Abstract
Abstract. Ferrybox measurements have been carried out between Tallinn and Helsinki in the Gulf of Finland (Baltic Sea) on a regular basis since 1997. The system measures autonomously water temperature, salinity, chlorophyll a fluorescence and turbidity and takes water samples for further analyses at a predefined time interval. We aimed to show how the Ferrybox technology could be used to study the coastal upwelling events in the Gulf of Finland. Based on the introduced upwelling index and related criteria, 33 coastal upwelling events were identified in May–September 2007–2013. The number of events, as well as the frequency of their occurrence and intensity expressed as a sum of daily average temperature deviations in the 20 km wide coastal area, were almost equal near the northern and southern coasts. Nevertheless, the wind impulse, which was needed to generate upwelling events of similar intensity, differed between the northern and southern coastal areas. It is suggested that the general thermohaline structure adapted to the prevailing forcing and the estuarine character of the basin weaken the upwelling created by the westerly to southwesterly (up-estuary) winds and strengthen the upwelling created by the easterly to northeasterly (down-estuary) winds. Two types of upwelling events were identified – one characterized by a strong temperature front and the other revealing gradual decrease in temperature from the open sea to the coastal area, with maximum temperature deviation close to the shore.
Highlights
Unattended monitoring of marine environments using ships of opportunity has been implemented in many regions of the world ocean (e.g., Paerl et al, 2009; Hardman-Mountford et al, 2008), including the Baltic Sea and the Gulf of Finland (Rantajärvi, 2003)
We showed that Ferrybox data from the Tallinn–Helsinki ferry route could be successfully employed to describe the characteristics of coastal upwelling events in the Gulf of Finland
An advantage of the geographical location of the ferry route across the relatively narrow gulf and the schedule consisting of two crossings a day allowed one to control the quality of the data and introduce the upwelling index based on the data from a single crossing and the cumulative upwelling index
Summary
Unattended monitoring of marine environments using ships of opportunity has been implemented in many regions of the world ocean (e.g., Paerl et al, 2009; Hardman-Mountford et al, 2008), including the Baltic Sea and the Gulf of Finland (Rantajärvi, 2003). The commonly used Ferryboxes measure temperature, salinity, and chlorophyll a fluorescence in the seawater pumped through the system from the surface layer along the ship track. First trials of using ships of opportunity for environmental monitoring in the Gulf of Finland were made by Estonian and Finnish scientists between Tallinn and Helsinki in 1990– 1991 (Rantajärvi, 2003). The Gulf of Finland (GoF) lies in the northeastern part of the Baltic Sea (Fig. 1). It is an elongated basin with a length of about 400 km and a maximum width of 135 km (Alenius et al, 1998). The longterm residual circulation in the surface layer of the gulf is characterized by a relatively low speed and by a cyclonic pattern. A variety of mesoscale processes/features (fronts, eddies, coastal upwelling/downwelling), which sig-
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