Mapping Seasonal and Persistent Surface-Current Regimes Using Long-Term HF Radar Observations and Physics-Guided Clustering

Long-term high-frequency radar (HFR) observations provide a unique opportunity to examine how coastal surface currents organise into recurrent current-field configurations. This study uses the 2013–2024 HFR-Lisbon surface-current record to identify sub-inertial regimes in the Lisbon coastal region, a dynamically complex sector of the western Iberian margin. Hourly HFR currents were quality controlled, filtered with a centred 33 h rolling mean and restricted to a sub-inertial analysis domain defined from long-term coverage and residual semidiurnal-energy criteria. A physics-guided K-means clustering approach was then applied to seven physically interpretable descriptors of current intensity, directional coherence, spatial contrast, temporal change and bathymetry-based current-speed distribution. Four recurrent regimes were identified: a low-energy regime, a shelf-intensified regime, a moderate slope/offshore regime and an energetic offshore-intensified regime. The regimes show coherent spatial structures, marked seasonal organisation, persistence over several days and preferential temporal pathways. ERA5 winds, analysed a posteriori, show the strongest contrast between the low-energy regime, associated primarily with weaker-than-average winds, and the energetic offshore-intensified regime, associated with stronger and more northerly wind conditions. The results demonstrate the value of long-term HFR observations and physics-guided clustering for identifying and interpreting recurrent coastal surface-current configurations.