A reduced model for surface wave-current interactions without spatial scale separation

Abstract: We propose a reduced asymptotic model for the mutual interaction between a weakly nonlinear surface gravity wave field and a slowly evolving incompressible current in a homogeneous rotating fluid. The formulation builds on the Craik-Leibovich theory for the wave-averaged momentum equation, but the Stokes drift is not prescribed externally. Instead, it is determined by a companion amplitude equation for a narrow-band wave field concentrated near the wavenumber circle associated with a prescribed carrier frequency. The derivation combines a multiple-time-scale expansion in wave steepness with a phenomenological closure that neglects quartic wave-wave interactions while retaining the third-order Stokes correction. Importantly, no spatial-scale separation is imposed on the wave-current interaction, allowing the wave equation to represent current-induced advection, refraction, and multidirectional scattering. The resulting equations conserve wave action and admit closed energy and momentum budgets for the coupled wave-current system. The model thus provides a tractable bidirectional extension of the classical Craik-Leibovich framework for regimes in which current-induced wave evolution feeds back significantly on the mean flow.