Theoretical and Numerical Investigations into the role of Bora Winds in generating Transversal Seiches in the Adriatic Sea

Abstract:

Field observations of waves and currents collected on the western shore of the Adriatic in January 2014 show low-intensity oscillations of surface elevation and cross-shore flows persisting for two days, in the wake of a strong bora storm (Brocchini et al. 2017), which is a north-easterly low-level wind (Grisogono et al. 2008), often exceeding 20 m/s and can persist for days. The discrete spectral distribution of the oscillations, with characteristic periods between 1.5 hr to 20 min, suggests that they are the shoreline expression of Adriatic transversal seiches (i.e., oscillations with an East-West standing-wave structure). The mechanisms of excitation of Adriatic transversal seiches may be multiple and are not well understood. While the relaxation of bora storm surge likely plays a role, other mechanisms are possible, such as excitation of long waves by convective cells through Proudman resonance, or through oscillatory wind surface stresses. We present theoretical (linear shallow water waves theory) and numerical investigations into the source of the seiches. Numerical simulations based on 2D FUNWAVE-TVD Boussinesq model (Shi et al. 2012) show a seiche response to both pressure perturbations and oscillatory wind surface stresses. The simulations are compared to observations. The study highlights aspects of the relation between bora events and transversal seiches that are not well documented and poorly understood, but relevant in relation with other air-sea interaction processes that have a significant shoreline impact, such as wave activity, meteotsunamis, and flooding induced by storm surges.