4
S. Schwegmann & J. Holfort: Baltic sea ice volume 1982-2019
Meteorol. Z. (Contrib. Atm. Sci.)
PrePub Article. 2020
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Trend ASIC
(%mVy)
M-'OO
-0 50
I 1-0 23
HZ]-0.10
I l-O.OI
□□ 0.01 -010
010 -0 25
0 26 - 0 60
Hoso-060
Trend ASIV
(m/y)
H<-2
■■ -2 00- -100
■I -100- -0 60
-0 50--0 2S
□D-02S--010
I I -0 .10 • -0 01
-0 01 -001
CZD 0.01 -010
CD o 10 - 0 25
■■026 - 0 60
Hoso-060
Figure 3: Linear trend of accumulated sea ice concentration (left) and accumulated sea ice volume over the entire winter for the period
1982 to 2019. Blue areas show negative, red areas positive trends. The trend in accumulated sea ice concentration varies between -2.98 and
+3.26 % rrT 2 per year and the trend for accumulated sea ice volume ranges from -2.25 to +1.14 m per year. However, the majority of grid
cells has a trend between -2 and +0.6 % rrT 2 per year (m per year) for ASIC (ASIV).
data are available only for this period. To keep the com
parison consistent, also the impact of air temperature
changes on sea ice is analysed for this time span. The
most important influencer is the air temperature (7^,),
which also links to sea surface temperature (SST). Over
simplifying, with SST well above freezing, no sea ice is
present (or is melting). Around the freezing point sea
ice formation can occur and with sea ice present SST
is at the freezing point. So there is no direct linear re
lationship between SST and sea ice, but we expect that
monthly means arc negatively correlated. Therefore, we
correlated the detrended SST and winter ASIC (ASIV),
and, as expected, the correlation is usually negative. In
regions, where a large amount of ice forms every win
ter, coefficients of down to -0.9 arc reached. However,
in regions where ice occurs only in few years (in par
ticular the central Sea of Bothnia) and along the east
ern coast of the Gulf of Bothnia, correlation is mostly
weak. Trends for monthly SST arc shown in Fig. 5 for
the months January to April. In January, SST increases
by between 0.1 °C to 0.5 °C per decade in the Gulf of
Bothnia. However, in the northernmost Bay of Both
nia, along the coast, a light decrease of up to 0.1 °C
per decade occurs. Further south, trends he between
-0.1 °C per decade close to the coasts in the east and
up to +0.25 °C per decade towards the Swedish coast.
with the majority of grid cell having trends between
-8 m and 0 m per decade over the winter season. As for
ASIC, also in the ASIV product some grid cells show
an increase. These positive trends arc proven to be not
realistic (see discussion).
In order to see, when the largest changes in ASIV oc
cur within a season, trends were calculated for monthly
ASIV. The results for the months January to April arc
shown in Fig. 4. In December and May, any trends arc
small ah over the Baltic Sea. From February to April,
ASIV decreases strongest in the Bay of Bothnia while in
the Sea of Bothnia, the highest negative trends occur in
March and April. In the Gulf of Finland and Riga, ASIV
changes show hardly any seasonal variability. Monthly
trends vary between -3.5 m and +2.2 m per month.
4 Corresponding changes in sea
surface temperature and
meteorological conditions
from 1982 to 2011
Basically, changes in sea ice concentration and sea ice
thickness arc caused by atmospheric and oceanographic
conditions. Here we analyse the impact of these condi
tions on sea ice for the period 1982 to 2011, as the SST