Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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An Overview of Long-Term Time Series of Temperature, Salinity and
Oxygen in the Baltic Sea
Karin Wesslander 1 , Philip Axe 2 , Mattias Green 1 , Anders Omstedt 1 and Artur Svansson 1
1 Göteborg University, Department of Earth Sciences: Oceanography. Box 460, SE-405 30 Göteborg Sweden
2 SMHI Oceanographic Unit, Byggnad 31, Nya Varvet, Västra Frölunda SE-426 71, Göteborg, Sweden
1. Introduction
One of the new objectives within BALTEX Phase 2 is to
analyze climate variability and change since 1800. Improved
understanding of the Baltic Sea climate requires a set of
observations that are of known quality and well distributed
in space and time. Marine data however, are often sampled
irregularly both in space and time, are of variable quality
and thus difficult to interpret. To meet the new BALTEX
objectives we therefore need to put more efforts in
oceanographic data handling, data correction, time series
reconstruction and data quality assessment. To initiate this
work we summarise here the available salinity, temperature,
and oxygen data from the Baltic Sea. We also discuss efforts
needed for making long-term data sets available for the
research community with assessments of the data quality.
Finally, we outline a work strategy that could be used within
the BALTEX research community.
2. Data sources
Temperature, salinity and oxygen profile data are mainly
collected during research vessel cruises. Data are then stored
at various national and international centres. These centres
often exchange data between themselves. However, there are
occasionally time delays and differences in the data stored at
the different data banks.
When using data from the different centres and putting longterm
data sets together, one has to compare observations
from different ships and countries, despite the fact that intercalibrations
are very rare. Different measurement techniques
must be considered when assembling long time series.
Correction of systematic errors and data homogenization are
therefore important. Furthermore, the temporal and spatial
resolution of data should be considered.
2.1 Special Cruises and data sets
The very first Swedish research cruise in the Baltic Sea was
conducted in 1877 by F. L. Ekman. Several sections of
temperature and salinity were measured in the Skagerrak,
Kattegat and Baltic Sea, Ekman (1893). Instrumental
offshore data before 1877 are thus difficult to obtain. Instead
proxy data need to be used when we are to extend time
series back to 1800.
Björk and Nordberg (2003) present an example of a long
time series with good data quality. From 1930-1989, daily
hydrographic observations were made from a suspension
bridge at Bornö Oceanographic Station, inside Gullmar fjord
on the Swedish west coast, resulting in a long and dense
time series.
2.2 Lightships observations
Several lightships were in operation in the Baltic Sea during
the 20 th century, although operation ceased during the latter
half of the century. Lightship crews made daily observations
of meteorology, sea-ice thickness and extent, sea-surface
temperature, current velocity, and at least weekly
observations of salinity. Most lightship observations have
been digitized but there is still work to do. Data from
Swedish lightships have been digitized from 1923
onwards. Around this time the method for measuring
salinity was changed from areometer to titration. The
titration method is considered to be more accurate.
2.3 Databases
The Baltic Environmental Database (BED) is an extensive
database with almost 1.5 million observations sampled
between 1900-1995 (Sokolov et al., 1997, see also
http://data.ecology.su.se/models/bed.htm).
The Swedish Ocean Archive SHARK (Svenskt
HavsARKiv) is a database that is run by SMHI (Swedish
Meteorological and Hydrological Institute, www.smhi.se).
This database contains mainly Swedish, but also
international, observations from the Skagerrak, Kattegat
and Baltic Sea. Data from 1952 onwards, from selected
stations are freely available online at
http://www.smhi.se/sgn0102/nodc/datahost/datahost.html#
Datahost. The Oceanographic Data Centre for BALTEX is
located at SMHI.
ICES (International Council for the Exploration of the
Sea) was founded in 1902 and coordinates and promotes
marine research in the North Atlantic. Nineteen member
countries submit data to ICES and oceanographic profile
data from 1900 (and surface data from 1890) are available
on their website, www.ices.dk. Data may be retrieved free
of charge either by searching for special cruises, ocean
weather ships or by selecting data in geographical squares.
In addition, through its data centre work, ICES supports
the work of the HELCOM and OSPAR marine
conventions. These promote, and coordinate, data
collection in the Baltic, Kattegat and Skagerrak. ICES
fisheries work coordinates the International Bottom Trawl
Survey, which has produced high spatial resolution
hydrographic data annually (every January-February)
since about 1970.
3. Baltic Sea data
When working with long time series from regions such as
the Baltic Sea it is important to consider both temporal and
spatial resolution. Data are distributed unevenly in both
time and space. When sampling, for example, occurs only
once a year it is not obvious that the same month was used
from year to year. While the sampling frequency increased
enormously from the early 1900s to the present, there are
periods where the number of observations was
significantly reduced, for example during the two World
Wars.