Temporal and spatial characteristics of sea surface height variability in the North Atlantic Ocean

We investigate the spatial and temporal variability of sea surface height (SSH) in the North Atlantic basin using altimeter data from October 1992–January 2004. Our primary aim is to provide a fuller description of such variability, particularly that associated with propagating signals. We also investigate possible correlations between SSH variability and climate indices. We first investigate interannual SSH variations by deriving the complex empirical orthogonal functions (CEOFs) of altimeter data lowpass-filtered at 18 months. We determine the spatial structure of the leading four modes (both in amplitude and phase) and also the associated principal components (PCs). Using wavelet analysis we derive the time-varying spectral density of the PCs revealing when particular modes are strongest between 1992–2004. The spatial pattern of the leading CEOF, comprising 30% of the total variability, has a 5-year period. Signal propagation with a 5-year period is also observed in the Labrador Sea. The second mode, with a dominant 3-year signal, has strong variability in the eastern basin. We next focus on the Azores subtropical frontal region. The leading mode (35%) is strong in the south and east of this region. The second mode (21%) has a near-zonal band of low variance between ~22°–27° N sandwiched between two regions of high variance. We then lowpass filter the altimeter data at a cutoff of 30 days, instead of 18 months, in order to retain signals associated with propagating baroclinic Rossby waves. The leading mode is the annual steric signal, around 46% of the SSH variability. The third and fourth CEOFs, 11% of the remaining variability, are associated with westward propagation which is particularly dominant in a ''waveband'' between 32°–36° N. No significant cross-correlation is found between the North Atlantic Oscillation index and the amplitude of the leading two principal components of interannual SSH variability. The East Atlantic Pattern index, however, is correlated with the principal components of the two leading modes of SSH variability, particularly with PC2 in the Azores subtropical frontal region. Further investigation of forcing mechanisms is suggested using hindcasts from ocean general circulation models.