A dynamic ocean-atmosphere model of the tropical Atlantic decadal variability

A linear model that couples an ocean mixed layer with a simple dynamic atmo sphere is used to study the mechanism for decadal variability over the trop ical Atlantic. An unstable mode with a dipole sea surface temperature (SST) pattern similar to observed decadal variability in the tropical Atlantic e merges in the time integration of the model. A wind-evaporation-SST feedbac k is responsible For the growth and oscillation of the unstable mode wherea s the mean state of the Atlantic climate is essential For maintaining the s patially quasi-standing dipole structure. The oscillation period ranges fro m several to a few tens of years and is sensitive to coupling strength. The oscillation is not self-sustainable as the realistic damping rate excee ds the growth rate. In response to white noise forcing, the model produces a red SST spectrum without a peak at finite frequencies. Therefore it is su ggested that the tropical dipole's preferred timescales, if any, arise from the forcing by or interaction with the extratropics. In a model run where the forcing is confined to the extratropics, a dipole SST pattern still dom inates the forcing-free Tropics, in support of the proposed linkage between the Tropics and extratropics.