Spatial-temporal evolution of the low frequency climate variability in the tropical Atlantic

Simulated sea surface temperature (SST) and sea surface height (SSH) anomalies are used to diagnose the low frequency variability in the tropical Atlantic. Multichannel Singular Spectrum Analysis (M-SSA) is performed to detect oscillations at different time scales during the 1979-1993 study period. Three major oscillations are simultaneously found for both variables: one on semi decadal (5 years) time scale, and the two others on shorter interannual (1.5 to 3 years) time scale. At the lowest frequency, the SSH patterns show strong equatorial structures, while the SST patterns exhibit dipole structures. These spatio-temporal oscillations appear to be controlled by both the thermodynamic air-sea interaction and by the equatorial dynamics, and correspond to a meridional redistribution of the oceanic heat content. The 1.5-to-3-year SST and SSH oscillations are primarily associated with a dynamic response to fluctuations of the equatorial easterlies. The related SSH oscillation shows marked zonal propagation of waves, along and on either side of the equator. The 1.5-to-3-year oscillation pattern of the SST also exhibits dipole structures. These results show that the meridional SST mode and the equatorial dynamic mode, the two main modes of the climate variability in the tropical Atlantic, oscillate at different time scales, and are not confined to specific time scales (the lowest period for the SST meridional mode, the fastest period for the equatorial mode) as previously believed. This result from the M-SSA explains the good relationships recently founded between these modes from observations [Servain et al., 2000].