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Slopes of glacier ELAs in the Southern Alps of New Zealand in relation to atmospheric circulation patterns
Global and Planetary Change 22 Ž1999. 209–219 www.elsevier.comrlocatergloplacha
Slopes of glacier ELAs in the Southern Alps of New Zealand in relation to atmospheric circulation patterns G.N. Lamont a , T.J. Chinn b, B.B. Fitzharris a b
a,)
Department of Geography, UniÕersity of Otago, PB 56, Dunedin, New Zealand Institute of Geological and Nuclear Sciences, PB 1930, Dunedin, New Zealand Received 29 September 1997; accepted 19 February 1999
Abstract The links between climate and glaciers of the Southern Alps are investigated by examining the relationships between atmospheric circulation and glacier mass balance changes, as measured by end-of-summer snowlines ŽELAs. over a 21-year period on 48 index glaciers. Trend surfaces are fitted to the ELAs data for each mass balance year, and the elevation and slope of each surface is computed. These are compared to atmospheric pressure anomaly maps generated for the southwest Pacific from long-period climate station data. The results show that atmospheric circulation patterns exert a strong control on elevation and slope of the trend surfaces. A simple ‘‘sloping roof’’ model is suggested to represent the ELAs surface, with its tilt across the Southern Alps varying from year to year, depending upon the nature of the atmospheric circulation. Steeper sloping trend surfaces across the Southern Alps are associated with anomalous southwest to westerly flow, whereas less steep slopes are associated with anomalous airflow from the south, southeast, and easterly directions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: glacier mass balances; ELAs; atmospheric circulation; Southern Alps; New Zealand
1. Introduction In the current debate on global climate change, temperate glaciers such as those in New Zealand, are receiving increased attention. They are sensitive indicators of climate change and are an important source
) Corresponding author. Tel.: q64-3-479-8779; fax: q64-3479-9037. E-mail address: [email protected] ŽB.B. Fitzharris.
for any future rise in global sea level ŽMeier, 1984; Oerlemans and Fortuin, 1992.. While much attention has been given to response of individual glaciers to climate variability and change, there are few studies as to the overall response of a mountain system. With the real possibility of global warming, this knowledge is now crucial for prediction of future trends. One such mountain system is the Southern Alps of New Zealand. An inventory compiled by Chinn Ž1991. indicates that it has over 3000 glaciers exceeding 0.01 km2 in area. Anderton Ž1973. estimates
0921-8181r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 9 9 . 0 0 0 3 8 - 7
210
G.N. Lamont et al.r Global and Planetary Change 22 (1999) 209–219
that the ice volume is 53 km3 , with an area of 810 km2 , although more detailed recent work suggests an area of 1159 km2 ŽChinn, 1991.. New Zealand’s location means it is very sensitive to atmospheric circulation changes. It is surrounded by vast areas of ocean. The Southern Alps trend across the prevailing westerlies, while the subtropical high-pressure zone influences the northern part of the country. Small shifts in either of these circulation systems appear to be critical in governing glacier behaviour ŽFitzharris et al., 1992.. Following Oerlemans Ž1992., the exceptionally heavy precipitation, which exceeds 10,000 mm ay1 means that the maritime glaciers of the Southern Alps are at the high end of the sensitivity range. Mass balance measurements are demonstrably the most effective indicators of glacial behaviour and climate, for they provide the immediate, unmodified link between the two. However, measurements of mass balance are difficult in the extensive, underpopulated and very wet Southern Alps, and few such observations have ever been made. Instead, the altitude of the end-of-summer snowline on the glacier is measured and used as a surrogate for annual mass balance values. The annual end-of-summer snowline lies at the altitude on a glacier where the net mass balance in that year is zero. It has been defined as the ‘‘equilibrium line altitude’’ for that specific year by Meier and Post Ž1962., and so is equivalent to the annual equilibrium line altitude or ELA. The steady-state ELA, measured over many years, is the position of the ELA for the glacier to remain in equilibrium ŽMeier, 1962.. The annual ELA value provides an index of mass balance fluctuations and was first investigated by Mercer Ž1961.. It gives a single, direct measurement for the climate at the glacier, integrated over the past year, without the signal being distorted by passage through the dynamics of a glacier system ŽChinn, 1995.. The main climatic factors which determine this elevation are total precipitation and summer temperature. Ohmura et al. Ž1992. argue that for glaciers of the mid latitude and polar regions, the vertical shift of annual ELAs is approximately linear with both temperature and precipitation changes, and that a precipitation change of 300–400 mm water equivalent corresponds to about 18C temperature change. For any given year, a high ELA value indicates a small
residual snowpack and a negative mass balance, while a low ELA value indicates an overall gain to the glacier and a positive mass balance. The first study of New Zealand ELAs was made by Willett Ž1950., and of the related glaciation limit Žthe altitudes of the highest peaks without glaciers and the lowest peaks with glaciers. by Porter Ž1975.. Flights have monitored annual ELAs of the Southern Alps glaciers since 1977 ŽChinn, 1995, 1997; Chinn and Whitehouse, 1980.. The results of the second flight, made in 1978, were used to map the regional trend surface of ELAs throughout the Southern Alps ŽChinn and Whitehouse, 1980.. It was found to descend southwards, from 1900 m at 438S, to 1600 m at 458S ŽFig. 1.. ELAs in the central Southern Alps rise steeply from 1600 m west of the Main Divide to 2200 m on the eastern most glaciers. It was also found that the ELAs for glaciers with north-facing aspects were some 200 m higher than those for south-facing glaciers ŽChinn and Whitehouse, 1980.. Several researchers have uncovered relationships between glacier behaviour and atmospheric circulation patterns over New Zealand. Hessell Ž1983. noted that the general retreat of the Franz Josef glacier since about 1930 can be linked to changes in atmospheric circulation, particularly to a pressure gradient term related to the general strength of the westerlies. The relationship between glacier ablation, terminus position and larger scale atmospheric features has been investigated by Hay and Fitzharris Ž1988. and Fitzharris et al. Ž1992.. The behaviour of New Zealand glaciers was shown to be sensitive to the frequency of certain synoptic weather types, especially the relative strengths of the westerlies and of blocking anticyclones. Subsequently, Fitzharris et al. Ž1997. have related the overall deviations of annual ELAs from their long-term means to atmospheric circulation patterns. They found that fluctuations in annual ELAs of the Southern Alps are due to atmospheric circulation changes that are linked with large-scale centres of action in the Pacific. Key features are the size of the anticyclone over Australia in the accumulation season, the position of the subtropical high-pressure zone to the north of New Zealand in the ablation season and the strength of the westerlies. Fluctuations in glacier mass balance of the Southern Alps are determined by anomalous airflow direction, and
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Fig. 1. Distribution of the Index Glaciers of the Southern Alps, New Zealand, with isolines of mean elevations of end-of-summer-snowlines.
some of these are associated with extremes of the Southern Oscillation Index.
Here we extend these findings by examining the relationship between annual trend surfaces of ELAs
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across the Southern Alps and atmospheric circulation patterns. The objectives are: Ø to compute annual trend surfaces of ELAs across the Southern Alps Ø to produce maps of atmospheric sea level pressure, and their anomalies, for the glacier accumulation and ablation seasons of each mass balance year Ø to examine the relationships between trend surfaces and atmospheric circulation patterns for both the accumulation and ablation seasons of each mass balance year.
2. Methods Survey methods that are used to determine annual ELAs, as measured by simple oblique aerial photography of end-of-summer snowlines are described in
Chinn Ž1995, 1997.. Annual ELAs are observed near the end of each mass balance year ŽMarch. for 48 index glaciers for the period 1977–1997. A mass balance year is 1 April to 31 March. The index glaciers are arranged in a series of transects across the Southern Alps as shown in Fig. 1, which also illustrates the surface of the ELAs for the mass balance year 1977–1978. This year is used as an example by Chinn Ž1995., and shows how ELAs are at much higher altitudes on the leeward side of the Southern Alps compared with the windward. There is a strong gradient in ELAs from northwest to southeast. A very much weaker gradient can be seen from southwest to northeast. Thus the surface of ELAs values is analogous to a tilted plane or ‘‘sloping roof’’, which lies across the Southern Alps, with the edge parallel to the Main Divide. To test this ‘‘sloping roof’’ model, multiple linear regression is applied to the data of annual ELAs for the 48 index glaciers of the Southern Alps. A trend surface is computed for each mass balance year.
Fig. 2. Variation of annual ELAs Žvalues averaged for 48 glaciers. over the Southern Alps for the period 1977–1997.
G.N. Lamont et al.r Global and Planetary Change 22 (1999) 209–219
Regression analysis produces an equation relating the ELAs to their location, as follows: ELA s a q bX q cY , where b is the slope of the trend surface from northwest to southeast, and c is the slope from northeast to southwest. X,Y is a coordinate system used to locate the glaciers throughout the Southern Alps. A line is drawn parallel to the Main Divide of the Southern Alps starting at the point Ž165.48E, 46.078S. and going through the point Ž171.498E, 42.018S.. This line is where X s 0 and each glacier’s X coordinate, or position across the Alps, is given in kilometres from this. A further line perpendicular to the first, and starting at point Ž165.48E, 42.018S. forms the Y axis. The intercept of the two lines is the origin for the X,Y coordinate system. The Y coordinate, or position along the Alps, for each glacier is also given in kilometres.
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The gradient of the trend surfaces of ELAs changes from year to year. It is postulated that the changes in slope of these surfaces are caused by differences in atmospheric circulation patterns from one mass balance year to the next. The slopes of the trend surface for each year are compared to the direction of airflow over the Southern Alps for the glacier accumulation season Ž1 April to 31 October. and ablation season Ž1 November to 31 March.. The idea is to relate slopes of the trend surface of annual ELAs, both across and along the Southern Alps, to specific atmospheric circulation patterns. Past fields of monthly sea level pressures over wide areas can be reconstructed from long, homogeneous series of pressure observations made at meteorological stations. The mathematical details of these pressure reconstructions are described by Jones Ž1987. and Jones and Wigley Ž1988.. In the Southwest Pacific region, a total of 65 grid points are used on an alternating 10-degree grid. The pressure data
Table 1 Slopes Žm kmy1 . of linear trend surfaces for ELAs across and along the Southern Alps and the direction of anomalous airflow for each mass balance year ELAs is the mean for each year of all 48 glaciers. All R 2 values are significant at the 95% confidence level. Mass balance year
1976–1977 1977–1978 1978–1979 1979–1980 1980–1981 1981–1982 1982–1983 1983–1984 1984–1985 1985–1986 1986–1987 1987–1988 1988–1989 1989–1990 1990–1991 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 a b
ELAs Žm. a
1842 1872 1861b 1827 1815 1859 1744 1754 1774 1825 1814 1849 1842 2064 b 1796 b 1758 a 1742 1782 1728 1788 1748
R 2 Ž%.
Slope Across
Along
1.6 6.6 – 6.6 10.4 10.9 9.7 8.2 8.5 7.0 7.3 10.2 8.4 – – 9.1 5.6 5.1 5.5 4.3 4.5
0.3 0.0 – y0.4 y0.5 y0.3 y0.3 y0.2 y0.1 y0.4 y0.1 y0.7 y0.1 – – y0.8 0.3 0.5 0.4 0.5 0.3
Estimates based on less than 16 glacier observations. Estimates based on less than three glacier observations.
50.0 57.2 – 58.9 66.1 76.2 69.5 62.9 67.7 52.7 70.2 62.9 61.3 – – 60.8 46.5 46.3 46.3 35.9 48.6
Anomalous airflow Acc. Season
Abl. Season
SE S – NW SW W SW S N NE S SW W – – SW S W S E E
E NW – W SW NE SW S NE E SW SW S – – SW E SE S NE E
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are grouped into the glacier accumulation season and ablation season. Pressure maps for accumulation and ablation seasons of each mass balance year are then compared with the long-term mean pressure maps Ž1911–1994. for these seasons. This procedure gave pressure anomaly maps, which are acknowledged as sensitive indicators of changes in atmospheric circulation patterns ŽBarry and Perry, 1973.. In all, 42 anomaly maps were prepared: two for each mass balance year, representing the accumulation and ablation seasons. It is not practical to show all of these, but they can be summarised by extracting the direction of anomalous airflow over the Southern Alps.
3. Results A time series of annual ELAs for mass balance years 1976–1977 to 1996–1997, as averaged for the 48 index glaciers, is shown in Fig. 2. There is a high frequency of years with ELAs below the long-term mean, indicating relatively positive mass balances, during this period. Results of the multiple linear regression analyses used to determine the gradients of the annual trend surfaces of annual ELAs are given in Table 1. Slopes of trend surfaces across and along the Southern Alps are given for each mass balance year. Also shown is R 2 Žthe coefficient of determination., which mea-
Fig. 3. Sea level pressure anomaly maps for a steep trend surface of ELAs across the Southern Alps for Ža. accumulation season and Žb. ablation season.
G.N. Lamont et al.r Global and Planetary Change 22 (1999) 209–219
sures the goodness of fit of each trend surface to the ELAs data. Trend surfaces for 1976–1977 and 1991–1992 are based on only 14 or 15 glacier observations, so are not considered to be as reliable. Trend surfaces for the mass balance years 1978– 1979, 1989–1990 and 1990–1991 could not be produced, because bad weather prevented sufficient observations of all but a few glacier ELAs. The trend surfaces explain some 36–76% of the variation. In most years, this ‘‘sloping roof’’ model explains more than half of the variation in ELAs. In the five most recent years, the explanation is less than this, mainly because of variation caused by local spatial variation, such as aspect differences.
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The slopes along the Southern Alps of the trend surfaces are very small, and not statistically different from zero, and are not discussed further. By contrast, the slopes across the Southern Alps are marked and significantly different from zero. The trend surfaces of ELAs always slope upwards from northwest to southeast. The slopes were steepest in 1980–1981, 1981–1982, 1982–1983, 1987–1988, and 1991– 1992. These are mass balance years with enhanced southwest to westerly flow during both the accumulation and ablation seasons, except for 1981–1982 which has enhanced northeasterly flow during the ablation season. For mass balance years with the least steep slopes, airflow over the Southern Alps is
Fig. 4. Sea level pressure anomaly maps for the least steep trend surface of ELAs across the Southern Alps for Ža. accumulation season and Žb. ablation season.
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Fig. 5. Frequency roses of anomalous airflow for accumulation seasons of Ža. steepest and Žb. least-steep slopes of annual ELAs across the Southern Alps.
more from the south to easterly quarter Žmass balance years 1976–1977, 1992–1993, 1993–1994, 1994–1995, 1995–1996, 1996–1997.. The only exception is 1994–1995, which has more westerly flow during the accumulation season. Fig. 3 shows sea level pressure anomaly maps for a mass balance year Ž1987–1988. with a steep trend surface of ELAs across the Southern Alps. Positive anomalies indicate higher than normal pressure and negative anomalies lower than normal pressure. During the accumulation season, pressures are higher
than normal over eastern Australia and relatively low to the east and south of New Zealand. Consequently, there is a much enhanced southwesterly airflow over the Southern Alps. This situation is repeated in the ablation season, except that the pressure anomalies are not as marked. Sea level pressure anomaly maps for a mass balance year Ž1976–1977. with the least steep trend surface across the Southern Alps are given in Fig. 4. During the accumulation season, pressures are higher than normal south of Western Australia and rela-
Fig. 6. Frequency roses of anomalous airflow for ablation seasons of Ža. steepest and Žb. least-steep slopes of annual ELAs across the Southern Alps.
G.N. Lamont et al.r Global and Planetary Change 22 (1999) 209–219
tively low to the east of the North Island of New Zealand. Consequently, there is a much enhanced southeasterly airflow over the Southern Alps. In the ablation season, these easterly anomalies continue, as pressures are relatively low over New Zealand and the Tasman Sea. Fig. 5 shows frequency roses of enhanced airflow for trend surfaces of years with the six steepest and six least steep slopes across the Southern Alps for the accumulation season. Fig. 6 does likewise for the ablation season. For the accumulation season, there is a higher frequency of anomalous southwest flow for the steepest trend surfaces, whereas there is more south, southeast, and easterly flow for the least steep slopes. For the ablation season, there is again a higher frequency of anomalous southwest airflow for the steepest trend surfaces, whereas a regime dominated by more easterly quarterly flow is apparent for the least steep trend surfaces.
4. Discussion The ELAs of the Southern Alps range in altitude from 1500 m in the south and west, to 2200 m in the
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east. The marked slope across the Alps is generally a result of the high precipitation gradient, with greater accumulation on the western side of the Main Divide. This is a direct result of the prevailing westerly flow over the South Island. This gradient of ELAs across the Southern Alps varies from year to year. The gradient along the Alps is small. This suggests a simple ‘‘sloping roof’’ model for the ELAs of the Southern Alps, with the cross tilt varying from year to year, depending upon the nature of the atmospheric circulation. The tilt is steeper, when winds from the southwest and west, in both the accumulation and ablation seasons, are stronger than normal. Years with less tilt have anomalous airflow from the south, southeast and easterly directions. This latter situation advects colder air and brings an increase in snowfalls to the eastern Southern Alps, hence decreasing the magnitude of the gradient of ELAs from northwest to southeast. The ‘‘sloping roof’’ model is illustrated by Fig. 7. This shows how the average trend surface across the Southern Alps not only tilts from year to year, but also fluctuates in altitude. For comparison, trend surface slopes for a year with high ELAs and a year with low ELAs, and for the steepest and least steep
Fig. 7. Schematic summary of the ‘‘sloping roof’’ model: topographic profile across the central Southern Alps showing trend surface slopes for Ža. a year with high ELAs, Žb. a year with the steepest slope, Žc. long-term mean ELAs, Žd. a year with the least steep slope, and Že. a year with a low ELAs.
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slopes are also shown. It can be seen that the year with high ELAs has a relatively steeply tilted trend surface, whereas, the year with low ELAs has less tilt. The correlation between slope and ELAs is 0.4, Ž1976–1977 and 1991–1992 are removed from this analysis because of fewer observations; see Table 1.. These outcomes reinforce past findings on the behaviour of New Zealand glaciers and atmospheric circulation. The importance of the relative strength of the westerlies to the slope of trend surfaces of ELAs across the Southern Alps indicates that they play a crucial role in glacier mass balance of this mountain system. This is in line with Hessell Ž1983., Hay and Fitzharris Ž1988., Fitzharris et al. Ž1992., Fitzharris et al. Ž1997., and Hooker Ž1995.. Ž1988., both Armstrong Ž1989. and Letreguilly ´ argue that the glacier response of a mountain system is a result of variations in large-scale weather patterns. The results support this general idea, in that the overall response of ice in the Southern Alps, as measured by regional changes in ELAs, is controlled by atmospheric circulation.
5. Conclusions There is a marked slope of annual trend surfaces of glacier ELAs across the Southern Alps, but little gradient along the Alps. A ‘‘sloping roof’’ model for ELAs is suggested for the mountains of New Zealand, with its tilt across the Southern Alps varying from year to year, depending upon the nature of the atmospheric circulation. When airflows from the southwest to west are stronger than normal, the tilt is steeper. Years with less tilt have greater than normal airflows from the south through to the east. Such a model provides a potentially useful approach for assessing the overall glacier mass balance and response of the ice volume of a mountain system, such as the Southern Alps, to climate change.
Acknowledgements This work was carried out under contract No. CO5522 of the New Zealand Foundation for Research, Science and Technology. We are indebted to Dr. P.D. Jones of the Climatic Research Unit, Uni-
versity of East Anglia, for supplying the reconstructed gridded pressure data.
References Anderton, P.W., 1973. The significance of perennial snow and ice to the water resources of the South Island of New Zealand. N. Z. J. Hydrol. 12 Ž1., 6–18. Armstrong, R.L., 1989. Mass balance history of Blue Glacier, Washington, USA. In: Oerlemans, J. ŽEd.., Glacier Fluctuations and Climatic Change, Proceedings of the Symposium on Glacier Fluctuations and Climatic Change, held in Amsterdam, 1–5 June 1987. Kluwer Academic Publishers, Dordrecht, pp. 183–192. Barry, R.G., Perry, A.H., 1973. Synoptic Climatology. Methuen, London. Chinn, T.J., 1991. Glacier Inventory of New Zealand. Institute of Geological and Nuclear Sciences, unpublished. Chinn, T.J., 1995. Glacier fluctuations in the Southern Alps of New Zealand determined from snowline elevations. Arct. Alp. Res. 27, 187–198. Chinn, T.J., 1997. Glacier snowline survey 1997. Institute of Geological and Nuclear Sciences, New Zealand, Science Report, 97r18, 22 pp. Chinn, T.J., Whitehouse, I.E., 1980. Glacier snow line variations in the Southern Alps, New Zealand. In: World Glacier Inventory, Proceedings of the Riederalp Workshop, September 1978 126 International Association of Hydrological Sciences Publication, pp. 219–228. Fitzharris, B.B., Hay, J.E., Jones, P.D., 1992. Behaviour of New Zealand glaciers and atmospheric circulation changes over the past 130 years. The Holocene 2 Ž2., 97–106. Fitzharris, B.B., Chinn, T.J., Lamont, G.N., 1997. Glacier balance fluctuations and atmospheric circulation patterns over the Southern Alps, New Zealand. Int. J. Climatol. 17, 745–763. Hay, J.E., Fitzharris, B.B., 1988. The synoptic climatology of ablation on a New Zealand glacier. J. Climatol. 8, 201–215. Hessell, J.W.D., 1983. Climatic effects on the recession of the Franz Josef Glacier. N. Z. J. Sci. 26, 315–320. Hooker, B.L., 1995. Advance and Retreat of Franz Josef Glacier in Relation to Climate. Unpublished BSc ŽHons.., Dissertation at Department of Geography University of Otago, Dunedin, 62 pp. Jones, P.D., 1987. The early twentieth century arctic high — fact or fiction? Climate Dynamics 1, 63–75. Jones, P.D., Wigley, T.M.L., 1988. Antarctic gridded sea level pressure data: an analysis and reconstruction back to 1957. J. Climate 1, 1199–1220. Letreguilly, A., 1988. Relation between the mass balance of ´ western Canadian mountain glaciers and meteorological data. J. Glaciol. 34 Ž116., 11–18. Meier, M.F., 1962. Proposed definitions for glacier mass budget terms. J. Glaciol. 4 Ž33., 252–263. Meier, M.F., 1984. Contribution of small glaciers to global sea level. Science 226, 1418–1421.
G.N. Lamont et al.r Global and Planetary Change 22 (1999) 209–219 Meier, M.F., Post, A.S., 1962. Recent variations in mass net budgets of glaciers in western North America. In: Proceedings of Obergurgl Symposium 58 International Association of Hydrological Sciences Publication, pp. 63–77. Mercer, J.H., 1961. The estimation of the regimen and former firn limit of a glacier. J. Glaciol. 3 Ž30., 1053–1062. Oerlemans, J., 1992. Climate sensitivity of glaciers in southern Norway: application of an energy-balance model to Nigardsbreen, Hellstugubreen and Alfotbreen. J. Glaciol. 38 Ž129., 223–232.
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Oerlemans, J., Fortuin, J.P.F., 1992. Sensitivity of Glaciers and Small Ice Caps to Greenhouse Warming. Science 258, 115– 117. Ohmura, A., Kasser, P., Funk, M., 1992. Climate at the equilibrium line of glaciers. Journal of Glaciology 38 Ž130., 397–411. Porter, S.C., 1975. Glaciation limit in New Zealand’s Southern Alps. Arct. Alp. Res. 7 Ž1., 33–37. Willett, R.W., 1950. New Zealand Pleistocene snow line, climate conditions and suggested biological effects. N. Z. J. Sci. Technol. B 32 Ž1., 18–48.
Slopes of glacier ELAs in the Southern Alps of New Zealand in relation to atmospheric circulation patterns G.N. Lamont a , T.J. Chinn b, B.B. Fitzharris a b
a,)
Department of Geography, UniÕersity of Otago, PB 56, Dunedin, New Zealand Institute of Geological and Nuclear Sciences, PB 1930, Dunedin, New Zealand Received 29 September 1997; accepted 19 February 1999
Abstract The links between climate and glaciers of the Southern Alps are investigated by examining the relationships between atmospheric circulation and glacier mass balance changes, as measured by end-of-summer snowlines ŽELAs. over a 21-year period on 48 index glaciers. Trend surfaces are fitted to the ELAs data for each mass balance year, and the elevation and slope of each surface is computed. These are compared to atmospheric pressure anomaly maps generated for the southwest Pacific from long-period climate station data. The results show that atmospheric circulation patterns exert a strong control on elevation and slope of the trend surfaces. A simple ‘‘sloping roof’’ model is suggested to represent the ELAs surface, with its tilt across the Southern Alps varying from year to year, depending upon the nature of the atmospheric circulation. Steeper sloping trend surfaces across the Southern Alps are associated with anomalous southwest to westerly flow, whereas less steep slopes are associated with anomalous airflow from the south, southeast, and easterly directions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: glacier mass balances; ELAs; atmospheric circulation; Southern Alps; New Zealand
1. Introduction In the current debate on global climate change, temperate glaciers such as those in New Zealand, are receiving increased attention. They are sensitive indicators of climate change and are an important source
) Corresponding author. Tel.: q64-3-479-8779; fax: q64-3479-9037. E-mail address: [email protected] ŽB.B. Fitzharris.
for any future rise in global sea level ŽMeier, 1984; Oerlemans and Fortuin, 1992.. While much attention has been given to response of individual glaciers to climate variability and change, there are few studies as to the overall response of a mountain system. With the real possibility of global warming, this knowledge is now crucial for prediction of future trends. One such mountain system is the Southern Alps of New Zealand. An inventory compiled by Chinn Ž1991. indicates that it has over 3000 glaciers exceeding 0.01 km2 in area. Anderton Ž1973. estimates
0921-8181r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 9 9 . 0 0 0 3 8 - 7
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that the ice volume is 53 km3 , with an area of 810 km2 , although more detailed recent work suggests an area of 1159 km2 ŽChinn, 1991.. New Zealand’s location means it is very sensitive to atmospheric circulation changes. It is surrounded by vast areas of ocean. The Southern Alps trend across the prevailing westerlies, while the subtropical high-pressure zone influences the northern part of the country. Small shifts in either of these circulation systems appear to be critical in governing glacier behaviour ŽFitzharris et al., 1992.. Following Oerlemans Ž1992., the exceptionally heavy precipitation, which exceeds 10,000 mm ay1 means that the maritime glaciers of the Southern Alps are at the high end of the sensitivity range. Mass balance measurements are demonstrably the most effective indicators of glacial behaviour and climate, for they provide the immediate, unmodified link between the two. However, measurements of mass balance are difficult in the extensive, underpopulated and very wet Southern Alps, and few such observations have ever been made. Instead, the altitude of the end-of-summer snowline on the glacier is measured and used as a surrogate for annual mass balance values. The annual end-of-summer snowline lies at the altitude on a glacier where the net mass balance in that year is zero. It has been defined as the ‘‘equilibrium line altitude’’ for that specific year by Meier and Post Ž1962., and so is equivalent to the annual equilibrium line altitude or ELA. The steady-state ELA, measured over many years, is the position of the ELA for the glacier to remain in equilibrium ŽMeier, 1962.. The annual ELA value provides an index of mass balance fluctuations and was first investigated by Mercer Ž1961.. It gives a single, direct measurement for the climate at the glacier, integrated over the past year, without the signal being distorted by passage through the dynamics of a glacier system ŽChinn, 1995.. The main climatic factors which determine this elevation are total precipitation and summer temperature. Ohmura et al. Ž1992. argue that for glaciers of the mid latitude and polar regions, the vertical shift of annual ELAs is approximately linear with both temperature and precipitation changes, and that a precipitation change of 300–400 mm water equivalent corresponds to about 18C temperature change. For any given year, a high ELA value indicates a small
residual snowpack and a negative mass balance, while a low ELA value indicates an overall gain to the glacier and a positive mass balance. The first study of New Zealand ELAs was made by Willett Ž1950., and of the related glaciation limit Žthe altitudes of the highest peaks without glaciers and the lowest peaks with glaciers. by Porter Ž1975.. Flights have monitored annual ELAs of the Southern Alps glaciers since 1977 ŽChinn, 1995, 1997; Chinn and Whitehouse, 1980.. The results of the second flight, made in 1978, were used to map the regional trend surface of ELAs throughout the Southern Alps ŽChinn and Whitehouse, 1980.. It was found to descend southwards, from 1900 m at 438S, to 1600 m at 458S ŽFig. 1.. ELAs in the central Southern Alps rise steeply from 1600 m west of the Main Divide to 2200 m on the eastern most glaciers. It was also found that the ELAs for glaciers with north-facing aspects were some 200 m higher than those for south-facing glaciers ŽChinn and Whitehouse, 1980.. Several researchers have uncovered relationships between glacier behaviour and atmospheric circulation patterns over New Zealand. Hessell Ž1983. noted that the general retreat of the Franz Josef glacier since about 1930 can be linked to changes in atmospheric circulation, particularly to a pressure gradient term related to the general strength of the westerlies. The relationship between glacier ablation, terminus position and larger scale atmospheric features has been investigated by Hay and Fitzharris Ž1988. and Fitzharris et al. Ž1992.. The behaviour of New Zealand glaciers was shown to be sensitive to the frequency of certain synoptic weather types, especially the relative strengths of the westerlies and of blocking anticyclones. Subsequently, Fitzharris et al. Ž1997. have related the overall deviations of annual ELAs from their long-term means to atmospheric circulation patterns. They found that fluctuations in annual ELAs of the Southern Alps are due to atmospheric circulation changes that are linked with large-scale centres of action in the Pacific. Key features are the size of the anticyclone over Australia in the accumulation season, the position of the subtropical high-pressure zone to the north of New Zealand in the ablation season and the strength of the westerlies. Fluctuations in glacier mass balance of the Southern Alps are determined by anomalous airflow direction, and
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Fig. 1. Distribution of the Index Glaciers of the Southern Alps, New Zealand, with isolines of mean elevations of end-of-summer-snowlines.
some of these are associated with extremes of the Southern Oscillation Index.
Here we extend these findings by examining the relationship between annual trend surfaces of ELAs
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across the Southern Alps and atmospheric circulation patterns. The objectives are: Ø to compute annual trend surfaces of ELAs across the Southern Alps Ø to produce maps of atmospheric sea level pressure, and their anomalies, for the glacier accumulation and ablation seasons of each mass balance year Ø to examine the relationships between trend surfaces and atmospheric circulation patterns for both the accumulation and ablation seasons of each mass balance year.
2. Methods Survey methods that are used to determine annual ELAs, as measured by simple oblique aerial photography of end-of-summer snowlines are described in
Chinn Ž1995, 1997.. Annual ELAs are observed near the end of each mass balance year ŽMarch. for 48 index glaciers for the period 1977–1997. A mass balance year is 1 April to 31 March. The index glaciers are arranged in a series of transects across the Southern Alps as shown in Fig. 1, which also illustrates the surface of the ELAs for the mass balance year 1977–1978. This year is used as an example by Chinn Ž1995., and shows how ELAs are at much higher altitudes on the leeward side of the Southern Alps compared with the windward. There is a strong gradient in ELAs from northwest to southeast. A very much weaker gradient can be seen from southwest to northeast. Thus the surface of ELAs values is analogous to a tilted plane or ‘‘sloping roof’’, which lies across the Southern Alps, with the edge parallel to the Main Divide. To test this ‘‘sloping roof’’ model, multiple linear regression is applied to the data of annual ELAs for the 48 index glaciers of the Southern Alps. A trend surface is computed for each mass balance year.
Fig. 2. Variation of annual ELAs Žvalues averaged for 48 glaciers. over the Southern Alps for the period 1977–1997.
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Regression analysis produces an equation relating the ELAs to their location, as follows: ELA s a q bX q cY , where b is the slope of the trend surface from northwest to southeast, and c is the slope from northeast to southwest. X,Y is a coordinate system used to locate the glaciers throughout the Southern Alps. A line is drawn parallel to the Main Divide of the Southern Alps starting at the point Ž165.48E, 46.078S. and going through the point Ž171.498E, 42.018S.. This line is where X s 0 and each glacier’s X coordinate, or position across the Alps, is given in kilometres from this. A further line perpendicular to the first, and starting at point Ž165.48E, 42.018S. forms the Y axis. The intercept of the two lines is the origin for the X,Y coordinate system. The Y coordinate, or position along the Alps, for each glacier is also given in kilometres.
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The gradient of the trend surfaces of ELAs changes from year to year. It is postulated that the changes in slope of these surfaces are caused by differences in atmospheric circulation patterns from one mass balance year to the next. The slopes of the trend surface for each year are compared to the direction of airflow over the Southern Alps for the glacier accumulation season Ž1 April to 31 October. and ablation season Ž1 November to 31 March.. The idea is to relate slopes of the trend surface of annual ELAs, both across and along the Southern Alps, to specific atmospheric circulation patterns. Past fields of monthly sea level pressures over wide areas can be reconstructed from long, homogeneous series of pressure observations made at meteorological stations. The mathematical details of these pressure reconstructions are described by Jones Ž1987. and Jones and Wigley Ž1988.. In the Southwest Pacific region, a total of 65 grid points are used on an alternating 10-degree grid. The pressure data
Table 1 Slopes Žm kmy1 . of linear trend surfaces for ELAs across and along the Southern Alps and the direction of anomalous airflow for each mass balance year ELAs is the mean for each year of all 48 glaciers. All R 2 values are significant at the 95% confidence level. Mass balance year
1976–1977 1977–1978 1978–1979 1979–1980 1980–1981 1981–1982 1982–1983 1983–1984 1984–1985 1985–1986 1986–1987 1987–1988 1988–1989 1989–1990 1990–1991 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 a b
ELAs Žm. a
1842 1872 1861b 1827 1815 1859 1744 1754 1774 1825 1814 1849 1842 2064 b 1796 b 1758 a 1742 1782 1728 1788 1748
R 2 Ž%.
Slope Across
Along
1.6 6.6 – 6.6 10.4 10.9 9.7 8.2 8.5 7.0 7.3 10.2 8.4 – – 9.1 5.6 5.1 5.5 4.3 4.5
0.3 0.0 – y0.4 y0.5 y0.3 y0.3 y0.2 y0.1 y0.4 y0.1 y0.7 y0.1 – – y0.8 0.3 0.5 0.4 0.5 0.3
Estimates based on less than 16 glacier observations. Estimates based on less than three glacier observations.
50.0 57.2 – 58.9 66.1 76.2 69.5 62.9 67.7 52.7 70.2 62.9 61.3 – – 60.8 46.5 46.3 46.3 35.9 48.6
Anomalous airflow Acc. Season
Abl. Season
SE S – NW SW W SW S N NE S SW W – – SW S W S E E
E NW – W SW NE SW S NE E SW SW S – – SW E SE S NE E
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are grouped into the glacier accumulation season and ablation season. Pressure maps for accumulation and ablation seasons of each mass balance year are then compared with the long-term mean pressure maps Ž1911–1994. for these seasons. This procedure gave pressure anomaly maps, which are acknowledged as sensitive indicators of changes in atmospheric circulation patterns ŽBarry and Perry, 1973.. In all, 42 anomaly maps were prepared: two for each mass balance year, representing the accumulation and ablation seasons. It is not practical to show all of these, but they can be summarised by extracting the direction of anomalous airflow over the Southern Alps.
3. Results A time series of annual ELAs for mass balance years 1976–1977 to 1996–1997, as averaged for the 48 index glaciers, is shown in Fig. 2. There is a high frequency of years with ELAs below the long-term mean, indicating relatively positive mass balances, during this period. Results of the multiple linear regression analyses used to determine the gradients of the annual trend surfaces of annual ELAs are given in Table 1. Slopes of trend surfaces across and along the Southern Alps are given for each mass balance year. Also shown is R 2 Žthe coefficient of determination., which mea-
Fig. 3. Sea level pressure anomaly maps for a steep trend surface of ELAs across the Southern Alps for Ža. accumulation season and Žb. ablation season.
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sures the goodness of fit of each trend surface to the ELAs data. Trend surfaces for 1976–1977 and 1991–1992 are based on only 14 or 15 glacier observations, so are not considered to be as reliable. Trend surfaces for the mass balance years 1978– 1979, 1989–1990 and 1990–1991 could not be produced, because bad weather prevented sufficient observations of all but a few glacier ELAs. The trend surfaces explain some 36–76% of the variation. In most years, this ‘‘sloping roof’’ model explains more than half of the variation in ELAs. In the five most recent years, the explanation is less than this, mainly because of variation caused by local spatial variation, such as aspect differences.
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The slopes along the Southern Alps of the trend surfaces are very small, and not statistically different from zero, and are not discussed further. By contrast, the slopes across the Southern Alps are marked and significantly different from zero. The trend surfaces of ELAs always slope upwards from northwest to southeast. The slopes were steepest in 1980–1981, 1981–1982, 1982–1983, 1987–1988, and 1991– 1992. These are mass balance years with enhanced southwest to westerly flow during both the accumulation and ablation seasons, except for 1981–1982 which has enhanced northeasterly flow during the ablation season. For mass balance years with the least steep slopes, airflow over the Southern Alps is
Fig. 4. Sea level pressure anomaly maps for the least steep trend surface of ELAs across the Southern Alps for Ža. accumulation season and Žb. ablation season.
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Fig. 5. Frequency roses of anomalous airflow for accumulation seasons of Ža. steepest and Žb. least-steep slopes of annual ELAs across the Southern Alps.
more from the south to easterly quarter Žmass balance years 1976–1977, 1992–1993, 1993–1994, 1994–1995, 1995–1996, 1996–1997.. The only exception is 1994–1995, which has more westerly flow during the accumulation season. Fig. 3 shows sea level pressure anomaly maps for a mass balance year Ž1987–1988. with a steep trend surface of ELAs across the Southern Alps. Positive anomalies indicate higher than normal pressure and negative anomalies lower than normal pressure. During the accumulation season, pressures are higher
than normal over eastern Australia and relatively low to the east and south of New Zealand. Consequently, there is a much enhanced southwesterly airflow over the Southern Alps. This situation is repeated in the ablation season, except that the pressure anomalies are not as marked. Sea level pressure anomaly maps for a mass balance year Ž1976–1977. with the least steep trend surface across the Southern Alps are given in Fig. 4. During the accumulation season, pressures are higher than normal south of Western Australia and rela-
Fig. 6. Frequency roses of anomalous airflow for ablation seasons of Ža. steepest and Žb. least-steep slopes of annual ELAs across the Southern Alps.
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tively low to the east of the North Island of New Zealand. Consequently, there is a much enhanced southeasterly airflow over the Southern Alps. In the ablation season, these easterly anomalies continue, as pressures are relatively low over New Zealand and the Tasman Sea. Fig. 5 shows frequency roses of enhanced airflow for trend surfaces of years with the six steepest and six least steep slopes across the Southern Alps for the accumulation season. Fig. 6 does likewise for the ablation season. For the accumulation season, there is a higher frequency of anomalous southwest flow for the steepest trend surfaces, whereas there is more south, southeast, and easterly flow for the least steep slopes. For the ablation season, there is again a higher frequency of anomalous southwest airflow for the steepest trend surfaces, whereas a regime dominated by more easterly quarterly flow is apparent for the least steep trend surfaces.
4. Discussion The ELAs of the Southern Alps range in altitude from 1500 m in the south and west, to 2200 m in the
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east. The marked slope across the Alps is generally a result of the high precipitation gradient, with greater accumulation on the western side of the Main Divide. This is a direct result of the prevailing westerly flow over the South Island. This gradient of ELAs across the Southern Alps varies from year to year. The gradient along the Alps is small. This suggests a simple ‘‘sloping roof’’ model for the ELAs of the Southern Alps, with the cross tilt varying from year to year, depending upon the nature of the atmospheric circulation. The tilt is steeper, when winds from the southwest and west, in both the accumulation and ablation seasons, are stronger than normal. Years with less tilt have anomalous airflow from the south, southeast and easterly directions. This latter situation advects colder air and brings an increase in snowfalls to the eastern Southern Alps, hence decreasing the magnitude of the gradient of ELAs from northwest to southeast. The ‘‘sloping roof’’ model is illustrated by Fig. 7. This shows how the average trend surface across the Southern Alps not only tilts from year to year, but also fluctuates in altitude. For comparison, trend surface slopes for a year with high ELAs and a year with low ELAs, and for the steepest and least steep
Fig. 7. Schematic summary of the ‘‘sloping roof’’ model: topographic profile across the central Southern Alps showing trend surface slopes for Ža. a year with high ELAs, Žb. a year with the steepest slope, Žc. long-term mean ELAs, Žd. a year with the least steep slope, and Že. a year with a low ELAs.
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slopes are also shown. It can be seen that the year with high ELAs has a relatively steeply tilted trend surface, whereas, the year with low ELAs has less tilt. The correlation between slope and ELAs is 0.4, Ž1976–1977 and 1991–1992 are removed from this analysis because of fewer observations; see Table 1.. These outcomes reinforce past findings on the behaviour of New Zealand glaciers and atmospheric circulation. The importance of the relative strength of the westerlies to the slope of trend surfaces of ELAs across the Southern Alps indicates that they play a crucial role in glacier mass balance of this mountain system. This is in line with Hessell Ž1983., Hay and Fitzharris Ž1988., Fitzharris et al. Ž1992., Fitzharris et al. Ž1997., and Hooker Ž1995.. Ž1988., both Armstrong Ž1989. and Letreguilly ´ argue that the glacier response of a mountain system is a result of variations in large-scale weather patterns. The results support this general idea, in that the overall response of ice in the Southern Alps, as measured by regional changes in ELAs, is controlled by atmospheric circulation.
5. Conclusions There is a marked slope of annual trend surfaces of glacier ELAs across the Southern Alps, but little gradient along the Alps. A ‘‘sloping roof’’ model for ELAs is suggested for the mountains of New Zealand, with its tilt across the Southern Alps varying from year to year, depending upon the nature of the atmospheric circulation. When airflows from the southwest to west are stronger than normal, the tilt is steeper. Years with less tilt have greater than normal airflows from the south through to the east. Such a model provides a potentially useful approach for assessing the overall glacier mass balance and response of the ice volume of a mountain system, such as the Southern Alps, to climate change.
Acknowledgements This work was carried out under contract No. CO5522 of the New Zealand Foundation for Research, Science and Technology. We are indebted to Dr. P.D. Jones of the Climatic Research Unit, Uni-
versity of East Anglia, for supplying the reconstructed gridded pressure data.
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