Numerical Modelling of the Barotropic Tides in the Gulf of Aden

Abstract

The Gulf of Aden (GA) is a strategically dynamic water basin linking the Red Sea basin with the Arabian Sea via the Bab el Mandeb Strait. In spite of its importance for oil transportation globally and regional hydrodynamics, tidal propagation remains relatively understudied. Thus, in this study, a depth-averaged 2D barotropic hydrodynamic model based on the Delft3D modelling system with a uniform resolution of 5 km was set up, calibrated, and validated to model the tidal propagation within the GA. The open boundaries were forced with eight primary astronomical tidal constituents, four semidiurnal (M2, S2, N2, K2) and four diurnal (K1, O1, P1, Q1) components. The model performance was assessed using hourly water level observations as well as current measurements. Sensitivity analyses were carried out on key parameters, with the Chézy bottom roughness coefficient of 35 m1/2 s−1 providing the optimal performance. Statistical parameters revealed a strong match between the modelled and the observations. Mean Absolute Error (MAE) was observed to vary from 0.07 to 0.08 m (about 2.7–5.1% of the local mean tidal range), with large inconsistencies found at ME and MW stations, while the Root Mean Square Error (RMSE) varied from 0.08 to 0.10 m (about 3.1–6.4% of the local mean tidal range). The Index of Agreement (IOA) was found significant across all stations (0.91 to 0.95). Among all constituents, the semidiurnal M2 and diurnal K1 tidal waves exhibit the largest amplitudes. The amplitudes of S2 and N2 attain approximately 43% and 30% of M2, while O1 represents about 38% of K1. The co-tidal charts show that semidiurnal and diurnal tides propagate as coastally influenced long waves with a double-maximum structure along coastal boundaries, reaching maximum surface elevation amplitudes near Bab el Mandeb Strait and the eastern boundary (~0.48 m to 0.60 m for M2, and up to 0.48 m for K1). Rossby radius of deformation and quarter-wave resonance indicate that the semidiurnal band lies close to the fundamental resonant period of the GA, generating a co-oscillation-like response with small phase gradients, while the diurnal band more closely resembles a freely propagating Kelvin wave. Form Factor values within the GA range between 0.7 and 0.9, indicating a predominantly mixed, mainly semidiurnal tidal regime throughout the Gulf.