How Currents Trigger Extreme Sea Waves. The Roles of Stokes Drift, Eulerian Return Flow, and a Background Flow in the Open Ocean

A deterministic system of ocean surface waves and flow in the oceanic boundary layer is key to understanding the dynamics of the upper ocean. For the description of such complex systems, a higher-order shear-current modified nonlinear Schrodinger equation is newly derived and then used to physically interpret the interplay between Stokes drift, Eulerian return flow due to a passing wave group, and an open-ocean vertically sheared flow in the extreme sea wave generation. The conditions for the suppression or enhancement of the modulation instability in the rogue wave dynamics in the presence of a background flow are reported, whose relevance and influence to the Craik-Leibovich type 2 instability in triggering a Langmuir-type circulation is discussed. The findings highlight the need for future studies to establish and assess the energy transfer from waves to currents or in the reversing order, asserting a plausible physical mechanism for the dissipation of the surface wave energy through wave-current interactions in the open ocean. The dynamics of the upper-ocean involve many complex processes, including for instance the interplay between wind, waves, currents, and global circulation systems. Such interactions can give rise to instabilities and extreme events with far-reaching consequences. In this letter, we use a newly derived weakly nonlinear wave framework accounting for the presence of shear currents to quantify the requirements to trigger modulation instability, giving rise to long-crested rogue waves. Our investigation also provides combined conditions for the occurrence of both, modulation and Craik-Leibovich (type 2) instabilities, and demonstrates the possibility of energy transfers between waves as well as between waves and currents in the ocean. An advanced shear-current modified nonlinear Schrodinger-type equation is derived for surface waves in a background open-ocean flow The interplay between Stokes drift, background flow, and Eulerian return flow by a wave group, in extreme waves generation is revealed How a background flow suppresses the modulational instability is explained and its relevance to the CL2 instability is discussed