With a mean annual air temperature of 7°C and a mean annual precipitation of 383 mm (data period 1961-1990, SMHI station Östergarnsholm) the climate is characterized after Köppen as a Cfb-climate, i.e. humid temperate with mild summers and mild winters.

The cold water in spring and summer gives dominantly stable atmospheric stratification during April to July. Despite low winter temperatures, the sea surrounding Östergarnsholm has ice free conditions throughout the year. Higher rates of cyclonic activity in winter result generally in higher wind speeds in winter than in summer.

Figure 1: Climate diagram Östergarnsholm (data period 1961-1990, SMHI station Östergarnsholm). The diagram displays monthly averages for temperature and precipitation. When the precipitation curve undercuts the temperature curve, this indicates dry season. When the precipitation curve exceeds the temperature curve, this indicates moist season

The island is small and flat covered by small rocks and pebbles. The vegetation on the island consists of a few single trees (mainly in the N-E corner) as well as low herbs and grass together with small bushes.

Figure 2: Photograph of the vegetation at Östergarnsholm including parts of the site infrastructure. The photo was taken before the updated power supply system was installed (in November 2016).

The carbon cycle represented by the Östergarnsholm site shows a strong seasonal variation with marine carbon uptake from April to October, and emissions from November to March. Algae blooms with resulting biofouling on water-side sensor is fairly common in summer time.

Figure 3: Modeled partial pressure of CO2 (pCO2) in atmosphere and sea at Östergarnsholm.