Within Baltic Sea ocean modelling context observational data are required for regional data-assimilation and validation. The Baltic Sea is bordered by nine countries (Sweden, Finland, Estonia, Latvia, Lithuania, Russia, Poland, Germany and Denmark; http://en.wikipedia.org/wiki/Baltic_Sea) and every issue related to the sea is of mutual interest for these countries. Consequently, marine research in the Baltic Sea is extensively structured and several platforms exist for collaboration (e.g., within HELCOM; see http://www.helcom.fi/) and exchange of oceanographic data. In 2000 the Baltic Operational Oceanographic System (BOOS; http://www.boos.org/) was created. BOOS is a formal association of institutes from the countries bordering the Baltic Sea. These institutes take national responsibility for operational oceanographic services to support the protection of lives and properties and the promotion of the development of society. The main focus of BOOS is on observations, analysis and model predictions of sea level, currents, waves, hydrography, sea ice, biochemistry, algae and chlorophyll. The Programme for a bAltic network to assess and upgrade an oPerational observing and forecAsting System (PAPA; http://www.boos.org/papa/) was a project under BOOS and funded for the period 2002-2005 under EU's 5th Framework Thematic Network. The aim of PAPA was to develop operational ocean monitoring, and modelling activities within the Baltic Sea and to enhance expertise in setting up and running observing platforms and forecasting systems. In the Baltic Sea, ECOOP builds upon the activities and results created in the above mentioned projects and the project Marine EnviRonment and Security for the European Area (MERSEA; http://www.mersea.eu.org/).

Along the Baltic shoreline an extensive network of tide gauges exist. Sea level in the Baltic Sea is monitored by national institutions and made available to partners via collaborating projects, such as BOOS. At present, data from around 45 tide gauges in the Baltic Sea is available at Near Real Time (NRT) at http://www.boos.org/index.php?id=29. Figure 1 is taken from this web site, and shows sea level in NRT on a given date and hour along the Baltic Sea shoreline.

Figure 1. Available tide gauge observations in the Baltic Sea at NRT from the BOOS homepage (http://www.boos.org/index.php?id=29).

Currents and hydrography are measured at descrete depths in NRT at few localities in the Baltic Sea system. In the Danish Straits buoy stations are operated by the Danish Maritime Safety Administration (DMSA), in the Fehmarn Belt and western Baltic Sea buoys are operated by the Federal German Maritime and Hydrographic Agency (BSH) and GKSS-Forschungszentrum Geesthacht Gmbh (GKSS). In the Kattegat and Baltic Sea buoys are operated by the Swedish Meteorological and Hydrological Institute (SMHI). The location of the buoy stations are shown in Figure 2, and details of measured parameters and periods are given in Table 1. Figure 2. Locations of buoy stations operated by BSH (green dots), DMSA (red dots) and SMHI (blue dots).

Table 1. Details of measured parameters and measuring periods on buoy stations operated by BSH, DMSA and SMHI.

In addition to these buoy stations, Danish national authorities operates buoy stations on a limited number of locations as part of the national environmental surveillance program. These stations are operated by the so called regional environmental centres. The stations, however, are occasionally taken out of service for rather long periods of time and data records therefore may contain large gabs. Time series and data of measured parameters are available from http://www.havmodellen.dk/. Figure 3 and Table 2, respectively, shows the locations of these buoy stations and gives an overview of measured parameters and periods.

Figur 3. Locations of buoy stations operated by some of the Danish regional environmental centres. Læsø W maintained by Environmental Centre Aalborg (green dot), Hjelm Bugt maintained by Environmental Centre Nykøbing (red dots) and Little Belt maintained by Environment Centre Ribe (blue dot).

Tabel 2. Details of measured parameters and measuring periods on buoy stations operated by the Danish regional environmental centres.

The BSH moorings in the Baltic Sea is part of their marine environmental monitoring network called MARNET. Time series plots at NRT as well of historical data from the BSH moorings are available from http://www.bsh.de/en/Marine_data/Observations/MARNET_monitoring_network/. Historical data from DMSA can be obtained from http://www.frv.dk/data_produkter/stroem_dk.htm. Plots of NRT mooring data (going back up to one month) from the SMHI moorings can be found on http://www.smhi.se/cmp/jsp/polopoly.jsp?d=7818&l=sv.

The ocean surface state is monitored from wave rider buoy stations at different locations in the Baltic Sea system (see Figure 4). Details of measured parameters and periods are given in Table 3.

Figure 4. Locations of wave rider buoys operated by GKSS (red dots), BSH (green dots) and SMHI (blue dots).

Table 3. Details of measured parameters and measuring periods on wave rider buoy stations operated by BSH, GKSS and SMHI.

Time series plots of surface wave parameters at NRT from the BSH mooring are available from http://www.bsh.de/en/Marine_data/Observations/MARNET_monitoring_network/ and from the GKSS mooring at http://www.coastlab.org/. These data are also available from the BOOS homepage under products and observations.

A large number of hydrographic and biochemical profile data (CTD and bottle data) have been taken in the Baltic Sea system. Most of these data have been collected as part of national monitoring programs and from ships of opportunity, and are archived in national and/or international databases. These data have usually been quality checked and may be available with delays of up to several years since their date of sampling essays.

The International Council for the Exploration of the Sea (ICES) database contains more than 9000 station profiles taken in the Baltic Sea system between 1960 and now. The ICES database (http://www.ices.dk/ocean/) includes data collected under the intergovernmental cooperation of countries bordering the Baltic Sea, formalised in HELCOM. The HELCOM database includes about 400 stations http://www.ices.dk/ocean/asp/helcom/helcom.asp?Mode=1. Figure 5 shows the locations of HELCOM stations as of today.

Figure 5. Locations of hydrographic stations in the HELCOM data base.

Since 1972 the Danish National Environmental Research Institute (NERI) has archived hydrographic and biochemical data in the Danish national database for marine data (MADS; http://www.dmu.dk/International/Water/Monitoring+of+the+Marine+Environment/MADS/). This database includes data from around 152 stations in the Baltic Sea system sampled by NERI and Danish regional authorities with the most recent data being about one year old. The data sampling frequency of the different stations is variable. Prior to 2004 most stations in MADS were sampled every 1-6 months. Since 2004, with the start of the Danish national environmental surveillance project (NOVANA), the sampling frequency has been increased for some selected stations, new stations have been added to the sampling scheme and some of the old stations have been taken out. Within NOVANA, a total of 109 stations are sampled in the Baltic Sea system with a frequency of up to one week during spring, summer and early autumn. During the winter time the frequency is about once a month. NOVANA data can be obtained from http://www.dmu.dk/Overvaagning/NOVANA/Det+Marine+Modelkompleks/NRT-data/. BSH has sampled profile data in the Baltic Sea system since the 19th century. Only some of these data are available via the HELCOM data base. The complete database is managed by the German Oceanographic Datacenter and data can be obtained from http://www.bsh.de/de/Meeresdaten/Beobachtungen/DOD-Datenzentrum/. SMHI manages the national and regional Swedish database for hydrographic and biochemical observation in the Baltic Sea system. These data can be obtained from http://www.smhi.se/cmp/jsp/polopoly.jsp?d=5431&l=sv. Baltic Sea data archived by the Finnish Institute of Marine Research (FIMR) from 1979 till now is available at http://www.fimr.fi/en/palvelut/en_GB/palvelut/.

The use of oceanographic instruments mounted on private, merchant and military vessels on an opportunity basis is a relatively cost efficient way of obtaining data of the upper most layer of the ocean. The Baltic Sea has a dense network of ships following regular schedules and some of these perform ocean measurements. At present FIMR has mounted oceanographic instruments, known as Ferryboxes, which measures upper ocean physical and biochemical parameters on a number of commercial vessels trafficking the Baltic Sea (Lips et al., 2007). These measurements are part of the ALGALINE project (Figure 6), and data can be obtained from http://www.fimr.fi/en/tietoa/algaline_seuranta/en_GB/algaline_seuranta/.

Figure 6. Ferrybox routes in 2006 as part of the ALGALINE project.

DMI produces and presents daily satellite sea surface temperature (SST) maps on an operational basis. Observations in the infrared band from a number of different satellites in the Ocean and Sea Ice Satellite Facility (O&SI SAF; http://www.osi-saf.org) project are combined to obtain a robust product. Missing observations due to clouds are filled using a spatial and temporal optimal interpolation technique, using local and empirically estimated statistics (Høyer and She, 2004). Daily (and going back up to one month) SST maps of the Baltic Sea are available from http://ocean.dmi.dk/satellite/. Figure 7 is taken from this web site, and shows SST for a specific date in the Baltic Sea-North Sea region. The data used to produce the maps can be obtained upon request to DMI.

Figure 7. SST in the Baltic Sea and North Sea on August 6th, 2008. This figure is an example of the daily SST maps that DMI produces and which can be downloaded from http://ocean.dmi.dk/satellite/.

BSH publishes weekly SST maps of the North Sea and Baltic Sea (and other regions) based on satellite observations. These maps can be obtained from http://www.bsh.de/en/Marine_data/Observations/Remote_sensing/.

DMI displays maps of remotely sensed chlorophyll-a concentrations in Danish waters and the western Baltic Sea based on data from the European Satellite Agency's (ESAs) ENVISAT MERIS satellite. These maps are updated 4-5 times a week and can be obtained from http://www.dmi.dk/dmi/index/danmark/algekort.htm. Daily and/or weekly maps of chlorophyll-a, Secchi disk depths and SST for the same area is also available at http://marcoast.dmi.dk/. These products are supplied to the public as part of the MARine & COASTal environmental information services (MarCoast) which is an operational service funded by ESA. FIMR and the Finnish Environment Institute (SYKE) monitors the chlorophyll-a and algae situation in the summer season in the Bothnian Sea and Gulf of Finland. Maps from these regions can be obtained from http://www.fimr.fi/en/palvelut/levatiedotus/en_GB/levatiedotus/ and http://www.ymparisto.fi/default.asp?node=11761&lan=en.

Sea ice monitoring in the Baltic Sea is based on ground truth, visual air-borne data, and satellite remote sensing data. High resolution, daily sea ice data are available from the AMSR-E sensor that was launched on the AQUA satellite platform in 2002. These data are processed by the University of Bremen (http://www.gmes-bremen.eu/productsservices/seaicemapsfromsatellite/) and have a horizontal resolution of approximately 6 km. Another daily sea ice product that includes the Baltic Sea area is produced by the High Latitude Processing Facility of the Ocean & Sea ice Satellite Application Facility (http://saf.met.no/) which is operated jointly by the Norwegian and Danish Meteorological Institutes. This product has been available since 1978 and has a resolution of approximately 10 km. Sea ice charts for the Baltic Sea area, based on data from various sources, are produced on an operational basis by BSH (http://www.bsh.de/en/Marine_data/Observations/Ice/Publications.jsp), FIMR (http://www.fimr.fi/en/itamerinyt/en_GB/jaatilanne/), SMHI (http://www.smhi.se/oceanografi/iceservice/is_prod_en.php), and the Admiral Danish Fleet (http://forsvaret.dk/SOK/Nationalt/Istjenesten/).

Høyer, J. L. and She, J., Validation of satellite SST products for the North Sea-Baltic Sea region, DMI technical report, 04-11, 2004.

Lips, I., U. Lips, V. Fleming, S. Kaitala and A. Jaanus, Use of Ferrybox measurements for the Baltic Sea environment assessment, Environmental Research, Engineering and Management, 3, 3-8, 2007.

• ICES: http://www.ices.dk/
• BOOS: http://www.boos.org/

1. Denmark:
   • NERI (MADS): http://www.dmu.dk/International/Water/Monitoring+of+the+Marine+Environment/MADS/
   • DMSA: http://www.frv.dk/data_produkter/stroem_dk.htm
2. Germany:
   • GKSS: http://www.coastlab.org/
   • BSH: http://www.bsh.de/en/Marine_data/Observations/MARNET_monitoring_network/
3. Sweden:
   • SMHI: http://www.smhi.se/cmp/jsp/polopoly.jsp?d=7806&l=sv
4. Finland:
   • FIMR: http://www.fimr.fi/en/palvelut/en_GB/palvelut/
   • SYKE: http://www.environment.fi/syke/