Coastal upwelling in the Black Sea as derived from satellite remote sensing

In the surface water of the Black Sea a strong thermal stratification, which controls processes of vertical exchange develops during summer months. Coastal wind driven upwelling phenomena play a significant role for the biogeochemistry of the marine ecosystem of the Black Sea. The temperature difference between upwelling areas and the surrounding water can be as high as 12 C and therefore suggests thermal infrared satellite remote sensing as an ideal tool for the investigation of this phenomenon. AVHRR / Sea Surface Temperature (SST) data from 1996 to 2003 were analyzed for the recognition of upwelling phenomena in the Black Sea with emphasis on the summer months (15 May -- 15 September). The area directly affected by upwelling can reach 8% of the total surface of the Black Sea. Upwelling events show a strong variability in the different regions of the Black Sea. However a two years periodicity in upwelling events can be detected during the extended summers from 1996 to 2003. Typical size and temperature differences were estimated for zones with different coastline orientations. Satellite data evaluation shows, that the South Crimea coast, the Western part of the Turkish coast, as well as the Bulgarian and Romanian coasts are the regions with frequent upwelling events. NCEP wind data were used for the correlation with upwelling events as well as for the explanation of the generation of upwelling in the different areas of the Black Sea. The pattern of the upwelling areas is highly affected by mesoscale processes in the shelf zone, which are related with the ``rim'' current. ``Cut off'' of waterbodies from shore induced upwelling by interaction with mesoscale eddies occurs rather frequently near the Crimea coast. In the rim current upwelling can be generated by jets, and eddies as can be demonstrated on SST images of the Black Sea especially during summer month. The significance of these phenomena for cross-shelf transport processes and thermocline ventilation is discussed. Animations on the base of subsequent satellite images were made to demonstrate the generation and evolution of upwelling in the Black Sea. Mutual analysis of data of the optical sensors SeaWiFS, MODIS with AVHRR - SST data shows that upwelling events can decrease, but also increase the chlorophyll concentration in the surface water layer. The processes responsible for these observations are discussed.