Meteorol. Z. (Contrib. Atm. Sci.) 
PrePub Article. 2020 
S. Schwegmann & J. Holfort: Baltic sea ice volume 1982-2019 
3 
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winter ASIC variability 
winter ASIV variability 
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(j 
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40- 
20- 
Kaskinen 
Central 8ay of Bothnia 
Central Gulf of Finland 
1985 1990 1995 2000 2005 2010 2015 
Figure 2: Temporal variability of total winter ASIC and ASIC, exemplarily for the central Bay of Bothnia (2: 
(representing near coastal ice, 21° E/62.5° N) and the central Gulf of Finland (26°E/60°N). 
be quite high, depending on the winter strength, and 
differs regionally, as Fig. 2 illustrates. Shown are the 
SIC and SIV accumulated over the entire winter for each 
winter season, exemplarily for a grid cell in the central 
Bay of Bothnia, central Gulf of Finland and at Kaskinen, 
which represents a grid cell close to the coast in the 
Sea of Bothnia. As is expected, ASIC and ASIV are the 
highest in the central Bay of Bothnia, as in this northern 
most region, ice growth stalls early and ice disappears 
only in the end of the winter. The other two positions 
show varying patterns: in some years, Kaskinen, which 
is further north than the Gulf of Finland, has less ice, in 
other years, more ice can be found compared to the Gulf 
of Finland. This reflects the impact of the underlying 
atmospheric patterns on ice growth, as in some years, 
cold air is advected from the eastern continent (earlier 
ice growth in the Gulf of Finland) and in other years 
from the Polar Regions (earlier ice growth in the Gulf of 
Bothnia). 
2.2 Sea surface and air temperatures 
Sea surface temperature (SST) data was obtained from 
the Copernicus Marine Environment Monitoring Ser 
vice Baltic Sea- Sea Surface Temperature Reprocessed 
data set (H0yer and Karagali, 2016). These data arc 
available for the period January 1982 to December 2011. 
SST calculations arc based on infrared satellite obser 
vations from NOAA AVHRR and ERS/Envisat. Spatial 
resolution is with 0.03° x 0.03° higher than in other SST 
products, temporal resolution is 24 hours. In order to ac 
count for sea ice in winter months, which has a much 
lower temperature than the water, a sea ice concentration 
mask based on the high resolution ice information from 
the Swedish Meteorological and Hydrological Institute 
(SMHI) has been used to set grid cells with more than 
30 percent of sea ice to a constant SST of -1 °C. For 
more detailed information on the SST data set, please 
see H0yer and Karagali (2016). As we have analyzed 
monthly accumulated SIC and SIV and their changes in 
this study, we calculated monthly fields from the daily 
SST before trends were calculated. 
& 
N), Kaskinen 
For an analysis of the atmospheric drivers of sea ice 
changes we have examined monthly mean 2-m air tem 
peratures from the National Center for Environmental 
Predictions/National Center for Atmospheric Research 
(NCEP/NCAR, hereinafter referred to as NCEP data) 
Reanalysis 1 Project (Kalnay etal., 1996). The respec 
tive mean fields in the NCEP data arc composites of 
observations - when and where available - and model 
simulations (where no observations arc available).These 
data have a coarser resolution (2.5° x 2.5°) than the sea 
ice and SST data. However, the temporal resolution cov 
ers the entire study period in contrast to other products 
with higher spatial resolution, like ECMWF ERA-40 
(only to 2002) or COSMO-REA (only from 1995). 
3 Observed changes in sea ice coverage 
and accumulated sea ice volume 
The trends in winter seasons ASIC and ASIV for the 
Baltic Sea over the whole period arc shown in Fig. 3, the 
30-year period from 1982 to 2011 shows similar trends. 
In most regions, the trend is negative. Trends in ASIC 
vary between -33 % m 2 dec 1 to +41 % m 2 dec 1 , 
with the majority of grid cells showing a decrease be 
tween 0 % m 2 dec -1 and 18 %m 2 dec -1 . The strongest 
decrease is observed in the Gulfs of Bothnia, Finland 
and Riga. In the central Baltic Sea, trends arc lower, cer 
tainly due to the fact that the absolute values arc smaller 
to staid with. Along the coasts, there arc regionally some 
grid cells which show an increase in ASIC, which is 
caused by a very low data coverage in the beginning of 
the observation period. Some positive trends arc most 
probably due to the fact, that with newer satellite data 
new ice is better detected; and with 100 % concentra 
tion have a strong impact on the trend. Due to the small 
thickness of new ice, the effect is not very pronounced 
on the sea ice volume. 
ASIV shows in most regions a decrease and the 
highest trends in the same regions as ASIC. Mostly, 
the trends vary between -22.5 m and 11.5 m per decade