Swell-induced atmospheric Ekman dynamics in the marine boundary layer: analytical modeling and field observations

The historical physical description of the coupled atmosphere-ocean system focuses on Ekman dynamics (EK-D) driven by wind stress on the ocean. This thesis addresses a lack of symmetry by hypothesizing the existence of an "inverse" Ekman dynamics. In this scenario, ocean swell waves act as a forcing mechanism, transferring energy into the lower Marine Atmospheric Boundary Layer. Although literature confirms swell can act as a source of momentum, the possibility of an inverse EK-D has been rarely explored. A theoretical model, HB-M (Hanley and Belcher, 2008), modified the Ekman equations by including a swell-induced stress component with the opposite sign to the turbulent one. This model predicts a Low Level Jet with a wind speed maximum near the interface and a spiral pattern opposite to the usual atmospheric EK-D. However, HB-M was purely theoretical, lacking observational support. This thesis provides the first experimental evidence for wave-induced Ekman dynamics using data from the WFIP3 campaign, conducted south of Martha's Vineyard Island, MA, USA. The dataset includes wind profiles (from sonic anemometers and lidars) and high-frequency stress measurements, allowing decomposition into turbulent and wave-induced fractions. Analysis on specific days revealed significant swell-induced stresses on the atmosphere. Concurrently, the predicted spiral pattern was observed in the wind profiles. Fitting these profiles to the HB-M model yielded satisfactory results: the model accurately reproduced the measured profiles and provided wave direction estimates consistent with observations. Conversely, a thermal wind balance model failed in reproducing the same data. In conclusion, these analyses support the existence of a wave-induced Ekman dynamics generated by swell in the Atmospheric Boundary Layer. Future studies should compare these findings with measurements from locations farther offshore, where the influence of coastal processes is minimized.