Abstract
<title>Abstract</title> <p>The Gulf of Tonkin has one of the world’s most strongly diurnal tidal regimes, yet spatially continuous descriptions of that regime, and of its surface kinetic energy, remain scarce. We combine harmonic synthesis of the EOT20 global tide model, daily GLORYS12 reanalysis currents, hourly CMEMS operational surface currents, ERA5 winds, and tide-gauge records to characterise the diurnal regime and surface current energy of the Gulf. The first spatially continuous Form Factor map for the basin places the F = 3 contour near 19°N, with F reaching 12.7 in the northern interior. Synthesised elevations reproduce hourly records at Hon Dau and Beihai, matching the dominant constituent amplitudes to within seven percent in vector terms, with a Nash–Sutcliffe efficiency of 0.916 at Hon Dau. Mean surface power density is 2.86 W/m² in the daily reanalysis, against 121.35 W/m² for a frictionless progressive-wave scaling of the EOT20 tide, an apparent 42-fold gap. Harmonic analysis of a 365-day hourly record at four sites shows that this gap is not a pointwise frictional ratio: daily averaging alone suppresses power density by factors of 3.8–8.9, while the ratio of scaled to realised tidal amplitude (0.1–2.6) traces the standing-wave structure of the near-resonant diurnal oscillation, with K₁ and O₁ exceeding M₂ at every site. Combined diurnal current amplitudes approach 1.3 m/s near the Qiongzhou Strait, a busy shipping lane and the only part of the Gulf meeting first-order tidal-stream screening thresholds; its single daily high-water cycle shapes both navigation and energy use.</p>