Open-sea and coastal upwelling in the Adriatic Sea

An experiment has been carried out in the Adriatic Sea, in the framework of the Middle Adriatic Upwelling and Downwelling (MAUD) project. The CTD and ADCP data were collected by the yo-yo and shipborne measurements performed during the cruises whereas the temperature, pressure and dissolved oxygen time series were recorded by the probes deployed at the sea bottom. Additionally, the SST satellite data and the meteorological time series originating from permanent coastal stations were considered. Moreover, the high-resolution, 2-km meteorological (ALADIN) and 2.5-km oceanographic (ROMS) models were used to reproduce and interpret the experimental findings. Analysis of the data has concentrated on the end of May 2017, when a dense water dome was documented by the CTD measurements in the area between the island of Blitvenica (close to the east coast) and the island of Jabuka (in the open sea). Its center was observed at a distance of about 20 km from the coast. The dome left its mark on the sea surface, with the temperature above its center being slightly lower than in the surrounding areas as documented by both in situ and remotely-sensed data. The vmADCP measurements suggested that the surface circulation around the dome was cyclonic. At the time, a decrease of temperature close to the east coast was documented by the bottom probes and satellite images. The meteorological data and modeling results showed that the northern winds prevailed during the May 2017 experiment, suggesting that the open-sea and coastal upwelling occurred at the same time. In order to verify the interpretation, several schematized numerical experiments were conducted. The modelled wind fields were first decomposed into the curl and curl-free components, using the Helmholtz-Hodge decomposition. The components were then used to impose the forcing on the Adriatic model, assuming flat bottom and realistic bathymetry. Schematized simulations revealed that the wind curl was responsible for the offshore rising of pycnocline through Ekman pumping and therefore for the open-sea upwelling. On the other hand, in simulations with the curl-free wind component the pycnocline rose only close to the east coast and thus the coastal upwelling was reproduced.