Surface gravity waves in the presence of an unsteady uniform current: application to tsunami warning

International audience; In recent years, there has been an increased interest in the possibility for tsunami radar remote sensing, owing tothe effects of tsunami-induced changes on the propagation of short sea waves. Well before microwave radars, HFradars have been investigated to detect tsunami induced- current effects on ocean surface waves (Barrick, 1979).Nonetheless no warning system has been proposed with a sufficiently short warning time, say between 5 and20min depending on the shelf width, which is more adapted to tsunami early warning system in coastal regions.Alternatively, the potential use of Ultra High Frequency (UHF) radar technology has been suggested based onrecent observations of modulation processes in radar echoes, which indicates the possibility for the short windwaves to be affected by the tsunami (Troïtskaya and Ermakov, 2005).In addition to current-induced effects on short gravity waves, like for instance current-induced Dopplershift in the apparent wave frequencies, it is well known that short waves modulations could also arise fromother mechanism interactions, such as wind-wave interactions and nonlinear wave-wave interactions. Thus theidentification and quantification of the interaction between short waves and current-induced tsunami is verychallenging, in particular for waves in the submetric range (high frequency).To achieve these tasks, we use a Higher Order Spectral (HOS) method to simulate numerically the nonlinearevolution of gravity waves in the presence of a time-varying current but spatially homogeneous. The HOSformulation of the prognostic equations has been modified to account for the presence of a time-varying current,whereas numerical time integration has been improved by using both an integrating factor method (unconditionallylinearly stable) and an embedded Runge-Kutta method with variable time steps. This paper presentsinitial numerical results obtained with our modified HOS model. To explore the signature of tsunami inducedcurrents on surface waves, we have used our modified HOS model to study different wave dynamics in thepresence of a time-varying current. In this paper, we present the results on wave dynamics obtained with realistictsunami-induced currents that were computed using the tsunami propagation code GEOWAVES.