This study evaluates the intraseasonal variability associated with the South American Monsoon System (SAMS) in 14 coupled general circulation models (GCMs) participating in the Inter-governmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of each model’s 20th century climate simulation are analyzed. We focus on the two dominant intraseasonal modes associated with SAMS: the 40-day mode and the 22-day mode.

The results show that in the southern summer (November-April), more than 10 of the 14 models simulate realistic seasonal mean precipitation over central-east Brazil, but display a southward shift of the Atlantic Intertropical Convergence Zone (ITCZ), and underestimate the seasonal mean precipitation over the northeast Brazil and the South Atlantic Convergence Zone (SACZ). Most of the models capture both the 40-day mode and 22-day mode around Uruguay, but only half of the models reproduce the 40-day mode over central-east Brazil. The models also tend to underestimate the total intraseasonal (10-90 day) variance, the 40-day mode variance and the 22-day mode variance. For the 40-day mode, 10 of the 14 models simulate to some extent the three-cell pattern around the south America, and six models reproduce its teleconnection with precipitation in south central Pacific, but only one model simulates the teleconnection with the MJO in the equatorial Pacific, and only two models capture its northward propagation from the south Atlantic to the SACZ. For the seven models with three-dimensional data available, only one model reproduces well the deep baroclinic structure of the 40-day mode. For the 22-day mode, only six of the 14 models capture its northward propagation from the SACZ to the Atlantic ITCZ. It is found that models with some form of moisture convective trigger tend to produce large variances for the intraseasonal modes.

Last Updated: 2010-06-04