Mechanisms of tropical Atlantic decadal Climate Decadal variability

Interannual variability of tropical Atlantic SST and the location of the ITCZ display a strong mutual statistical relationship and perplexing decadal variability. Understanding the mechanisms governing this variability is of utmost importance to the semi arid regions of northeast Brazil and other locations along the rim of the tropical Atlantic. From experiments with an ocean GCM driven with observed winds we concluded that the dynamics governing SST variability in the region can be expressed as a balance between heat flux forcing due to changes in windspeed, and damping due to advection by the mean meridional ocean current. Based on these considerations we construct a simple model to describe the evolution of SST anomalies zonally averaged across the basin. The model provides a reasonable simulation of the observed zonally averaged SST anomalies when forced with observed zonally averaged winds. Because the location of the ITCZ is so strongly linked with the gradient of SST across the equator, and based on other studies explaining this phenomenon as the atmospheric response to the SST, we devise a statistical model that predicts ITCZ associated meridional wind across the equator based on SST. This statistical model enables us to create a simple coupled system whereby SST anomalies in the trade wind regions north and south of the equator are forced by random, "white-noise" wind fluctuations, generating a coupled atmospheric wind response close to the equator. The coupling leads to a considerable "reddening" of the SST spectrum with large power on decadal time scales. We use the simple model to explore the necessary conditions for such reddening of the spectrum.