Current dynamics, drivers, and vertical thermal structure around Sir Bu Nair Island (Arabian/Persian Gulf)

Abstract. Sir Bu Nair Island (SBN) in the central Arabian Gulf is one of the last locations in the UAE’s Gulf waters to retain extensive and well developed coral reefs, whereas many nearshore environments have experienced severe degradation following recurrent marine heatwaves. Although tides, winds and subtidal circulation are thought to influence reef-level thermal conditions at SBN, the local physical processes governing circulation and temperature variability have not previously been resolved from in situ observations. Here, we analyse a 17-month record (May 2021–October 2022) of currents, sea level, and temperature from bottom-mounted ADCPs and multi-depth temperature moorings, complemented by reanalysis of winds. Currents at SBN are strongly anisotropic and aligned along a dominant principal axis, reflecting topographic steering around the island. At the more exposed northwest site, upper-layer along-axis velocities commonly reach ~0.6 m s−1, with episodic peaks approaching 0.8–1.0 m s−1, while at the southern site, flows are weaker, with typical upper-layer velocities of ~0.3 m s−1 and peaks of ~0.5–0.7 m s−1. Tidal forcing dominates the instantaneous flow, explaining up to ~80 % of the along-axis current variance at the northwest site but substantially less in the cross-axis direction and at the southern site, leaving significant subtidal variability. Wind–current coupling is strongest at subtidal timescales, with rapid (0–2 h) responses consistent with local barotropic adjustment to wind forcing. Temperature observations reveal pronounced seasonal variability, with summer surface temperatures exceeding 35 °C and annual excursions of more than 15 °C across the water column. A persistent seasonal thermocline develops during late spring and summer, with vertical temperature differences of 2–4 °C. This stratification is repeatedly disrupted by rapid mixing events lasting 2–3 days, during which the water column becomes nearly homogeneous. Spectral and variance analyses show that stratification variability is dominated by diurnal processes, with additional modulation at synoptic (2–4 day) timescales. The results indicate that tidal mixing alone is insufficient to suppress stratification and that episodic destratification arises from the combined influence of wind forcing and subtidal variability acting on a preconditioned water column. These findings provide the first integrated observational description of circulation and thermal structure at SBN, demonstrating how topographic control, tides, winds, and subtidal variability interact to regulate stratification, vertical exchange and transport processes around an isolated reef system in the central Gulf.