Observation of long first-mode baroclinic Rossby waves in the North Atlantic and their connection with wind forcing

Residual sea surface height anomaly fields are obtained over the North Atlantic Basin from TOPEX/Poseidon altimeter data. Evidence of westward propagating anomalies characterized as first-mode baroclinic Rossby waves is presented. The sea surface height signal is divided into two spectral bands, the first with nearly annual periods and the second with nearly semiannual periods. Values for the zonal and meridional components of the phase speed and period are obtained on a 5$\\sp\\circ$ x 5$\\sp\\circ$ grid. Wavelengths are obtained from the phase speed and period. Zonally averaged westward phase speeds vary between 1 and 27 km/day, between 50$\\sp\\circ N$ and 5$\\sp\\circ N,$ increasing southward. The zonally averaged zonal wavelengths associated with the annual band are in the range of 400 to 4600 km and for the semiannual band, 270 to 2500 km. 5$\\sp\\circ N.$ Comparison with results from numerical models suggests that the baroclinic waves observed at mid-latitudes are generated at the eastern boundary by fluctuations of the wind stress curl. This hypothesis is tested by using a numerical version of the White (1977) analytical model. The annual component of the Ekman pumping field derived from the ERS-1 scatterometer data is used to force a 1${1\\over 2}$ layer, quasi-geostrophic long wave model. Numerical results are compared to the altimeter derived observations at $12\\sp\\circ N,\\ 21\\sp\\circ N,\\ 28\\sp\\circ N,\\ 37\\sp\\circ N$ and 42$\\sp\\circ N.$ A significant agreement is found in all selected latitudes. The westward amplification of the baroclinic waves is correctly reproduced in the absence of topographic effects. Further numerical simulations indicate that direct forcing by the Ekman pumping away from the coast is the most important contributor to the observed wave field, with the coastal forcing as a secondary effect.