150
JJ. Haapala et al.
8.3 Ice Duration
Jevrejeva et al. (2004) has compiled the most extensive
analysis of ice season changes in the Baltic Sea to date,
using time series at 37 coastal stations from the Bothnian
Bay to the Arkona Basin. In general, observations showed a
tendency towards milder ice conditions. For the variables
studied, the greatest change occurred in ice season length,
which has decreased by 14-44 days over the past 100 years,
mostly due to earlier ice break-up.
A recent study by Ronkainen (2013) confirmed the trend
towards a shorter ice-covered period in the Finnish coastal
zone (Fig. 8.6 and Table 8.2). In the Bothnian Bay (Kemi),
where the ice season is clearly longer than in the eastern Gulf
of Finland (Loviisa), the trend is -18 d 100 year -1 . In the
eastern Gulf of Finland, where sea ice is also formed every
winter, larger changes have been observed. The length of the
ice season has decreased by 41 d 100 year \ while the trend
based on the last 50 observations is -62 d 100 year \
In the southern Baltic Sea, the first sea ice is formed in the
coastal lagoons between 10 and 20 December, near the coast
around 1 January, in the sea area of the western Baltic and
off the Polish coast in mid-January, and in the German
coastal areas at the beginning of January (Schmelzer et al.
2008, 2012; Giijatowicz 2011). Comparing the different
periods analysed, there has been no major change in the
average date at which ice formation begins (Fig. 8.7). In
terms of ice break-up, on average, the last ice in the inner
coastal waters has disappeared by late February or early
March, while residual ice has been observed in offshore
waters until mid-March. The longest duration of ice in the
coastal waters was observed in the 1961-1990 winters. In
the past 30-year period, ice disappeared from the eastern
coastal waters up to ten days earlier. In the Baltic Sea offshore
waters, no change has been observed in this parameter,
and in the western Baltic Sea, there has even been a weak
trend towards a later break-up date.
Ice duration decreases from east to west, and from the
inner waters towards the sea areas. A weak trend towards
shorter ice duration was found for the 1981-2010 period.
The maximum ice duration occurs in particularly cold and
long winter periods. There has been at least one cold winter
with strong ice formation in each of the 30-year periods
analysed, for example 1962/63, 1978/79 and 1995/96. A
significant decrease (30 days) has only been observed in the
sea area west and east of Bornholm Island in the period
1971-2000. For the period 1981-2010, ice duration during
very cold winters is nearly identical to that in 1971-2000.
8.4 Ice Thickness
For climate studies, the sea ice thickness or, preferably, the
large-scale sea ice thickness distribution should be the main
indicator of change in sea ice, since it is essentially the same
as the mass of the ice pack. However, interpreting the causes
of change in sea ice thickness is not straightforward, since in
addition to the atmosphere-ocean energy balance, sea ice
thickness depends on snow thickness and ice dynamics.
Monitoring activity in the Baltic Sea is limited to the landfast
ice regions, where the sea ice could be thinner than in the
drift ice regions. According to Jevrejeva et al. (2004), the ice
thickness time series around the Baltic Sea coast does not
show any consistent trend: both decreasing and increasing
trends were reported. A recent study by Ronkainen (2013)
supported these conclusions. In the Bothnian Bay (station
Fig. 8.6 Observed changes in
a length of ice season in Kemi
and b in Loviisa, and c ice
thickness in Kemi and d in
Loviisa (Ronkainen 2013)
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