Effects of Upwelling Events on the Atmosphere

Abstract: During an upwelling event the cold bottom-water is brought to the sea surface. This cools the atmosphere from below and the stratification becomes more stable. When the atmosphere is more stable the turbulence is reduced and, as a consequence, so are the turbulent fluxes. This study is investigating four periods of upwelling from the Östergarnsholm-site, in the Baltic Sea east of Gotland, during the summer of 2005. The air measurements are taken at a tower at the southernmost tip of Östergarnsholm while the measurements in the water are from a buoy moored 1 km south-southeast of the tower. During all the upwelling events the wind is south-westerly, along the coast of Gotland. This means that the buoy is not within the flux footprint area and is perhaps not always representative of what happens there. All the periods show a stabilization of the atmosphere as the SST (Sea Surface Temperature) decreases. The heat fluxes, especially the latent heat flux, decreases as the SST decreases. The amount of CO2 in the atmosphere, in the summer, is usually higher than the amount in the surface water of the seas because the oceans are a net sink of CO2. The air-sea flux of CO2 is to a large extent controlled by this difference. Therefore the flux of CO2 is usually directed to the sea. The deep-water contains more CO2 than the surface water because the phytoplankton near the surface removes CO2 through photosynthesis. The deep-water is also colder and can solve more CO2. During an upwelling event this CO2-rich water is brought to the surface. As an upwelling event progresses the difference in CO2-concentration between the air and the sea is reduced, sometimes reversed, and the flux decreases. This is what happens in three of the investigated periods in this study. During the fourth period a counter gradient flux is observed.