Anthropogenic changes in the Walker circulation and their impact on the extra-tropical planetary wave structure in the Northern Hemisphere

A robust change in the tropical circulation induced by anthropogenic warming in CMIP3 models is a weakening of the Walker circulation. This weakening affects the upper tropospheric divergence thereby modifying the propagation of Rossby waves from the tropics into the extra-tropics. It can be modeled by the barotropic vorticity equation forced with a Rossby wave source that is computed from the upper tropospheric divergence. Using the BVE as a diagnostic tool it is demonstrated for the CMIP3 models that the weakening of the Walker circulation has a significant impact on the extra-tropical planetary wave structure and to a large extent explains the projected changes in the mid tropospheric meridional wind in the CMIP3 models. The dominant response is a wave number five pattern similar to the circumglobal waveguide pattern. This analysis implies that a correct simulation of the Walker circulation and its response to anthropogenic changes are crucial for a correct simulation of the anthropogenic change in the extra-tropical planetary wave structure. Structure and intensity of the Walker circulation of the CMIP3 models show significant deviations from the Walker circulation as diagnosed from the ERA-interim and NCEP/NCAR reanalysis. Improving the simulation of the Walker circulation is a prerequisite to narrow the uncertainty in the projected anthropogenic change in the extra-tropical planetary wave structure.