Dynamics of the Atlantic Meridional Mode, and implications for tropical cyclones

The Atlantic Meridional Mode (AMM) is the dominant mode of large-scale ocean-atmosphere interaction in the tropical Atlantic, and has a substantial influence on tropical cyclone activity. While the AMM is easily identified in the observational record, it also emerges as a dominant growing structure in simple models of the tropical ocean and atmosphere. Tropical meridional mode-like structures in these simple models propagate equatorward and westward and are sensitive to forcing in subtropical latitudes (both of these features are consistent with AMM variations in nature). The propagation and transient growth of meridional-mode structures depend on the meridional and zonal phasing of the atmospheric and oceanic components of the coupled disturbance, and can be explained via the steady, forced atmospheric quasi-geostrophic potential vorticity equation. Growth and propagation characteristics of meridional mode-like variations are also sensitive to the mean surface winds (which affect latent heat fluxes) and surface moisture convergence (which affect the atmospheric sensitivity to SST variations). The growth and propagation characteristics of meridional mode variations in these coupled models provides an explanation for why meridional modes emerge as dominant modes of ocean-atmosphere variability in the tropics, and provide guidance for understanding connections between the subtropics and tropics. The characteristics of observed and modeled meridional mode variations will be compared, and implications for seasonal tropical cyclone activity will be discussed.