Space-time structure of long ocean swell fields

Matthias Delpey (1, 2) , Fabrice Ardhuin (1) , Fabrice Collard (2) , Bertrand Chapron (3)

1 SHOM - Service Hydrographique et Océanographique de la Marine
2 CLS - Collecte Localisation Satellites [Toulouse]
3 LOS - Laboratoire d'Océanographie Spatiale

The space-time structure of long period ocean swell fields is investigated, with particular attention given to features in the direction orthogonal to the propagation direction. This study combines space-borne Synthetic Aperture Radar (SAR) data with numerical model hindcasts and time series recorded by in situ instruments. In each dataset the swell field is defined by a common storm source. The correlation of swell height time series is very high along great circle paths, with a time shift given by the deep water dispersion relation of the dominant swells. Correlations are also high for locations situated on different great circles. Given the Earth radius R, we define the distance from the source Rα and the transversal angle β so that α and β would would be equal the the colatitude and longitude for a storm center on at the North pole. Outside of land influence, the swell height field at time t, Hss(α,β,t) is well approximated by a function Hss0(t-R α/Cg)/√(α sinα) times another function r2(β), where Cg is a representative group speed. r2 derived from SAR data is very broad, with a width at half the maximum that is larger 70{degree sign}. Land shadows introduce further modifications so that in r2 is a function of β and α. This separation of variables and the smoothness of the Hss field, allows the estimation of the full field of Hss from sparse measurements, such as wave mode SAR data, combined with one time series, such as provided by a single buoy.

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]

Matthias Delpey, Fabrice Ardhuin, Fabrice Collard, Bertrand Chapron. Space-time structure of long ocean swell fields. 2009. ⟨hal-00422578v1⟩