Structures and Mechanisms of the Northward Propagating Boreal Summer Intraseasonal Oscillation*

The spatial and temporal structures of the northward-propagating boreal summer intraseasonal oscillation (BSISO) are revealed based on the analysis of both the ECHAM4 model simulation and the NCEP-NCAR reanalysis. The BSISO structure and evolution characteristics simulated by the model bear many similarities to those derived from the NCEP-NCAR reanalysis. The most notable features are the remarkable meridional asymmetries, relative to the BSISO convection, in the vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center. The maximum specific humidity also shows a clear northward shift in the lower troposphere.