Jökull - 01.01.2001, Side 16

Helgi Björnsson et al.




(11)
where

is the inflow to the lake (m s  ), 
 "! J kg # C  is the specific heat capacity of
water and


is the cooling of inflowing water.
The annual average sea–surface temperature along
the southern coast of Iceland is close to 7 # C (Stef-
ánsson, 1999, p. 404–405). The volume of seawater
inflow to the lake is not accurately known nor are its
tidal and seasonal fluctuations. Measurements of the
lake level in February, when melting is at minimum,
show that the water level changes by 20 cm due to the
tides (Zóphóníasson and Freysteinsdóttir, 1999). The
average inflow of seawater to the 15 km $ lake is thus
near 70 m s  in a 12 hour tidal cycle. An average
tidal inflow of 200 m s  was measured during two
hours of tidal high in the summer of 1998 (25 July;
Árnason, 1998). This is equal to 30 m s  over a
tidal period during which 9 # C warm seawater flowed
into the lake and water at 0 # C flowed out of the lake.
Thus, all the heat was used for melting calving ice
within the lake. If the above–measured inflow is used
(30–70 m s  and sea temperature of 7 # C) the aver-
age heat flux received by the lake from warm seawater
would be 900–2000 MW. Hence, the total flow of en-
ergy, according to these calculations is approximately
2100–3200 MW. The volume of icebergs flowing out
to sea is negligible. This rough estimation suggests
that the lake absorbs enough energy (2100–3200 MW)
to melt all the calving ice (requiring 2500 MW).
Our estimates suggest that approximately half to
two–thirds of the heat presently used to melt ice in
Jökulsárlón is derived from warm seawater flowing
into the lake. Assuming that all of the energy compo-
nents are directly proportional to lake area their totals
to melt since the formation of the lake are described
in Figure 14b. The contribution of heat from inflow-
ing seawater, however, may have increased at a more
rapid rate as the lake volume expanded. While the
lake was still small the inflow of melt water from the
glacier might have kept the lake level high enough to
prevent the enchancement of seawater. In contrast at
present, even vigorous melting of the glacier surface
has little influence on the lake level.
CONCLUSIONS
Jökulsárlón, first detected at the outlet of the river
Jökulsá á Breiðamerkursandi in 1934, is now 15 km $
in area. At present it is expanding at a rate of
0.5 km $ yr  and the calving is 0.3 km yr  . Flow of
ice toward the lake and calving increased significantly
over the last two decades and are expected to continue
to increase at similar rates. The energy used for melt-
ing floating ice in the lake is presently 2500 MW, half
or even two–thirds of which is transported to the lake
by tidal inflow. If this seawater flow into the lake were
to be stopped by filling of the present Jökulsá river
channel, the icebergs melt rate would decline. Further
study of the energy balance of the lake and ice flow
modeling are recommended to estimate the impact of
ice flow, and the mass and energy balance on the size
of Jökulsárlón.
ACKNOWLEDGEMENTS
This work was supported by a research grant from
the Public Roads Administration. We thank Helgi
Jóhannesson for discussion of the work and comments
on an earlier draft of the manuscript. Eyjólfur Guð-
mundsson and Gunnar Páll Eydal collected maps and
aerial photos of Jökulsárlón. We thank Niels Reeh for
useful discussions of the evaluation of ice flow veloc-
ity at a calving front and Gwenn Flowers for construc-
tive review of the manuscript.
ÁGRIP
Jökulsárlón á Breiðamerkursandi: myndun þess,
vöxtur á 20. öld og framtíðarhorfur
Frá því Jökulsárlón á Breiðamerkursandi hóf að myn-
dast á þriðja áratug 20. aldar hefur framburður aurs
undan Breiðamerkurjökli sest í lónið og aðeins lítill
hluti þess borist með Jökulsá til strandar. Fram-
burður undan jöklinum hefur því ekki náði að bæta
upp landbrot við ströndina svo að hún hörfaði mest-
an hluta 20. aldar. Samanburður á kortum frá 1904,
1945 og 1989 sýnir að strandlínan hefur hörfað á 8 km
löngum kafla við mynni Jökulsár. Mest hefur rofist
af ströndinni um 700 m á 85 árum, eða að meðaltali
8,5 m á ári (Skúli Víkingsson, 1991; Helgi Jóhann-
esson, 1994, 1995). Enn er ekkert lát á landbrot-
inu svo stefnir í að innan fárra ára rofni vegurinn yfir
16 JÖKULL No. 50