Use of the ELA As a Practical Method of Monitoring Glacier Response to Climate in New Zealand's Southern Alps

Journal of Glaciology, Vol. 51, No. 172, 2005 85 Use of the ELA as a practical method of monitoring glacier response to climate in New Zealand’s Southern Alps Trevor J. CHINN,1 Clive HEYDENRYCH,2 M. Jim SALINGER2 120 Muir Road, Lake Hawea, RD2 Wanaka 9192, New Zealand E-mail: [email protected] 2National Institute of Water and Atmospheric Research Ltd, PO Box 109695, Auckland, New Zealand ABSTRACT. In lieu of direct glacier surface mass-balance measurements, equilibrium-line altitudes (ELAs) have been measured over a 28 year period at 50 selected glaciers distributed along the glacierized length of New Zealand’s Southern Alps. Analysis of the data shows that ELAs are a useful measurement of glacier response to annual climate fluctuations, although there is much variability in the degree of response between glaciers in any given year. Comparisons of individual glacier annual ELA with the mean for all annual ELAs of the Southern Alps show a large variation of individual glacier response, with coefficients of variation (r2) ranging from 0.53 to 0.90. The ELA data show detailed, but qualitative, annual mass-balance variations on both regional and individual glacier scales. The ELA record closely predicts glacier termini responses that follow after appropriate response time delays. The recorded variability in climate response for the Southern Alps suggests no single glacier is truly representative for detailed studies of glacier–climate relationships, and that a large number of ELA measurements may be as good an indicator of climate as a few mass-balance measurements. Given the appropriate mass-balance gradient, mass-balance values may be calculated for any of the monitored glaciers. 1. INTRODUCTION of dust on firn and the clean snow of the previous winter, is the zero mass-balance value for that year, i.e. the ELA is the 1.1. The ‘index’ glaciers average elevation at which accumulation exactly balances The aim of this paper is to present results of measurements of ablation over a 1 year period (Paterson, 1994). This annual the glacier equilibrium-line altitude (ELA) to show that ELA variable differs from the ‘steady-state ELA’, which is the monitoring may be a simple and effective alternative to average taken over many years of annual ELA values at a glacier surface mass-balance measurements for measuring glacier in equilibrium. The steady-state ELA indicates the the response of glaciers to climate (e.g. Chinn and Salinger, mean position of the ELA for the glacier to remain in 1999, 2001). This study uses data from the Southern Alps of equilibrium (Meier, 1962). To make the distinction between New Zealand where an inventory compiled by Chinn (2001) the annual ELA and the steady-state ELA, in New Zealand the includes some 3150 glaciers exceeding 0.01 km2 in area, annual ELA is also termed the end-of-summer snowline that collectively have an area of 1158 km2 and a volume of (EOSS) (Clare and others, 2002) while this paper employs the 53.3 km3. notation of the Glacier Mass Balance Bulletins (Haeberli and Arising from comprehensive aerial photography under- Herren, 1991; Haeberli and others, 1993, 1994, 1996, 1999, taken for the compilation of the New Zealand Glacier 2001) in which the steady-state ELA for zero glacier mass Inventory commencing in 1977, 50 ‘index’ glaciers were change is given as ELA0. selected for systematic annual ELA surveys (Chinn, 1995). The index glaciers were chosen to align in six transects across the Southern Alps, to sample both the wet western 2. REVIEW OF BACKGROUND GLACIOLOGY and the arid east alpine climates (Fig. 1). Northern and 2.1. The New Zealand ELA trend surface southern extensions sample the full latitudinal range of The New Zealand mountains are exposed to a prevailing 750 km along the length of the Southern Alps. Individual moist westerly circulation. Interannual climate variability is glaciers were chosen for their simplicity of form, with even responsible for annual differences in the ELA in different gradients about the equilibrium line. However, to gain the parts of the Southern Alps. From year to year, the ELA trend maximum spread of the transects across the Alps, reaching a surface shifts vertically with uniform mass-balance changes, maximum of 60 km, some of the small glaciers selected have or tilts with different circulation patterns (Lamont and others, less than ideal form. This paper presents results from the 1999). Vertical shifts do not influence the representativity of programme to demonstrate that the annual ELA can provide an individual glacier, but tilting of the trend surface does. a simple and very effective means of monitoring glacier Lamont and others (1999) have demonstrated that in the response to climate throughout the Southern Alps. Southern Alps the tilt axis lies approximately along the Main Divide, and correlations between ELA and mass balance 1.2. The equilibrium-line altitude (ELA) should decline away from this axis. However, the tilt in the The lower margin of the transient snowline at the end of annual trend surface is found to be consistently small in the summer indicates the equilibrium line, and its altitude has Southern Alps, suggesting that, despite the very different been defined by Meier and Post (1962) as the ‘equilibrium- climate regimes of the wet western and arid eastern sides of line altitude’ (ELA) for that specific year. This line, normally the Main Divide, the entire Southern Alps behaves as a visible as the contrast between the discoloured concentration single climatic unit (Clare and others, 2002). Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 18:47:29, subject to the Cambridge Core terms of use. 86 Chinn and others: ELA to monitor glacier response to climate Fig. 1. The South Island, New Zealand, showing the distribution of the ELA index glaciers. A trend surface for New Zealand glacier ELAs was routine glacier observations have been continued since that compiled for a large number of glaciers in 1978 (Chinn and time by recording ELAs on the set of index glaciers. Whitehouse, 1980). This was effectively an ELA0 surface The mass-balance data available for 7 years from Ivory because in 1978 most glaciers were close to zero balance. Glacier were obtained as part of the International Hydro- The ELA0 trend surface is strongly warped upwards from the logical Decade (1970s) programme (Anderton and Chinn, high-precipitation western glaciers towards the drier eastern 1973, 1978; Hay and Fitzharris, 1988). This small cirque glaciers. Superimposed on this tilted surface is a north-to- glacier has an area of 0.8 km2 and a small altitude range south latitudinal gradient of 1 m km–1. 1400–1700 m (Fig. 2). It lies to the west of the Main Divide in the high-precipitation zone reaching 10 000 mm. Un- 2.2. Glacier mass balances fortunately the systematic ELA measurements reported here Full mass-balance measurements for a period of 7 years are commenced after the mass-balance measurement pro- available for one glacier only, Ivory Glacier (Anderton and gramme on this glacier was terminated and the two datasets Chinn, 1978), together with a similarly short series of mass- do not overlap in time. The Ivory Glacier balance measure- balance measurements at points along the profile of the ments also coincided with a period of rapid recession large Tasman Glacier (Anderton, 1975). These specific point accompanied by the accelerating growth of a proglacial measurements are not area-averaged for altitude intervals lake. During the measurement period the Ivory Glacier ELA and are referred to as ‘specific’ balance values following the was high, and in one year rose above the glacier upper notation of Østrem and Stanley (1969). These studies were limit. The rapid recession and effective demise of this terminated in the mid-1970s, due mainly to cost, and glacier by 1998 has prevented its inclusion with the index Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 18:47:29, subject to the Cambridge Core terms of use. Chinn and others: ELA to monitor glacier response to climate 87 Fig. 2. Ivory Glacier in April 1969, early in the period of balance measurements. glaciers. Measured mass balances and ELAs are listed in range in the cold desert of the Antarctic Dry Valleys, Table 1. gradients as low as 0.14, 0.36 and 0.55 mm m–1 for Jeremy The Ivory Glacier research has provided data for the Sykes, Alberich and Heimdall Glaciers, respectively, have mass-balance gradient of the humid western alps, essential been measured (Chinn, 1980). Mass-balance gradients for calculating changes in ice volumes, one of the objectives measured at Ivory Glacier are 20 mm m–1 (Fig. 3). The of this programme. The first and most significant differences extreme glaciers of New Zealand, with precipitation reach- in glacier responses are due to mass balance gradient, ing 15 m a–1, lie at the high end of the activity index range. termed the ‘activity index’ by Meier (1961), i.e. the change Tasman Glacier (Fig. 4) is New Zealand’s largest glacier, in annual net mass balance as a function of altitude. The some 28.5 km in length and covering 98.34 km2, making it larger the gradient, the greater the mass turnover and the impractical to attempt a full area-averaged mass-balance greater the climate sensitivity of the glacier. Mass-balance measurement. Specific (point) mass-balance measurements gradients increase with increasing humidity (thus snowfall) were made on Tasman Glacier from 1965/66 to 1974/75, at and for continental areas are normally <5 mm m–1, for seven sites along the longitudinal profile (Fig.

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