The Seasonal Footprinting Mechanism in the CSIRO General Circulation Models*

Abstract

An influence of midlatitude atmospheric variability on interannual ENSO and decadal ENSO-like variability is established and investigated in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled general circulation models (CGCMs). The effect of midlatitude atmospheric variability is felt in the Tropics via the previously hypothesized “seasonal footprinting mechanism” (SFM), in which a tropical circulation is forced during spring and summer by tropical SST anomalies that are generated by midlatitude atmospheric variability during the previous winter. The tropical circulation includes equatorial zonal wind stress anomalies that act as a stochastic forcing for the CSIRO CGCM's damped ENSO mode. Details of the SFM are investigated herein. A temporal analysis indicates that the SFM may explain 20%–40% of the model's interannual ENSO variability and nearly 70% of the model's decadal to interdecadal tropical variability. An analysis of the physical mechanisms that govern the SFM highlights the role of relaxed trade winds in producing tropical SST anomalies during winter, and identifies a weak positive coupled feedback between off-equatorial tropical SST anomalies and the atmospheric response to those anomalies.