Jökull - 01.12.1969, Side 73

Winds and Ice Drift North of Iceland,
Especially in the Year 1965
JÓNAS JAKOBSSON,
THE ICELANDIC METEOROLOGICAL OFFICE, REYKJAVÍK, ICELAND
ABSTRACT
The aim of the work presented here was to
investigate whether the onrush of drift ice to
the north coast of lceland in February 1965
was caused chiefly by winds, and io study the
correlation between the movernent of the ice
edge east of Greenland and the toinds in these
tracts.
Seconclly it ruas the purpose to folloiu the
rnovements of the ice in Icelandic waters after
it hacl arrived and to find how its drift zoas
related to winds.
INTRODUCTION
In the I9th century is was believed in north-
ern Iceland that westerly winds increased the
danger of ice. British meteorologists came to
the same conclusion after their experience in
Iceland during World War II, especially in
the spring of 1944. (British Hydrographic
Dept. 1946). Their research includecl seven
years of the period 1931 to 1945 and showed
that when southwesterly winds between Ice-
land and Greenland had lasted one to two
weeks, one coukl expect drift ice near Straum-
nes, ancl after a duration of three weeks it
would have passed Horn. The months August
to November were excluded, because in these
months drift ice near the Icelandic coasts is
an exception.
In the years 1951 to 1956 the author kept
a record of wincl components between North-
west Iceland and the Greenland coast. In these
years no considerable ice occurrecl in Icelandic
waters, so these observations were discontinu-
ed. They showed, however, that a few days of
southwesterly winds always preceded ice oc-
currence at the coast of Vestfirðir.
It may seem more natural that northwesterly
winds would bring ice to the coast. But due
to the effect of Greenland northwesterly winds
are an unstable phenomenon at the northwest
coast of Iceland. Southwesterly winds can,
however, last there for days, even weeks.
Therefore, it becomes their role to bring in
the ice to Vestfirðir, at least when the ice belt
along Greenland is relatively narrow.
After a few days of unusually strong south-
westerlies off the northwest coast, ice came in
the proximity of Straumnes and Horn on the
9th ancl lOth of January, 1952. It was apparent
from trawler reports that the ice edge had
moved 80 miles* towards the coast. By assum-
ing the surface wind to be 70% of the geo-
strophic one, it appears that the ice has drift-
ecl witli the speed of 4—5% of the surface
wind. This is about twice the speed H. U.
Sverdrup (1942) reports from the Arctic. Tliere
he found that the thick April ice drifts with
1.4% ol' the wind velocity against 2.4% for
the thinner August ice. Here off the northwest
coast of Iceland, we have altogether different
conditions. The floes at the ice edge are thin
and will, therefore, drift with a higher per-
centage of the wind velocity than thicker ice.
Secondly, in a southwesterly wincl over the
Greenland Sea in winter the air is in an un-
stable state, so the velocity of the air above
is easily carried down to the surface. Such
turhulence effect causing transport of mo-
mentum is always present over the partly open
waters at an ice edge when the wind blows
out from the solid or nearly solid, cold, main
drift ice. Tliis turbulence is probably the
greatest factor in forming a belt of open drift
and scattered floes along the edge of major
drifts.
*) Nautical miles are used throughout.
JÖKULL 19. ÁR 69