Sea-breeze forced diurnal surface currents in the Thermaikos Gulf, North-west Aegean

Energetic rotary diurnal surface currents of up to 40 cm/s are observed in the Thermaikos Gulf, North-west Aegean. They are principally confined above the pycnocline in the upper 3 m of the water column. Beneath this depth they are much weaker, less regular and oscillate in anti-phase with those at the surface. Averaged over 10-days, the current cycles are seen to vary sinusoidally with the northward component leading the eastward by 0.2 of a cycle (0.4π), corresponding to clockwise rotating current ellipses. The mean amplitude ratio between the northward and eastward component amplitudes (u/v) is 0.83. A simple analytical model, representing the dynamical balance of a homogeneous surface layer, predicts diurnal wind forcing, i.e. a sea-breeze, should drive diurnal currents whose amplitude will be particularly energetic close to 30° latitude, where the diurnal and inertial periods are similar. Assuming that a 3 m deep surface layer is subject to linear frictional drag, the model is used to hindcast the phase of a northward diurnal wind required to force the observed current components. A linear friction coefficient (of 0.6ω) is derived from the phase difference between the eastward and northward current components. Using this friction coefficient the phase of a diurnal northward wind is hindcast from the absolute phase of the current components. This hindcast wind phase of 1.70 rad is consistent with the observed northward wind of 1.66 rad with a phase difference of 0.04 rad (<10 min), which provides confirmatory evidence of the role of the sea-breeze wind stress cycle in forcing these motions. The model is finally used to predict the variation in the amplitude, ellipticity and rotation of these currents with latitude.