The Seasonal Cycle of Significant Wave Height: Local vs. Remote Forcing

Surface gravity waves play a fundamental role in mediating air-sea interactions including fluxes of momentum, heat, gasses and energy between the ocean and atmosphere. Significant wave height (SWH) may provide insight about these interactions. In the Northern and Southern Hemispheres, wave heights have been observed to undergo an annual sinusoidal cycle in response to seasonal changes in storm patterns. In the California current region, local expansion fan wind events lead to deviations from the SWH seasonal cycle during boreal spring and summer. Other coastal regions where supercritical channel flows occur during the early summer include localities with similar coastal topography and atmospheric forcing to California, here designated as summer wind anomaly (SWA) regions. The seasonal variability of surface gravity waves is analyzed globally using SWH and wind speed data, from over two decades of satellite-derived SWH and wind data. The location where surface waves are generated is used to assess the drivers of wave characteristics. In general, the seasonal cycle of SWH peaks in boreal winter in the Northern Hemisphere and in austral winter in the Southern Hemisphere, with an abrupt transition that is not aligned with the equator and that shows evidence that islands can shield some regions from propagating waves. In SWA regions, the fraction of wave variability attributed to local wind events depends on local conditions. Global maps of probability of swell based on wave age confirm that the wave field in SWA regions is typically dominated by locally forced waves (wind-seas) during the spring and summer months. In those regions, modulation of the wave field by strong wind events may lead to enhanced wave breaking which could have implications for air-sea fluxes.