Quantifying highly variable air-sea momentum flux using wavelet analysis

Abstract

Surfacewind stress is a crucial driver of upper-ocean processes, impacting air-sea gas flux,wind-wave development, andmaterial transport.Conventional eddy covariance (EC)processing requires imposing a fixed averagingwindow on thewind velocity time series in order to estimate the downward flux of momentum.While this method has become the standard means of directlymeasuring the wind stress, the use of a fixed averaging interval inherently constrains one's ability to resolve transient signals that may have net effects on the air-sea interactions. Here we utilize the wavelet transform to develop a new technique for directly quantifying thewind stressmagnitude fromthewavelet coscalogram products.The time averages of these products evaluated at the scale ofmaximumamplitude are highly correlatedwith the EC estimates (R2 5 0.99; 5-min time windows), suggesting that stress is particularly sensitive to the dominant turbulent eddies. By taking advantage of the new method's high temporal resolution, transient wind forcing and its dominant scales may be explicitly computed and analyzed. This technique will allow for more general investigations into air-sea dynamics under nonstationary or spatially inhomogeneous conditions, such aswithin the nearshore region.