A Global Climatology of Wind–Wave Interaction

TLDR

Ocean waves normally act as a drag on surface wind, transferring momentum downward, but recent observations show that long‑wavelength swell can transfer momentum upward, accelerating near‑surface wind into a low‑level wave‑driven jet, a process predicted by the inverse wave age. The study aims to map the global distribution of inverse wave age to identify ocean regions that are typically wind‑driven or wave‑driven. This mapping is performed using the 40‑year ECMWF Re‑Analysis (ERA‑40) dataset. The wind‑driven wave regime dominates mid‑latitude storm tracks with high winds, while the wave‑driven wind regime is prevalent in the tropics with light winds and swell from higher latitudes; the inverse wave age also indicates the degree of wind‑wave coupling and highlights the nonequilibrium between local wind and wave fields, underscoring swell’s global importance.

Abstract

Abstract Generally, ocean waves are thought to act as a drag on the surface wind so that momentum is transferred downward, from the atmosphere into the waves. Recent observations have suggested that when long wavelength waves—which are characteristic of remotely generated swell—propagate faster than the surface wind, momentum can also be transferred upward. This upward momentum transfer acts to accelerate the near-surface wind, resulting in a low-level wave-driven wind jet. Previous studies have suggested that the sign reversal of the momentum flux is well predicted by the inverse wave age, the ratio of the surface wind speed to the speed of the waves at the peak of the spectrum. Data from the 40-yr ECMWF Re-Analysis (ERA-40) have been used here to calculate the global distribution of the inverse wave age to determine whether there are regions of the ocean that are usually in the wind-driven wave regime and others that are generally in the wave-driven wind regime. The wind-driven wave regime is found to occur most often in the midlatitude storm tracks where wind speeds are generally high. The wave-driven wind regime is found to be prevalent in the tropics where wind speeds are generally light and swell can propagate from storms at higher latitudes. The inverse wave age is also a useful indicator of the degree of coupling between the local wind and wave fields. The climatologies presented emphasize the nonequilibrium that exists between the local wind and wave fields and highlight the importance of swell in the global oceans.