Atmosphere-ocean coupled variability in the South Atlantic

Abstract

The climate variability of the South Atlantic region is determined from 40 yr (1953-92) of Comprehensive Ocean-Atmosphere Data Set monthly sea surface temperature (SST) and sea level pressure (SLP) data using the empirical orthogonal function (EOF) and the singular value decomposition (SVD) analysis methods. The EOF method is applied to each field separately, whereas the SVD method is applied to both fields simultaneously. The significance of the atmosphere-ocean interaction is revealed by a strong resemblance between individual (EOF) and coupled (SVD) modes of SST and SLP. The three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail. The first coupled mode, which accounts for 63% of the total square covariance, represents a 14-16-yr period oscillation in the strength of the subtropical anticyclone, accompanied by fluctuations of a north-south dipole structure in the SST The atmosphere-ocean coupling is strongest during the southern summer. The second coupled mode (20% of the total square covariance) is characterized by east-west shifts of the anticyclone center, in association with 6-7-yr period fluctuations of SST off the coast of Africa. The coupling depicted by this mode is weaker than that found in the first and third modes. The third coupled mode (6% of the total square covariance) is characterized by north-south displacements of the anticyclone, accompanied by SST fluctuations over a latitudinal band in the central South Atlantic. These oscillations occur on a relatively short interannual timescale (∼4 yr). As with the first mode, the atmosphere-ocean coupling is strongest during the southern summer. This mode is found to be temporally and spatially correlated with the El Niño-Southern Oscillation phenomenon. The statistical robustness of the results is tested by using a Monte Carlo approach, which indicates that the presented results are highly significant.