A multiplatform analysis of wind- and river-driven surface circulation in the Gulf of Trieste (northern Adriatic Sea) using HF radar, drifters and numerical model data

Coastal circulation is governed by closely linked interactions between atmospheric forcing, fresh water inflow, and complex bathymetry. Understanding these dynamics requires an integrated observational approach to adequately resolve surface transport processes. In this context, a multiplatform observational framework combining remote sensing, in situ measurements and numerical modelling was employed to study surface circulation in the Gulf of Trieste (GoT).
A preliminary observational analysis carried out in October–November 2023 addressed the interplay between surface circulation, wind forcing and river discharge, and, using high-frequency (HF) radar measurements, Weather Research and Forecasting (WRF) atmospheric simulations and river flow observations. The results show that easterly (Bora) wind events strengthen the GoT’s prevailing cyclonic circulation, enhancing surface outflow from the basin. In contrast, intense southerly winds induce a circulation reversal towards an anticyclonic regime, driving surface currents towards the coast. When strong river discharge episodes coincide with southerly wind conditions, a more complex scenario develops, characterised by anticyclonic flow in the central part of the GoT and cyclonic circulation in its northern sector.
To further assess near-surface dynamics and validate HF radar-derived currents under specific meteo-marine conditions, dedicated surface drifter experiments were carried out between late 2024 and 2025. Drifter trajectories showed strong agreement with HF radar observations, with high correlations between the observed velocity components and those derived from the radar. However, comparison with numerical model outputs revealed weaker consistency, highlighting the value of HF radar data for model validation and improvement via data assimilation methods.
Drifter pathways were closely associated with the low-salinity plume of the Isonzo River, confirming its pivotal role in driving near-surface transport. Additional coordinated deployments in 2025, allowed for a direct comparison between CODE and Stokes drifters, released from the same locations. The two kinds of drifters exhibited distinct responses, reflecting their different sampling depths. CODE drifters track currents at a depth of around 1 m, while Stokes drifters follow the immediate surface layer and are more sensitive to wind and wave forcing.
Overall, integrating HF radar observations, surface drifters and numerical simulations provides a robust framework for resolving variability in coastal surface circulation and improving the representation and operational forecasting of transport processes in numerical models of the GoT. This kind of approach can be easily exploited in other coastal areas with similar meteo-marine and bathymetric features.