Jökull - 01.12.2007, Page 67

Recent variations in sea-ice extent off Iceland
concentration within the ice pack. For comparison,
spread sea ice was only observed slightly closer to the
Iceland coast than indicated by the graph on two oc-
casions. The graph reflects how fast and voluminous
the changes are in this region. Rapid oscillations (nar-
row peaks), such as in early July, are caused by mel-
ting ice tongues and the main ice edge retreating from
Iceland. More prolonged variations (broader peaks),
such as in August, are linked to patches of spread ice
flowing south along the Denmark Strait whereas the
main ice edge remains closer to the coasts of Green-
land. A total of four incidents of sea ice within 50 km
of Icelandic coasts were observed during this period.
Two of those events, in January and November, are
discussed below.
Atmospheric pressure difference from Scoresby-
sund to Bolungarvík indicates the strength and direc-
tion of wind in the Denmark Strait. The pressure is
usually higher in Scoresbysund and the wind direction
thus northeasterly, as are prevailingwinds in Denmark
Strait. When the pressure is higher in Bolungarvík
the wind direction is southwesterly. Negative pressure
difference disturbs ice flow with the East Greenland
current south through the Denmark Strait. A slow mo-
ving blocking high pressure system in the ocean so-
uth of Iceland sometimes causes southwesterly winds
for days or weeks. This tends to accumulate the ice
in the northern part of Denmark Strait, and can even
spread the ice far east north of Iceland, as was the case
in 2005 (Jónsdóttir, 2007). Another example occured
in 1965 when great negative pressure difference bro-
ught the sea ice to Iceland, after a long mild period
(Eiríkur Sigurðsson, personal communication). The
winter 1965 was quite severe, as were the next few
years in Icelandic waters.
January
A typical blocking high pressure weather system set-
tled to the south and southwest of Iceland on January
21st. Two days later, or on January 23rd, conditions
were favourable for a low pressure system to form in
an unusual place close to Scoresbysund. It became ra-
pidly deeper and moved to NE direction. On January
24th there was great negative pressure difference, or
almost 27 hPa (Figure 4). The wind direction over the
ice pack was northwesterly and quite strong, as the
pressure difference indicates. Northwesterly winds
are not very common in this region. As soon as this
unusual wind direction managed to break the compact
ice field up into more spread ice and ice bands in the
marginal ice zone, sea ice was brought very fast to-
wards Vestfirðir. The blocking high pressure system
south of Iceland dissipated after this event, but south-
westerly winds continued for the next few days, or
until February 8th when northeasterly directions took
over for most of the rest of the month.
Under these unusual weather conditions, the
Greenland sea ice moved rapidly towards Iceland and
spread ice came up to the Western Fjords on January
27th (Figure 5). Satellite imagery made it possible to
Figure 4. January pressure difference be-
tween Scoresbysund and Bolungarvík. –
Loftþrýstingsmunur á milli Scoresbysunds
og Bolungarvíkur í janúar 2007. Fimm
daga keðjumeðaltöl.
JÖKULL No. 57 65