Space-time structure of long ocean swell fields

• Published in: Journal of Geophysical Research
• Main Authors: Delpey, Matthias T., Ardhuin, Fabrice, Collard, Fabrice, Chapron, Bertrand
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Geophysical Union 2010
• Subjects: North Pole
• Online Access: https://archimer.ifremer.fr/doc/00025/13647/10881.pdf; https://doi.org/10.1029/2009JC005885; https://archimer.ifremer.fr/doc/00025/13647/

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 data set the swell field is defined by a common storm source. The correlation of swell height time series is very high along a single great circle path with a time shift given by the deep water dispersion relation of the dominant swells. This correlation is also high for locations situated on different great circles in entire ocean basins. Given the Earth radius R, we define the distance from the source R alpha and the transversal angle beta so that alpha and beta would be equal the colatitude and longitude for a storm centered on the North Pole. Outside of land influence, the swell height field at time t, H-ss(alpha, beta, t) is well approximated by a function H-ss,H-0(t - R alpha/C-g)/root(alpha sin(alpha)) times another function r(2) (beta), where C-g is a representative group speed. Here r(2) (beta) derived from SAR data is very broad, with a width at half the maximum that is larger than 70 degrees, and varies significantly from storm to storm. Land shadows introduce further modifications so that in general r(2) is a function of beta and alpha. This separation of variables and the smoothness of the H-ss field, allows the estimation of the full field of H-ss from sparse measurements, such as wave mode SAR data, combined with one time series, such as that provided by a single buoy. A first crude estimation of a synthetic H-ss field based on this principle already shows that swell hindcasts and forecasts can be improved by assimilating such synthetic observations.