Multi-technique analysis of ocean tides and ocean tidal loading in the Northern Adriatic using GNSS PPP, tide gauges, and GNSS reflectometry

The northern Adriatic Sea constitutes a unique natural laboratory for the investigation of ocean tides and their geodetic and oceanographic consequences. Unlike most of the Mediterranean basin, which is characterized by microtidal conditions, the northern Adriatic exhibits the largest tidal ranges in the region, frequently exceeding 1 m during spring tides. This anomalous behaviour is primarily controlled by the basin’s shallow bathymetry, wide continental shelf, elongated geometry, and semi-enclosed configuration, which together favor resonant amplification of both diurnal and semidiurnal tidal constituents (Janeković et al., 2005; Medvedev et al., 2020). In addition, the interaction between astronomical tides, basin-scale seiches, and meteorological forcing (e.g., Bora and Scirocco winds, atmospheric pressure variations) produces complex sea-level variability and extreme events such as “acqua alta”, with significant societal and economic impacts along the densely populated coastline (Camuffo et al., 2023). In this contribution, we present a comprehensive and multi-technique analysis of tidal dynamics and ocean tidal loading (OTL) in the northern Adriatic region, integrating tide gauge (TG) records, GNSS reflectometry (GNSS-R) and high-rate GNSS precise point positioning (PPP) solutions for crustal deformation (see Figure 1 for station map distribution). TG observations from multiple stations spanning the northern Adriatic confirm the progressive amplification of semidiurnal energy toward the head of the basin, with the principal lunar semidiurnal tide M_2and the solar semidiurnal tide S_2becoming increasingly dominant relative to the diurnal constituent K_1. Harmonic analysis of TG time series shows good overall agreement with the FES2014b global ocean tide model (Carrère et al., 2015), while also revealing localized discrepancies in amplitude and phase, particularly at stations characterized by complex coastal geometry and shallow-water dynamics, consistent with previous regional studies (Tsimplis et al., 1995; Martens et al., 2020). GNSS reflectometry is employed to extend sea-level observations to sites lacking conventional tide gauges. Sea surface hight (SSH) time series derived from GNSS-R at several coastal stations along the northern Adriatic (VEN1, GARI, PORE, and an additional station located in Sistiana, close to Trieste) exhibit strong temporal coherence with nearby TG records, with correlation coefficients exceeding 90% (Figure 2). Spectral analyses demonstrate that both diurnal and semidiurnal tidal constituents are clearly resolved, with amplitude differences of only 2–3 cm relative to TG estimates. A dedicated case study at the Sistiana station further confirms the robustness of the technique, even under sub-optimal observational conditions such as limited azimuthal coverage and single-constellation GPS tracking. These results are consistent with recent applications of GNSS-R for coastal sea-level monitoring (Tabibi et al., 2020; Devoti et al., 2023) and highlight its potential as a cost-effective complement to traditional TG networks. To investigate the solid Earth response to ocean tides, GNSS PPP solutions were analyzed to estimate three-dimensional OTL displacements at multiple stations surrounding the Adriatic basin. Modelled OTL displacements were computed using the FES2014b tide model and elastic Earth Green’s functions and compared with GNSS-derived harmonic estimates. The vertical component (Figure 3) shows the clearest tidal signature, with the semidiurnal M_2constituent dominating the loading signal and exhibiting excellent agreement in both amplitude and phase between observations and model predictions. The secondary semidiurnal component N_2 is also consistently resolved. Diurnal constituents, particularly K_1, display larger discrepancies, including amplitude inflation and phase offsets, which are attributed to near resonance between the diurnal tidal period and the GNSS orbital repeat cycle, as well as residual orbit and multipath effects (King et al., 2006; Abbaszadeh et al., 2020). Horizontal OTL displacements further reveal a clear contrast between semidiurnal and diurnal behaviour. The M_2horizontal response is spatially coherent and well aligned with model predictions, reflecting the propagation of the semidiurnal tide from the Otranto Strait toward the northern Adriatic. In contrast, horizontal K_1displacements show greater scatter and systematic amplitude biases, reinforcing the known limitations of single-constellation GNSS for resolving diurnal loading signals. Overall, this study provides the first GNSS-based OTL analysis in the northern Adriatic region performed at an hourly temporal resolution. The GNSS PPP solutions enable the direct observation of sub-daily elastic crustal responses to ocean tides in a shallow, semi-enclosed basin, representing a significant methodological advancement for the region. The consistency between GNSS-derived OTL signals, GNSS-R sea level estimates, and traditional TG observations demonstrates the robustness of the integrated approach and confirms the capability of GNSS techniques to resolve both oceanographic and geophysical tidal processes at unprecedented temporal detail. These results not only improve the understanding of tidal dynamics and loading in the northern Adriatic but also establish a new benchmark for future high-resolution GNSS studies of coastal ocean–solid Earth interactions, with important implications for sea level monitoring, geodetic reference frame stability, and coastal hazard assessment under ongoing climate change.