Estimating Ocean Tidal Loading and Sea-Level Variability in the Northern Adriatic Using GNSS positioning, Tide Gauges, and GNSS Reflectometry

The northern Adriatic Sea provides an exceptional setting for investigating tidal processes because of its shallow bathymetry, elongated basin geometry, and semi-enclosed configuration. These characteristics produce some of the largest tidal amplitudes in the Mediterranean and favor the amplification of both diurnal and semidiurnal constituents, leading to complex sea-level variability and episodic extreme events such as high stands along the coastal cities. We analyze tidal dynamics and ocean tidal loading (OTL) in the northern Adriatic using a multi-technique approach that combines tide gauge (TG) observations, interferometric reflectometry (GNSS-IR) sea-level retrievals, and GNSS precise point positioning (PPP) solutions describing crustal deformation. Tide gauge records confirm the progressive increase of semidiurnal tidal energy toward the northern end of the basin, with the M2 and S2 constituents dominating over the diurnal band. These observations broadly agree with the FES2014b ocean tide model, although local amplitude and phase deviations are observed in shallow and geometrically complex coastal environments. Sea-level time series derived from GNSS-IR at several coastal sites show a high degree of agreement with nearby tide gauges, with correlations exceeding 90%. Both diurnal and semidiurnal constituents are well resolved, and amplitude differences remain within 4-5 cm, demonstrating the potential of GNSS-IR as an effective and low-cost complement to traditional TG networks. GNSS PPP solutions further allow the estimation of three-dimensional OTL displacements at hourly temporal resolution. The vertical component is primarily controlled by the semidiurnal M2 tide and closely matches model predictions in both amplitude and phase. Larger discrepancies are observed for diurnal constituents, particularly K1, likely related to interactions with GNSS orbital periods and remaining systematic effects in the processing. Overall, this work presents the first high-temporal-resolution GNSS-based assessment of ocean tidal loading in the northern Adriatic. The strong consistency among GNSS, GNSS-IR, and TG observations highlights the capability of integrated GNSS approaches to simultaneously capture oceanographic variability and solid Earth tidal deformation, opening new perspectives for coastal sea-level monitoring, geodetic stability studies, and hazard assessment in a changing climate.