Monitoring the Baltic Sea: Using Bathymetry to Guide Argo Floats

The Baltic Sea is a semi-enclosed brackish water basin where stratification is special compared with the overall conditions in the World Ocean. Finnish Meteorological Institute (FMI) is the first to successfully deploy and operate Argo floats in Baltic Sea. The area is very challenging for Argo activity due to very shallow depth, large horizontal and vertical gradients in salinity, and partial seasonal ice cover. FMI has been operating the Argo floats in Baltic Sea since 2012. With these Argo missions, we have obtained unique data series for the Baltic Sea. One float has been kept operational in the Gotland Deep continuously from 2013. Gotland deep is a 250 m deep basin in central Baltic. In this work we describe the monitored advancement of the major Baltic inflow during the measurement time, and describe the methods to operate Argo float in shallow waters, keeping it locked in relatively small area by controlling the dive depth. Argo floats used in the Gotland Deep have oxygen and turbidity sensors in addition to the typical CTD sensors. This combined with the long period of measurement, makes the obtained data set highly useful.