Jökull - 01.12.2007, Síða 64

Jónsdóttir and Sveinbjörnsson
ite difficult to get access to radar images at low cost.
This has changed recently, as ENVISAT images are
now available in somewhat reduced spatial resolution
(300 m) on the web in near-real time (Polar View,
2007). Passive microwave images have less spatial
resolution but other ice characteristics can be detected
on those images through analysis of different electro-
magnetic bands. Passive microwave images are also
important for sea-ice climatology as their data cove-
rage is still much greater than of active systems. One
of the downsides of using this type of images for Ice-
landic waters is that spread ice, i.e. ice concentration
less than 30%, is not always detectable. Thermal ima-
ges are easily obtained and give additional informa-
tion on sea-surface temperature, a factor that affects
the endurance of the ice, but are dependent on cloud
cover. Optical images are extremely good for ice de-
tection when illumination and cloud cover conditions
are favourable.
The sea-ice landscape is extremely variable. The
sea ice is of different ages which in turn affects its
thickness and salinity. Snow cover and melt pools
shape its surface. Wind stress and ocean currents in-
fluence the sea-ice coverage and can create pressure
ridges or leads. New sea-ice formation in the sea
changes wave amplitude in the ocean as the sea ice
develops rapidly from grease ice to pancake ice (Wa-
dhams, 2001). Seasonal changes do also affect the ice
cover. All these factors affect the electromagnetic pro-
perties of sea ice, radiation backscatter and emission,
and make remote sensing of sea ice challenging. It is
possible to compare different types of satellite images
in Geographical Information Systems (GIS), both vi-
sually and digitally. Since each type of image offers
information on different characteristics of the sea-ice
cover, it is very useful to be able to work with many
images simultaneously, without any concerns regar-
ding different scales or map projections.
This paper is focused on the variation in sea-ice
extent off Iceland in 2007 and considers briefly re-
cent trends and development in the Icelandic sea-ice
history. Monitoring the effect of atmospheric pres-
sure systems on sea-ice extent in the Greenland Sea is
of interest in an era where the amount of sea ice seems
to be diminishing rapidly.
GENERAL SEA-ICE CONDITIONS
A preliminary sea-ice index for Iceland, representing
variations in ice extent during the 20th century, has
been derived for four areas within 20 nm (37 km) of
the coast and four seasons of the year (Figure 1). An
annual sea-ice index of one indicates that sea ice has
been observed in one area during one season of a year
whereas an index number of three indicates that sea
ice has been observed in three areas during one se-
ason or one area during three seasons. Thus a single
occurrance of sea ice within 20 nm counts as one in
the index but the graph does not take into account sea-
ice concentration. A more detailed sea-ice index, co-
vering a larger area with higher temporal and spatial
resolution, is being constructed.
A severe sea-ice period extending from the 19th
century until 1920, was followed by a considerably
milder climate until the 1960’s when severe ice con-
ditions reigned for a few years. Following variable
sea-ice conditions during the 1980’s and 1990’s, mini-
mal sea-ice was observed in the beginning of the 21st
century with 2003–2004 being totally ice-free. The
sea ice returned in 2005 and again in 2007. Note, that
although a record low in ice extent and thickness was
observed in the fall of 2007 (NSIDC, 2007; Chapman,
2007), this year is in no way unique with respect to the
sea-ice index.
Following a winter and spring of 2007 where the
Northern Hemisphere sea ice remained slightly be-
low the average of previous years, a record low in ice
extent and thickness was observed in September 2007
(Figure 2. NSIDC, 2007; Chapman, 2007). This was
partly due to atmospheric pressure systems advancing
multi-year ice through the Fram Strait, between Spitz-
bergen and Greenland (The International Ice Char-
ting Working Group, 2007). This event caused great
concern amongst the cryospheric research community
and the national ice services, as these environmental
conditions had not been predicted to occur until ice
conditions had deteriorated more significantly, i.e. la-
ter this century (IICWG, 2007).
The sea-ice anomaly from the 1978–2007 shows
a striking record low at the end of 2007 (Figure 2).
The graph presents the sea-ice history of the satellite
era, from the launch of the first passive microwave sa-
62 JÖKULL No. 57