Jökull


Jökull - 01.12.2007, Side 63

Jökull - 01.12.2007, Side 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- JÖKULL No. 57 61
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