Representing El Nino in coupled ocean-atmosphere GCMs: The dominant role of the atmospheric component

A systematic modular approach to investigate the respective roles of the ocean and atmosphere in setting El Nino characteristics in coupled general circulation models is presented. Several state-of-the-art coupled models sharing either the same atmosphere or the same ocean are compared. Major results include 1) the dominant role of the atmosphere model in setting El Nino characteristics ( periodicity and base amplitude) and errors (regularity) and 2) the considerable improvement of simulated El Nino power spectra - toward lower frequency - when the atmosphere resolution is significantly increased. Likely reasons for such behavior are briefly discussed. It is argued that this new modular strategy represents a generic approach to identifying the source of both coupled mechanisms and model error and will provide a methodology for guiding model improvement.