B

1969

985 995 1,005 1,015

Pressure mmb (J-A)

"e

985 990 995 1,000 1,005

1,010 1,015

D)

A

\ /'

\ 1

V rf A ^

-2000

1990 1994 1998

Year AD

1986

2000

1000

-2000

1990 1994 1998

Year AD

2002

-3000

Figure 21.5 Modern Stykkisholmur data. (A) NAO winter index (Hurrell, 1995) versus the January-April average pressure at Stykkisholmur (Fig. 21.3). (B) Winter pressure versus winter precipitation (mm) at Stykkisholmur. (C) 'Net mass balance index' (winter precipitation (mm)/summer degree-days versus Stykkisholmur winter pressure). (D) The 'net mass balance index' (C) versus net mass balance data from Icelandic glaciers (Dyurgerov, 2002) for the period ad 1987-2002.

1990

How far can weather station data predict regional glacier variations? Monthly climatic data (temperature, precipitation, pressure) from Stykkisholmur in northwest Iceland (from www.vedur.is) (Fig. 21.3) from ad 1960 to 2002 were compiled for the winter accumulation season (taken as January to April) and the monthly degree-days for the summer ablation season (June, July, August). Barlow (2001) discussed the agreement between this temperature time-series and central Greenland ice-core isotope data and showed a 64% decadal-level agreement in the sign of the signal for the period ad 1830-1980. The explained variance between the average winter pressure values at Stykk-isholmur and the NAO winter index (Fig. 21.3) (Hurrell, 1995; Rodwell et al., 1999) is r2 = 0.61 indicating that the pressure variations at Stykkisholmur are closely coupled, as expected, to NAO variations (Fig. 21.5A). The winter pressure variations correlate with changes in the winter precipitation (Fig. 21.5B), with intervals of low pressure tending to be winters with higher accumulation. The data show large variations in precipitation (261 ±

105 mm) whereas summer degree-days have little variability (21 ± 2 degree-days). A simple index for the mass balance of glaciers in the region can be computed as: total winter precipitation/ summer monthly degree-days (units mm/degree-day). This index is strongly driven by the variations in winter accumulation and mirrors the winter NAO (Fig. 21.5C). Sigurdsson & Jonsson (1995) used a similar but more complex approach to compute from climatic data a 'snow-budget' index for a glacier in southern Iceland. They show that a change in both the index and termini variations occurred around ad 1970, the time of the Great Salinity Anomaly off north Iceland (Fig. 21.4A) (Malmberg, 1985; Olafsson, 1999). It coincided with an extreme negative NAO index (Fig. 21.5C) and indicates a coupling between the oceans and atmosphere, although this coupling is not simple (Rogers et al., 1998; Olafsson, 1999).

There are few long-term glacier mass balance observations from Iceland (Dyurgerov, 2002), although there is a long history of measuring the movement of termini (Sigurdsson, 1991; Sigurdsson & Jonsson, 1995). When the Stykkisholmur 'mass balance index' (Fig. 21.5B) is examined against the net mass balance data from nine glaciers starting in ad 1987 (Dyurgerov, 2002) there is a clear parallelism in the records, with a rapid increase in negative mass balance starting in 1992 to 1998 with a small upturn since then (Fig. 21.5D). A Stykkisholmur index of ca. 14 appears to delimit the move from positive to negative mass balance on the glaciers surveyed. This agreement (Fig. 21.5D) suggests that changes in winter accumulation are the dominant element in the sign of the mass balance, because the variations in summer (J, J, A) temperatures are slight.

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