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Recent field observations suggest that the air‐sea momentum flux (or the drag coefficient) is significantly reduced when the dominant wind‐forced surface waves are misaligned from local wind. Such conditions may occur under rapidly changing strong winds (such as under tropical cyclones) or in coastal shallow waters where waves are refracted by bottom topography. A recent Large Eddy Simulation (LES) study also shows that the drag coefficient is reduced by a misaligned strongly forced wave train (with a small wave age of 1.37). In order to investigate more realistic field conditions, this study employs LES to examine the effect of a misaligned (up to 90°) surface wave train over a wide range of wave age up to 10.95. For all wave ages examined, the drag coefficient is reduced compared to the flat surface condition when the misalignment angle exceeds around 22.5°–45°. The drag reduction may occur even if the form drag of the wave train is positive. Plain Language Summary: This study uses computer simulations to investigate turbulent wind blowing over ocean surface waves that are misaligned (propagate in different directions) from the wind. The study focuses on frictional wind forcing (wind stress) imparted on the ocean surface, because the wind stress is an important boundary condition for ocean, atmospheric, and surface wave models. Surface waves can be misaligned from wind when the wind field rapidly changes such as during tropical storms or in shallow water where wave directions are modified by varying water depth. When waves propagate slower than wind in the wind direction, the waves increase the wind stress compared to flat surface (no wave) conditions. However, this study finds that the wind stress is reduced, that is, the ocean surface becomes smoother, when the misalignment angle between wind and waves exceeds about 22.5°–45°. Key Points: Large eddy simulations are preformed to study wind over misaligned surface wavesDrag coefficient is reduced due to misaligned surface waves for all wave ages under wind‐driven wave conditionsDrag reduction can occur even when wave form drag is positive [ABSTRACT FROM AUTHOR]

Upper-ocean turbulence results from a complex set of interactions between submesoscale turbulence and local boundary layer processes. The interaction between larger-scale currents and turbulent fluctuations is two-way: large-scale shearing motions generate turbulence, and the resulting coherent turbulent fluxes of momentum and buoyancy feed back onto the larger flow. Here we examine the evolution and role of turbulence in the intensification, instability, arrest, and decay (i.e., the life cycle) of a dense filament undergoing frontogenesis in the upper-ocean boundary layer, i.e., cold filament frontogenesis (CFF). This phenomenon is examined in large-eddy simulations (LES) with resolved turbulent motions in large horizontal domains using 109 grid points. The boundary layer turbulence is generated by surface buoyancy loss (cooling flux) and is allowed to freely interact with an initially imposed cold filament, and the evolution is followed through the frontal life cycle. Two control parameters are explored: the initial frontal strength M2 = ∂xb and the surface flux Q * . The former is more consequent: initially weaker fronts sharpen more slowly and become arrested at a later time with a larger width. This reflects a competition between the frontogenetic rate induced by the secondary circulation associated with vertical momentum mixing by the turbulence and the instability rate for the along-filament shear flow. The frontal turbulence is energized by the shear production of the latter, is nonlocally transported away from the primary production zone at the filament centerline, and cascades to dissipation in a broad region surrounding the filament. The turbulent momentum fluxes arresting the frontogenesis are supported across a wide range of horizontal scales. [ABSTRACT FROM AUTHOR]