In 2017, I began working with Dr. Sarawh T. Gille on a project investigating the intra-annual variability of significant wave height (SWH) in regards to SWH relationship with local wind speed (WSP). Significant wave height (SWH) provides insight into the interactions between the ocean and the atmosphere. In the Northern and Southern Hemispheres, wave heights typically undergo a sinusoidal annual cycle, with larger SWH in winter in response to seasonal changes in high-latitude storm patterns that generate equatorward propagating swell waves. In the California coastal region, local expansion fan wind events occur in boreal spring and summer, leading to local deviations from the expected hemispheric-scale annual cycle in SWH. Other coastal regions with early summer expansion fan wind events occur in locations with coastal topography and atmospheric forcing similar to California. These regions, here are designated as summer wind anomaly regions (SWARs). In this study intra-annual variability of surface gravity waves is analyzed globally using two decades of satellite-derived SWH and wind speed data. The dividing line between swell originating in the Northern and Southern Hemispheres is slightly south of the equator and varies by location depending on a variety of factors, including topography and island shadowing effects. In SWARs, the fraction of wave variability attributed to local wind events varies depending on local conditions. Global maps of probability of swell based on wave age confirm that the wave-field in SWARs is typically dominated by locally forced waves during the spring and summer months. The phasing of the annual cycle is used to quantitatively identify SWARs. Wave-field modulation by strong wind events may lead to enhanced wave breaking which could have implications for air-sea fluxes.
Figure 1: (A) CCMP2 WSP Annual cycle phase map highlighting anomalous WSP phase regions, (B) Annual cycle phase difference between WSP and SWH with fraction of the world’s oceans that experience anomalous local surface winds and phase difference respectively (i.e. the annual cycle of WSP is approximately 4 to 6 months out of phase with the expected phase of the WSP set by hemispheric large-scale storm systems). Grid points with a phase value less than or equal to 2 standard deviations are considered insignificant and masked white.