Jökull - 01.12.2007, Síða 63

Reviewed research article
Recent variations in sea-ice extent off Iceland
Ingibjörg Jónsdóttir1 and Einar Sveinbjörnsson2
1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík, Iceland; ij@hi.is
2Veðurvaktin ehf. Eikarási 8, 210 Garðabæ
Abstract – Unusual variations in sea-ice extent were observed offshore W-Iceland during 2007. In January,
a blocking high pressure weather system south of Iceland caused prevailing southwesterly wind in Denmark
Strait for days. Consequently sea-ice reached the coasts of the NW peninsula, Vestfirðir, blocking Dýrafjörður
for a couple of days. Although sea-ice occurrences are frequent in the northern part of the Western Fjords, it
is very unusual to observe ice that far south in the region. During Spring and Summer, spread ice remained
relatively close to the coast, intermittently threatening to enter shipping routes. In September, sea-ice extent in
the Northern Hemisphere reached record low, partly due to great advance of multiyear ice through Fram Strait
but also as a result of increased solar radiation related to less cloud cover than normal. The conditions in
Greenland Sea were close to average and ice formation was rapid in the early Autumn. At the end of November,
sea-ice was again observed unusually close to Iceland, after a spell of southwesterly winds. Although the year
2007 is not considered a severe sea-ice year in Icelandic waters, the ice caused concern for ship navigation
and fish farming in NW-Iceland.
INTRODUCTION
Sea-ice conditions around Iceland are fairly well do-
cumented and environmental conditions that lead to
ice occurrences are reasonablywell understood (Thor-
oddsen, 1916–1917; Koch, 1945; Stefánsson, 1994,
1999; Wadhams, 1986, 2001; Ogilvie, 1981; Ogilvie
and Jónsdóttir, 2000). A rather complex interplay
of three environmental factors governs the ice con-
ditions at any given time; The volume of ice in the
East Greenland Current, oceanographic conditions in
the Greenland Sea and Iceland Sea, and atmospheric
pressure fields in the region. Although ice drift mo-
dels have been improving rapidly in recent years it is
still difficult to predict precisely where and when sea
ice will enter shipping routes or reach land. The mar-
ginal ice zone is a constantly changing environment,
new ice is formed and ice bands cut off from the main
ice edge can drift faster than the rest of the ice pack
into open water (Wadhams, 2001). This has special
relevance in Iceland as the first ice that reaches its wa-
ters typically consists of such bands. The longevity of
the sea ice in Icelandic waters is governed by oceano-
graphic conditions, again influenced by varying influx
of relatively warm and saline Atlantic waters to the
waters west and north of Iceland (Malmberg et al.,
1996). Until recently such ice bands could be hard to
detect using satellite imagery. Their behaviour is also
somewhat hard to predict.
Although information from ships and ice recon-
naissance flights is of importance for sea-ice monito-
ring and ground truth verifying, satellite images have
become the most important source for daily ice char-
ting. Various types of images are used for ice obse-
rvation on a daily basis, and each of them has their
pros and cons. Several books and papers discuss re-
mote sensing of sea ice, with a chapter by Shuchman
and Onstott (1990) being a classic. Radar imagery is
the only type that shows the ice in sufficient detail for
ship routing, whilst being independent of cloud co-
ver and light. They are therefore valuable for monito-
ring the polar environment. Until recently it was qu-
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