The central Gulf of Finland between Helsinki and Tallinn has been intensively monitored for last 100 years. There is lots of CTD-data available, though that is irregular in time.

In recent years, there have been Estonian profiling mooring stations at the Estonian coast and there is Ferry Box line between Tallinn and Helsinki. These data sets are the ones that will be used.

There are no Argo-floats in the Gulf of Finland.

Other area from the southern Baltic Sea that should be considered as test area is the Bornholm Basin. Argo Poland operates since 2016 Argo-floats there and that data is available from Coriolis.

This area is therefore comparable to the areas around Finland and the Baltic Sea Proper, because of the Argo-data.

The data types from the Arkona basin are similar, with the addition of German fixed buoy stations that measure temperature and salinity continuously.

That data has been available at least as time-series images.

There are lots of CTD data available from the Central Baltic Sea. Sweden conducts regular monitoring cruises there monthly. Data is open and goes to HELCOM database and is available from SeaDataNet and EMODnet.

There are Ferry Box lines crossing the area, too. Finland’s Euro-Argo project maintains Argo-float(s) in the Gotland Deep, too. Thus, there are both delayed mode data and near-real-time data available from the area.

Sweden has some regular stations in the Baltic Sea that measures salinity, too, and their data is open through SMHI Internet-pages.

Regular ship-based monitoring of the basin has been rather sparse in space and time. The basin is ice-covered part of the year. Because the environmental problems have been smaller than in other areas of the Baltic Sea the monitoring efforts have been smaller, too. However, Argo-floats have been used to monitor the basin about once a week for last seven years.

This routine is ongoing and is part of Finland’s Euro-Argo contribution. The data is freely available from Coriolis. There is also ship borne CTD-data available for the analysis of the vertical structure of the near surface salinity field. Most of the available CTD-data is publicly available through SeaDataNet and EMODnet.

There is seasonal cycle in the surface salinity because of melting of sea ice in the spring and fresh water supply from rivers when the snow cover on land melts and flows to the sea. Thus CTD-data from last 30-years are used in analysing the uncertainties of the extrapolation of the Argo-float subsurface salinity data to the very surface. This hold true for the Ferry Box data, too.

There is a continuous Ferry Box line along the central axis of the basin and that data gives good possibilities to estimate the horizontal salinity gradients within the basin. This Ferry Box is maintained by SYKE and is open data as well.