Abstract
Storms propagate over the ocean and create moving patches of strong
winds that generate swell systems. Here, we describe the dynamics of
wave generation under a moving storm by using a simple parametric model
of wave development, forced by a temporally- and spatially-varying
moving wind field. This framework reveals how surface winds under moving
storms determine the origin and amplitude of swell events. Swell systems
are expected to originate from locations different than the moving
high-wind forcing regions. This is confirmed by a physically-informed
optimization method that back-triangulates the common source locations
of swell using their dispersion slopes, simultaneously measured at five
wave-buoy locations. Hence, the parametric moving fetch model forced
with reanalysis winds can predict the displacement between the highest
winds and the observed swell source area when forced with reanalysis
winds. The model further shows that the storm’s peak wind speed is the
key factor determining swell energy since it determines surface wind
gradients that lead to the spatial convergence of wave energy into a
much smaller area than the wind fetch. This spatial wave energy
convergence implies enhanced wave energy dissipation in this focusing
area, slightly displaced from the maximum wind locations.
This analysis provides an improved understanding of fetches for
extra-tropical swell systems and may help to identify biases in swell
forecast models, air-sea fluxes, and upper-ocean mixing estimations.