Long baroclinic Rossby waves detected by TOPEX/POSEIDON

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 phase speeds are obtained from the analysis of the autocorrelation matrices of the residual surface height anomaly in three dimensions, on a 5 degrees x 5 degrees grid, Periods are calculated by least squares fitting a sinusoidal function to the time series at each grid point. Zonal and meridional wavelengths are obtained from the phase speed and period and are least squares fit to the data as a consistency check. Zonally averaged westward phase speeds vary between 1 and 27 km/d, between 50 degrees and 5 degrees N, increasing southward. Phase speeds are directed to the southwest north of approximately 25 degrees N and to the northwest south of this latitude. The sea surface height signal is divided into two spectral bands, the first with periods between 280 and 450 days and the second with periods between 120 and 280 days. For the first band the average of the periods taken over the whole basin is 370 days. In the second band a signal of period between 6 and 7 months is evident also throughout the basin. The zonally averaged zonal wavelengths associated with the first band are in the range of 400-4600 km between 50 degrees and 5 degrees N and for the second band are in the range 270-2500 km between 45 degrees and 5 degrees N. Longer wavelengths are found close to the southern boundaries. Shorter wavelengths are found in the northern portion of the basin. Comparison with results from numerical models suggests that the baroclinic waves observed at midlatitudes are generated at the eastern boundary by fluctuations of the wind stress curl. Waves observed south of 20 degrees N support the hypothesis of direct wind forcing.