Po River Effects on Adriatic Sea Dynamics: Temperature, Salinity, Circulation, Sea Level, and Inter-basin Transport

The Po River contributes one-third of the total fresh water into the Adriatic Sea. Located 40 to 50 kilometers from Venice, fluctuations in the Po's discharge have an impact on the sea level surface of Venice. Due to this importance, the study of the Po River and its impact on the Adriatic Sea’ hydrodynamics can be an interesting topic. This study investigates the sole effect of the Po River on the hydrodynamics of the Adriatic Sea. The Regional Ocean Modelling Systems (ROMS) is used to conduct a comprehensive analysis of 2018 based on two scenarios: one with (WITHPO) and one without the Po (NOPO) while other inputs of models are the same. Model layers are 25 and the grid spacing is 2 km. The hourly ERA5 data is used for atmospheric forcing, whereas the GEBCO data is used for making the grid file. Climatology data is used for the southern open boundary, located in the northern part of the Ionian Sea. In all analysis, the values WITHPO are subtracted from those NOPO. The surface temperature difference ranges from–2 to 2 °C, converging to zero near the bottom. Temperature differences are most noticeable in the northern basin in spring and autumn. The WITHPO has colder spring water, but warmer autumn water. Similar patterns are observed in the middle and southern basins. At the bottom layer, the temperature difference is minimal during the summer. In the southern basin, the temperature difference between layers 15 and 25 is less than 1°C all year round, while in the northern basin, it decreases from surface to bottom. Further, the Po River remarkably contributes to salinity reduction, particularly in surface waters. Salinity differences range from –1 to –0.35PSU at the surface, gradually diminishing to –0.2 PSU near the bottom layers in all basins. The presence of the Po River results in lower sea levels during spring (4–6 cm), contrasting with the rise of 8–10 cm observed in other seasons. Due to the difference in temperature and salinity caused by the WITHPO compared to the NOPO, the temperature and salinity (T–S) graphs differ as the WITHPO reduces salinity, it increases the area of the T–S graph. But the maximum density is almost the same in both scenarios mainly because higher salinity water is warmer, leading no remarkable changes in the maximum density. Among the important findings of this study is that the Po River strengthens the southern gyre in winter and autumn but weakens it in spring and summer. This is important because it complements our previous research on the southern Adriatic gyre. Another paper by the authors of this paper indicates that wind, friction, and flux through the strait are effective in transporting water volumes of the southern gyre and its core’s location. Moreover, the largest water exchange differences between WITHPO and NOPO occur between the northern and middle basins during summer, totaling approximately –5000 m3/s. June marks the period of greatest temperature difference exchange variations, showing a consistent pattern over time. During summer between middle and southern basin, the temperature flux difference ranges from –0.4 to 1×106 °C m3/s, while salinity flux exchange displays marked fluctuations, particularly in the daily and weekly time scale, ranging from –2 to 3 ×105 PSU m3/s. Between September and October, Otranto strait exhibits maximum positive flux at 11000 m3/s (daily/weekly) and maximum negative flux at –15000 m3/s from August to October. Consequently, the present study provides an important information into the dynamics of the Adriatic by showing how a major river system, the Po River, interacts with the Adriatic Sea's hydrodynamics.