Effects of the Po River on Hydrodynamics and Inter-basin Transport in the Adriatic Sea

Abstract: The Po River contributes one-third of the total fresh water into the Adriatic Sea. Fluctuations in the Po's discharge can affect the sea level surface of Venice, 50 km away, as well as regulate the salinity and circulation of the Adriatic Sea. To study the effect of the Po River on the hydrodynamics of the Adriatic Sea, the Regional Ocean Modelling Systems (ROMS) model is used based on two scenarios: one with (WITHPO) and one without the Po (NOPO). The horizontal grid spacing is 2 km with 25 model layers. Hourly ERA5 data is used for atmospheric forcing, and GEBCO data provides the bathymetry. Climatological temperature, salinity, and velocity data is used for the southern open boundary, located in the northern part of the Ionian Sea. In all our analyses, 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 Adriatic Sea 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 0–200 m (below the surface) is less than 1°C all year round, whereas in the northern basin, it decreases from surface to bottom. Salinity differences range from –1 to –0.35 PSU 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. The maximum density, however, is almost the same in both scenarios mainly because higher salinity water is warmer, leading to only small 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, resulting in shifting to the core of the southern gyre. This finding complements our previous research on the southern Adriatic gyre where we found that the wind contributes 10 % –14 % to the core displacement, while the influx from the Strait of Otranto contributes 37 %. 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, whereas the exchange of salinity flux displays marked fluctuations, particularly in the daily and weekly time scale, ranging from –2 to 3 ×105 PSU m3/s. Between September and October, the maximum positive flux through the Otranto Strait is 11,000 m3/s and maximum negative flux at –15,000 m3/s from August to October. Consequently, these variations in sea levels, salinity, and temperature in the Adriatic Sea are caused by fluctuations in the Po River discharge and signal the conditions that may exist in the future under reduced inflow from the Po River.

Biography: Javad is a third-year PhD student in atmospheric science at the University of Manchester. His thesis focuses on the circulation of the Mediterranean Sea, utilizing the ROMS model. His background is in physical oceanography, with a focus on geophysical fluid mechanics during his master's studies. He is interested in several topics: ocean-atmosphere interaction, overturning circulation, eddy and gyre dynamics, and the effects of climate change on the ocean.