Observation of swell dissipation across oceans

TLDR

Ocean swells can persist over distances exceeding 20,000 km, but the processes driving their dissipation remain uncertain. Satellite SAR data were used to quantify swell energy dissipation, revealing that steeper swells trigger a laminar‑to‑turbulent boundary‑layer transition at a Reynolds number around 100,000. The study found that steeper swells reduce the energy e‑folding scale to as little as 2,800 km, corresponding to a decay rate β = 4.2 × 10⁻⁶ s⁻¹, and that these results can improve wave‑forecasting models and constrain swell‑induced air‑sea momentum and energy fluxes.

Abstract

Global observations of ocean swell, from satellite Synthetic Aperture Radar data, are used to estimate the dissipation of swell energy for a number of storms. Swells can be very persistent with energy e‐folding scales exceeding 20,000 km. For increasing swell steepness this scale shrinks systematically, down to 2800 km for the steepest observed swells, revealing a significant loss of swell energy. This value corresponds to a normalized energy decay in time β = 4.2 × 10 −6 s −1 . Many processes may be responsible for this dissipation. The increase of dissipation rate in dissipation with swell steepness is interpreted as a laminar to turbulent transition of the boundary layer, with a threshold Reynolds number of the order of 100,000. These observations of swell evolution open the way for more accurate wave forecasting models, and provide a constraint on swell‐induced air‐sea fluxes of momentum and energy.