The Temporal Autocorrelation Structure of Sea Surface Winds

The temporal autocorrelation structures of sea surface vector winds and wind speeds are considered. Analyses of scatterometer and reanalysis wind data demonstrate that the autocorrelation functions (acf) of surface zonal wind, meridional wind, and wind speed generally drop off more rapidly in the middle latitudes than in the low latitudes. Furthermore, the meridional wind component and wind speed generally decorrelate more rapidly than the zonal wind component. The anisotropy in vector wind decorrelation scales is demonstrated to be most pronounced in the storm tracks and near the equator, and to be a feature of winds throughout depth of the troposphere. The extratropical anisotropy is interpreted in terms of an idealised kinematic eddy model as resulting from differences in the structure of wind anomalies in the directions along and across eddy paths. The equatorial anisotropy is interpreted in terms of the kinematics of large-scale equatorial waves and small-scale convection. Modelling the vector wind fluctuations as Gaussian, an explicit expression for the wind speed acf is obtained. This model predicts that the wind speed acf should decay more rapidly than that of at least one component of the vector winds. Furthermore, the model predicts a strong dependence of the wind speed acf on the ratios of the means of vector wind components to their standard deviations. These model results are shown to be broadly consistent with the relationship between the acfs of vector wind components and wind speed, despite the presence of non-Gaussian structure in the observed surface vector winds.