The Annual Cycle over the Tropical Atlantic, South America, and Africa*

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Abstract
The annual cycle over land can be thought of as being forced locally by the direct action of the sun and remotely by circulations forced by regions of persistent precipitation organized primarily by SST and, secondarily, by land. This study separates these two sources of annual variability in order to indicate where and when the remote effects are important. Two main sets of AGCM experiments were performed: one with fixed SST boundary conditions and seasonally varying insolation, another with fixed insolation and seasonally varying SST. For each experiment, the evolution of the annual cycle is presented as the differences from the reference month of March. The comparison of other months to March in the fixed-SST runs separates out the direct response of the land–atmosphere system to the annual insolation changes overhead. Similarly, the same comparison in the annual cycle of the fixed-insolation runs reveals the response of the land–atmosphere system to changes in SST. Over most of the domain, in... Abstract The annual cycle over land can be thought of as being forced locally by the direct action of the sun and remotely by circulations forced by regions of persistent precipitation organized primarily by SST and, secondarily, by land. This study separates these two sources of annual variability in order to indicate where and when the remote effects are important. Two main sets of AGCM experiments were performed: one with fixed SST boundary conditions and seasonally varying insolation, another with fixed insolation and seasonally varying SST. For each experiment, the evolution of the annual cycle is presented as the differences from the reference month of March. The comparison of other months to March in the fixed-SST runs separates out the direct response of the land–atmosphere system to the annual insolation changes overhead. Similarly, the same comparison in the annual cycle of the fixed-insolation runs reveals the response of the land–atmosphere system to changes in SST. Over most of the domain, in...