects on the ground remain obscure. In western Patagonia the weather is dominated by the westerly winds, with centres of cyclogenesis, according to Tal- jaard (1 967), at a maximum around 45”s shift- ing slightly polewards in summer. This latitude and the southwards seasonal shift in cyclogen- esis probably account for differences in the north-south seasonal patterns of precipitation along Chile’s southern seaboard, with summer maxima prevailing south of 46’50’s and winter maxima to the north (Winchester and Harrison 1996).

Significantly, Taljaard’s data were collected in the Southern Hemisphere between July 1957 and December 1958 during a strong El N S o event (Quinn et al. 1978). He noted that:

“in July 1957 altogether 18 lows moved over Southern Patagonia, the Drake Passage and the Antarctic Peninsula, and . . . the distribu- tion seems to be normal for the area. How- ever, in July 1958 only 10 systems moved through the area and their tracks were bunched together over the southern tip of Patagonia and over Tierra del Fuego . ..” with “the circulation and distribution of sys- tems ... [being] most unusual with many anticyclones and centres of cyclogenesis in high latitudes of the Indian and Pacific Oceans” (Taljaard 1967, pp. 980,985).

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48'

52'

i

80' 76' 72' 68' 64'

0 Patagonian Icefields Lakes nbn

0 Weather station m ,- Monthly precipitation (mm) 1951-1980

Andean Cordillera and archipelago

Fig. I Mean monthly precipitation (nirn) for the period 1951 -40 at meteorological stations on the west and east sides of the Andean Cordillera (adapted .frowr Eridlicher and Sarrranda (1 988) with data derived from the meteorological records from the Chileuri upid Argentinian Mercorolo&al Services)

The 4200 km' North Patagonian Icefield north-flowing Humboldt current, and the alti- caps the Andes between latitudes 46 30's and tude of the Andes. The climatic gradient from 47 30's. Its existence, and that of its west to east across the Andes is extremely 13 000 km' neighbour the South Patagonian steep. At sea-level precipitation at the Laguna Icefield (Aniya and Skvarca 1992), is related to San Rafael can total 4000 mm per year, while the interplay of three factors: the high precipi- at 700 m above sea-level on the icefield annual tation delivered by the westerly winds, the cold precipitation may reach as high as 10 000 mm

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(Fujiyoshi et al. 1987). Once across the divide, at altitudes between 1500 and 2500 my precipi- tation declines sharply.

However, climatic data from the North Patagonian Icefield and its glaciers are scarce (Kobayashi and Saito, 1985a,b; Inoue et al. 1987) and the only long-term data available for the region are from remote stations whose rele- vance to local conditions has been questioned (e.g. Warren and Sugden 1993). Nevertheless, good indirect evidence shows that marked change is in progress. Winchester and Harrison (1996) reported 100 m of recently exposed rock around the northern icefield's inner rim where the ice surface has been shrinking for a period, suggested by tree-ring counts, of more than 100 years; and, although the icefield's glaciers show variable behaviour, the general trend has been that of glacier retreat over a similar period (Warren and Sugden 1993; Winchester and Harrison 1996).

The Nef Glacier, now extending some 14 km down the valley from the icefield, has retreated approximately 4 km since the end of the nineteenth century and currently calves profusely into a proglacial lake (see front cover). Wind-blasted trees on the valley bottom and moraines testify to the strength of the local katabatic winds; and normal summer precipitation, although less than on the coast, is likely to be similar to that which we experi- enced on a previous expedition in 1996 to the next valley south of the Nef when there was rain on 17 days out of 42.

Observations

In the three weeks between 3 and 24 February 1998, only 1 mm of rain fell in the Nef valley. Light westerly to north-north-westerly kata- batic winds blew on most days. Haze devel- oped on the evening of the 15th; on the 16th the westerly wind strengthened a little and wave clouds formed over the Andes, light rain fell and it remained overcast into the 17th. The sun returned that afternoon and thenceforth the nights became much colder with ground frosts on three occasions. Autumn had come and the trees began to change colour. On 22 February around 1000 local time (LT) a thick yellow haze gathered over the icefield and

began to drift gradually eastwards. The air was still and moist with a grassland-country smell at odds with the westerly wind direction and our position under the icefield and in the lee of the Andes. On that day two of the authors who had climbed a 1500 m high ridge reported a strong inversion extending as far as the eye could see. The haze had cleared by mid-morn- ing on the 23rd, but returned as we started to walk out of the valley on 24 February; later that day the weather changed; a strong west wind started to blow and by 1500 LT wave clouds began to form; it became overcast and rain fell at night. This pattern was repeated in the next few days with increasingly threatening storm clouds gathering over the Andean divide behind us. On our return to Santiago we read in the national Press that on the night of 25 February in the Torres del Paine National Park (51'05's) in the aptly named "Provincia de Ultima Esperanza" there was 13 hours of tor- rential rainfall with strong winds, tourists benighted, and rivers flooded.

General circulation

To place these observations in a broader con- text, we compared events during January and February 1998 with composite anomalies for El Niiio events since 1960. We used the National Centers for Environmental Prediction (NCEP) reanalysis data (Kalnay et al. 1996) to construct composites for a variety of fields and compared these with anomalies for January and February 1998.

Figure 2 shows the composite anomalies of 1000 mbar geopotentials for all ENS0 events since 1960 for the domain 90"s to 20"s. The classic El Niiio anomaly pattern of above (below) normal sea-level pressure over Indo- nesia (the central Pacific) is readily apparent. Of interest to us is the anomalous anticyclone west of Patagonia that extends for some 70 degrees of longitude along 70°S, ending on the western edge of Patagonia. This feature is the most important signature in the Southern Oceans. Composite anomalies for January and February 1998 (Fig. 3) show an anomalous anticyclone centred on 54'S, 85"W with cen- tral pressure in excess of 50 gpm above the long term (1968-96) mean. The difference

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Fig. 2 January-February composite anomalies o j 1000 mbar geopotential heights (gpm) for El Nitio events 1966, 1973,1983, 1987, 1992,1998

140W 12ow 1 oow BOW 60W 40W 2ow 0

Fig. 3 January-February 1998 anomalies of 1000 mbar geopotential heights ( a m )

between this feature and the El Nino compos- ite discussed above is that the 1998 anticyclon- ic anomaly was closer to South America than the El Niiio composite. In addition, the near- surface pressure gradient south of the anomal- ous anticyclone was steeper over about 150 degrees of longitude. Inspection of near-sur- face pressure fields for the individual El Nino events since 1960 shows that the event in 1998 features the largest positive departure over Patagonia. The previous large El Niiio of 1983

also featured an extensive anticyclone, but its centre was well west of Patagonia (57 S, 120 W), although a ridge of pressure extended across southern South America to 40 W.

The January-February El Niiio composite anomalies for 1000 mbar winds are shown in Fig. 4. Anomalous anticyclonic circulation around the surface feature described above leads to the anomalous southerly flow west of Patagonia becoming a westerly flow to the south, along 70’s. The anomalous near-

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20 s

30s

40s

505

60s

70s

80s

90s 140W 120w 1 oow BOW 60W 40W 20w 0

Fig. 4 January-Februay composite anomalies of 1000 mbar w i d (m s-') for El NiGo events 1966, 1973, 1983, 1987, 1992,1998

20 s

40s

50s

60s

70s

80s

90s 14ow 120w 100w BOW 60W 40W 20w 0

Fig. 5 Februa y 1998 1000 mbar wind anomalies (m 5- l )

surface flow in February 1998 (Fig. 5) shows the same basic structure as in the El Nifio com- posite, except that the feature is shifted to the east. This, together with the mountain topo- graphy of the region, meant that downslope flow, and therefore adiabatic heating, led to stable air and clear skies. Figure 6 shows the upper tropospheric wind anomalies for Janu- ary-February 1998. These demonstrate the persistence of the anomalous anticyclonic cir- culation to the top of the troposphere. Sea sur- face temperature composites for El Niiio

events have also been calculated. These show a meridionally aligned structure of warm and cold pools in the eastern Pacific which are dis- cussed elsewhere (e.g. Karoly 1989; Trenberth etal. 1998).

Conclusions

The composite analysis suggests that the 1998 anomalies are consistent with the longer-term El Niiio composites, but that the 1998 event featured a particularly large surface anticyclone

29 1

20 s

30s

405

50s

60s

70s

80s

90s 140W 120w 1 oow 80W 60W 40W 20w 0

Fig. 6 January-February 1998 200 mbar wind anomalies (m sC')

whose position immediately west of Patagonia had maximum impact on the circulation in this sector. References to hazy conditions described earlier suggest that a non-surface subsidence inversion, associated with the anomalous anti- cyclonic circulation, was trapping aerosols in the lower and middle troposphere. Such inver- sions are highly unusual for this part of South America. What was also unusual in 1998 was that the anticyclone was strong (not necessarily the strongest) but, crucially, placed immedi- ately west of the North Patagonian Icefield. In other years these features have sometimes been a long way west so that their immediate influ- ence on the northern and southern icefields has been weak.

The number of strong to very strong El Niiio events over the last 120 years - 1877-79, 1917, 1925126, 1932, 1940141, 1957158, 19721 73, 1982183 (Grove 1998; Kennedy and Bras- sell 1992) and 1997198 - suggests that these events could have contributed to icefield thin- ning and the rapid retreat of most Patagonian glaciers. The implications of the 1997198 warmidry event for ablation on the icefield and its outlet glaciers are for accelerated downwast- ing and glacier retreat with flooding of river outlets.

The 1995 report by the Intergovernmental Panel on Climate Change directed attention towards assessment of the contribution of tem-

perate icefields and their outlet glaciers to global sea-level rise. The likely sensitivity of the Patagonian icefields to El Nifio events and the scale of ablation in this region, where ice caps cover some 17 000 km2, emphasises the over- whelming need for full mass-balance studies on the icefields and their glaciers.

Acknowledgements

The Nef valley project was funded by the Royal Society, the Carnegie Foundation and the Darwin Initiative through the Raleigh Interna- tional organisation. We thank the Corporacion Nacional Forestal for permission to work in the San Rafael National Park. Our work was made possible by the organisational and logistical skills of Raleigh International and excellent teams of Raleigh Venturers; we are most grate- ful to them all for their help.

References

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Aniya, M. and Skvarca, P. (1992) Characteristics and variations of Upsala and Moreno Glaciers, Southern Patagonia. Bull. Glaciol. Res., 10, pp. 39-53

Endlicher, E. and Santana Aguila, A. (1988) El

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clima sur de la Patagonia y sus aspectos ecologi- cos, un siglo de mediciones climatologicas en Punta Arenas. (The climate of southern Patagonia and its ecological aspects. A century of climatolo- gical measurements in Punta Arenas.) Anales Znsti- tuto de la Patagonia Punta Arenas (Chile), 18, pp.

Fujiyoshi, Y., Kondo, H., Inoue, J. and Yamada, T. (1987) Characteristics of precipitation and verti- cal structure of air temperature in the northern Patagonia. Bull. Glaciol. Res., 4, pp. 15-23

Grove, R. H. (1998) Global impact of the 1789-93 El Niiio. Nature, 393, pp. 3 18-3 19

Inoue, J., Kondo, H., Fujiyoshi, Y., Yamada, T. and Fukami, H. (1987) Summer climate of the North- em Patagonian Icefield. Bull. Glaciol. Res., 4,

Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Leetmaa, A., Rey- nolds, R., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Jenne, R. and Joseph, D. (1996) The NCEPNCAR 40-Year Reanalysis Project. Bull. Am. Meteorol. SOC., 77, pp. 437-471

Karoly, D. J. (1989) Southern Hemisphere circula- tion features associated with El Niiio-Southern Oscillation events. J. Clim., 2, pp. 1239-1252

Kennedy, J. A. and Brassell, S. C. (1992) Molecular records of twentieth-century El Niiio events in laminated sediments from the Santa Barbara basin. Nature, 357, pp. 62-64

Kobayashi, S. and Saito, T. (1985a) Meteorological observations on Soler Glacier. In: Nakajima, C., (Ed.) Glaciological studies in Patagonia Northern Zce- field, 1983-1984, Data Center for Glacier Research, Japanese Society of Snow and Ice,

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Roya I Meteor0 I og ica I Society Awards and prizes, nominations for 2000

The Awards Committee and Council invite nom- inations for the awards and prizes shown below, which will be presented at the 2000 Annual Gen- eral Meeting (AGM). Regulations for the awards are shown in the By-Laws, a copy of which can be obtained on request from the Society. For inform- ation, winners of these prizes for the last few years are shown in Table 1. Nominations should be sent to the Executive Secretary by 30 November for:

(i) The Buchan Prize, awarded annually to mem- bers of the Society for a paper or papers published

- (1985b) Heat balance on Soler Glacier. In: Nakajima, C. (Ed.) Glaciological studies in Patago- nia Northern Icefield, 1983-1 984, Data Center for Glacier Research, Japanese Society of Snow and Ice, pp. 46-5 1

Quinn, W. H., Zopf, D. O., Short, K. S. and Kuo Yang, R. T. W. (1978) Historical trends and statis- tics of the Southern Oscillation, El Niiio and Indonesian droughts. Fishery Bull., 76, pp. 663- 678

Rutllant, J. and Fuenzalida, H. (1991) Synoptic aspects of the central Chile rainfall variability associated with the Southern Oscillation. Znt. J. Climatol., 11, pp. 63-76

Taljaard, J. J. (1967) Development, distribution and movement of cyclones and anticyclones in the Southern Hemisphere during the 1GY. J. Appl. Meteorol., 6, pp. 973-987

Trenberth, K. E., Branstator, G. W., Karoly, D. J., Kumar, A., Lau, L.-C. and Ropelewski, C. (1 998) Progress during TOGA in understanding and mod- eling global teleconnections associated with tropi- cal sea surface temperatures. J. Geophys. Res., 103,

Warren, C. R. and Sugden, D. E. (1993) The Pata- gonian Icefields. Arct. Alp. Res., 25, pp. 316-331

Winchester, V. and Harrison, S. (1996) Recent oscil- lations of the San Quintin and San Rafael Gla- ciers, Patagonian Chile. Geogr. Ann., 78A, pp. 35- 49

Yamal, B. (1 985) Extratropical teleconnections with El Niiio/Southern Oscillation (ENSO) events. Prog. Phys. Geogr., 9, pp. 3 15-352

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Correspondence to: Dr V. Winchester, School of Geography, University of Oxford, Mansfield Road, Oxford OX1 3TB.

in the preceding five years in the Quarterly Journal or the International Journal of Climatology adjudged to contain the most important original contribu- tion or contributions to meteorology.

(ii) The L. E Richardson Prize, awarded annually for a meritorious paper published in the preceding four years in the Quarterly Journal or International Journal of Climatology or Meteorologkal Applications by an author under the age of 30 at the time of submission.

(iii) The Society Outstanding Service Award, awarded biennially for outstanding service to the Society by members or staff.

(iv) The Gordon Manley Weather Prize, awarded annually for any outstanding contribution to Weather through a paper or papers - or other out- standing service to Weather - in the preceding five years that has furthered the public understanding of meteorology and oceanography.

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