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Detailed assessment of climate variability of the Baltic Sea for the period
1950/70-2008
Andreas Lehmann, Klaus Getzlaff, Jan Harlass and Karl Bumke
Leibniz Institute of Marine Sciences at University of Kiel , alehmann@ifm-geomar.de
1. Introduction
The warming trend for the entire globe (1861-2002) is
0.05°C/decade. A specific warming period started around
1980 and continued at least until 2006. The temperature
increase of that period is about 1°C (0.4 °C/decade). This
trend is equally well evident for many areas on the globe,
especially on the northern hemisphere in observations
and climate simulations (IPCC 2007: WG1 AR4).
Consequently, this warming appeared also for the Baltic
Sea catchment. From 1960 to 1980 the air temperature for
the catchment was close or slightly below the long-term
mean with respect to the period 1871-2004, only between
1965-1975 the temperature was slightly above the mean.
Then with the beginning of the 1980s the annual mean
temperature increased by about 1°C until 2004.
Figure 1 Winter air temperature anomaly (DJFM)
at Kiel Lighthouse derived from SMHImeteorological
data base for the period 1970-2008
and winter NAO-index.
A similar warming trend could be observed for the SST
of the Baltic Sea (Siegel et al. 2006; MacKenzie and
Schiedek 2007). Since 1985, summer SSTs have
increased at nearly triple the global warming trend, and
for the period 1985-2002 summer SSTs have risen by
1.4°C, 2-5 times faster than in any other season. Even the
annual mean water temperatures averaged spatially and
vertically for the deep basins of the Baltic Sea show
similar trends (Hinrichsen et al. 2007). Figure 1 shows
the winter air temperature anomaly at Kiel Lighthouse for
the period 1970-2008 based on the SMHI-meteorological
data base. The number of anomalous warm winters has
strongly increased for the period 1988-2008. The winter
2006/07 was about 3.5°C warmer than the seasonal mean
over the period 1970-2008. The winters 2006/07 and
2007/08 were not only mild in the south-western Baltic
area. The maximum sea ice extent in these years was
extraordinarily small: for 2006/07 it was 139,000 km²,
and for 2007/08 a new low ice record was document, the
ice cover being only about 49,000 km² which is the
smallest since 1720 ( Vainio and Isemer, 2008).
2. Methods and concepts
As climate, to a large extent, controls patterns of water
circulation and biophysical aspects relevant for biological
production, such as the vertical distribution of
temperature and salinity, alterations in climate may
severely impact the trophic structure and functioning of
marine food webs. Since the mid-1980s an acceleration
of climate warming has occurred which agrees
remarkably well with a regime shift in the pelagic food
webs (Hinrichsen, et al. 2007). To answer which are the
processes linking changes in the marine environment and
climate variability it is essential to investigate all
components of the climate system. This will be
performed by a detailed analysis of about 2 decadesperiods
before and after the regime shift.
Most of the studies of climate change in the Baltic Sea
area have been restricted to the analysis of temperature
records. The detailed analysis of changes in variability of
atmospheric heat, radiation and momentum fluxes and
their impact on the Baltic Sea has not been studied in
detail. Her we will provide a detailed assessment of the
variability of atmospheric variables and the
corresponding response of the Baltic Sea including
temperature, salinity and circulation for different time
slices seasonally resolved within the period 1970-2008.
NCEP/NCAR re-analysis data are available for the
northern hemisphere for the period 1948-2008. However,
NCEP/NCAR re-analysis data are only poorly resolved
(2.5x2.5°, 6 hours) for the Baltic Sea area. Thus, the
approach is to use additionally atmospheric data from the
SMHI meteorological data base (1x1°, 3 hours, 1970-
2008) together with COADS-data (at present 1949-2004),
ICES Oceanographic data, IFM-GEOMAR atmospheric
and oceanographic measurements (1987-2008) and BSH
SSTs (1990-2008).
The main idea is to investigate in detail the climate
variability of the Baltic Sea area as a whole and for the
different sub-basins to assess the regional difference in
response to the large scale atmospheric forcing.
3. Results
We used statistical analysis including basic and higher
order statistics to discriminate the climatological
conditions between different time slices and identify
significant changes in atmospheric and oceanic variables.
As one example of our comprehensive analysis, Figure 2
shows the wavelet analysis of air temperature at the
position of Kiel Lighthouse and NAO DJFM-winter
indices (Figure 1). Interesting is the common structure of
the wavelet analysis for temperature and NAO which
reflects the high correlation between them. For the
periods 1970-1987 and 1988-2008 there is also a change
in the spectral characteristics of both temperature and
NAO winter index. After 1985, higher variability occur at
periods of about 2.5 and 5 years. Before 1985 highest