Spectral and empirical orthogonal function analysis of four years of European Center for Medium Range Weather Forecast wind stress curl over the North Atlantic Ocean

Abstract

The wind stress curl is calculated from a 4‐year European Center for Medium Range Forecast (ECMWF) data set over the North Atlantic. The mean wind stress curl appears as a northern positive cell of high amplitude and a large southern negative cell of lower amplitude. The separating zero contour shows large‐scale meanders and is highly variable at the seasonal and annual cycles. The rms of the wind stress curl is zonal and mainly contained in the short‐period band (less than 10 days). The frequency spectrum is nearly white north of 35°N. In the south, because of the lack of synoptic‐scale atmospheric storms, the spectrum shows a f−0.6 dependency at frequencies higher than 1/20 cpd. Wave number spectra are white for wavelengths larger than 1200 km and decay like k−4 at shorter wavelengths. The large‐scale variability is investigated by means of the empirical orthogonal function (EOF) analysis for the nonsynoptic time scales (periods of >10 days). The most significant EOFs display basin‐size oscillations with higher amplitude north of 35°N. The associated time series present high variability at the seasonal cycle and a larger amplitude in winter. These EOFs are related to important features of the North Atlantic climatology like the seasonal variability of the position of the Icelandic low, and the annual oscillation in intensity of the subpolar low, and the subtropical high. The first 10 EOFs are above the noise level. They are combined and compared to the initial data in order to produce realistic forcings for process studies in ocean modeling.