Simulation of the tropical Atlantic interannual variability

With the method of Singular Spectrum Analysis (SSA), we identified in the equatorial Atlantic sector a dominant quasi-biennial oscillation with a period of 28~30 months from the observational sea surface temperature (SST) and surface wind stress records. The oscillation exhibits features characteristic of equatorial coupled modes, and its correlation with ENSO is small. The SST and surface wind stress obtained in a 38-year long coupled general circulation model (CGCM) simulation without interannual variability in SSTs prescribed outside the tropical Atlantic also captures a leading oscillation with a quasi-biennial time scale (26~28 months). The associated modal distribution bears analogous coupled features with that of the observed. These results suggest that the observed quasi-biennial oscillation in the equatorial Atlantic result from coupled atmosphere-ocean interactions internal to this ocean basin. Outside the equatorial sector, Atlantic SST anomalies are only moderately correlated with ENSO. The largest correlation (r=+0.39) is found in the tropical North Atlantic Ocean for a 5-month lag. The application of SSA method on SSTs of this region also shows an oscillatory mode with an ENSO-like time scale. However, this oscillation does not pass the significance test, indicating that the imprint of ENSO onto this remote ocean region may not be systematic, and that the strength of the inter-connection may vary from event to event. For the 1987-88 Atlantic warm event, the ensemble CGCM simulation clearly demonstrates that the 1986-87 ENSO event played a role in forcing the Atlantic anomalies. In the simulation, when ENSO developed in the equatorial Pacific near the end of 1986, a thermally induced Rossby wave train propagated into the North America-North Atlantic sector with a pattern in the geopotential height field resembling the observed Pacific-North America (PNA) pattern. One of the centers of action of this pattern was a cyclonic circulation over North Atlantic and southeast United States. Such a cyclonic circulation is consistent with a weakened North Subtropical High and the associated northeasterly winds. The relaxed winds induced a reduction in the surface heat fluxes, particularly the latent heat flux component. About a season later, in spring 1987, significant surface warming developed in the North Atlantic, especially in the western side of the ocean. Along the equator, in the meanwhile, ENSO-induced changes in the atmospheric circulation altered the strength of the global Walker circulation. The associated surface zonal wind stress became anomalously strong over the equatorial Atlantic, which acted on the ocean to pile up warm waters toward the west since the onset of ENSO near the end of 1986. From spring 1987, the zonal circulation adjusted again in response to the reinforced SST warming in the equatorial Pacific. The anomalous surface easterlies relaxed and then changed to westerly anomaly. Warm waters subsequently spread eastward across the basin.