Jökull - 01.10.1998, Blaðsíða 55

standing due to anticyclonic circulation and had the
highest incoming solar radiation. This is why the
summer balance of 1995 does not correlate to summer
temperature in a similar way as for the other years
(Fig. 18). A similar approach of correlation between
the winter balance and the winter precipitation on
neighbouring stations shows that 1992 falls outside
the trend defined by the other years (Fig. 18); the
weather stations did not describe the heavy precipita-
tion on western Vatnajökull.
Although, further data are required to define aver-
age geographical conditions and expected climatic
variability, the data show geographical distribution
that is considered representative. A maximum in the
precipition extends from Síðujökull to Grímsvötn and
eastwards along the central ice cap. This reflects the
increase in precipitation with elevation when humid
southerly air masses pass the ice cap. In the central
areas of the ice cap, the winter balance has been about
2.0-2.5 m of water equivalent with small variation (0.3
m) from year to year. The mean winter balance was
2000
1986-1995 mean
.••
1 1

■i k
■ 1985-1995 mean
Brú
Hólar í Hornafirði
~r
~r
1993
Year
“1"
..I.
Fig. 15. Winter precipitation (September-May) and summer
temperature (May-September) at two meteorological sta-
tions compared with mean values over the period 1985-
1995.Veðráttan (1985-1995).
- Vetrarúrkoma (sept.-maí) og sumarhiti (maí-sept.) á tveimur
veðurstöðvum samanborið við meðalgildi 1985-1995.
l. 5 m on the westem and 0.5 m on the northem outlets
(Fig. 19). The average increase in the winter balance
with elevation was highest on Köldukvíslarjökull and
Brúarjökull, 150-200 mm water equivalent per 100 el-
evation, but about 250 mm/100 m on Tungnaárjökull
and Dyngjujökull. Some variation was observed in the
spatial distribution of the winter balance from year to
year, depending on the prevailing wind direction dur-
ing the winter but averaged over the various drainage
basins the winter balance was fairly constant over this
period of four years.
The summer balance varied in the central parts of
the ice cap from -0.5 m in sunny and relatively warm
summers (1995 and 1994) to +0.5 m in summers
when cold spells with northerly winds brought snow
to the northern and central part of the ice cap and
changed the surface albedo and retarded melting over
the whole summer (1992 and 1993). At the glacier
termini of 700-800 m elevation, the summer balance
was typically about -5 m with variations from -4 to -6
m. The net mass balance varied from 2 to 3 m in the
central ice cap, depending on the weather during the
summers. The average gradient of the summer bal-
ance was about 100 mm water equivalent per 100 m
elevation in the ablation area but three to four times
lower in the accumulation area.
The equilibrium line for the glaciers varied by
200-300 m over the period of four years and the accu-
mulation area ratio varied from 50% to 80% for the
glacier outlets (Figs. 20 and 21). For zero net balance
the equilibrium line is at about 1100 m for the south-
Fig. 16. Mass balance for various outlets of Vatnajökull,
1991-1995; bw: winter balance, bs: summer balance, b„: an-
nual net balance.
- Afkoma einstakra skriðjökla, 1991-1995. b„: vetraraf-
koma, bs: sumarafkoma, bn: ársafkoma.
JÖKULL, No. 45, 1998
53