Anthropogenic changes in the Walker Circulation and their impact on the extra-tropical planetary wave structure Reindert J. Haarsma and Frank Selten Royal Netherlands Meteorological Institute ABSTRACT: 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 barotropic vorticity equation 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 extend explains the obtained 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 (CWP). 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. Introduction In the extra-tropics the regional impact of anthropogenic warming is predominantly affected by the change in the structure of the large scale planetary waves. The position and amplitude of the planetary waves modify the large scale atmospheric surface circulation, temperature distribution and precipitation patterns. Anthropogenic climate projections differ substantially with respect to the simulated change in the planetary wave structure. This hampers the possibility to produce reliable climate projections. An important source of interannual variability of extra-tropical planetary waves resides in the tropics. Tropical variability affects the extra-tropical atmospheric circulation due to the generation of equivalent barotropic Rossby waves that propagate from the tropics into the extra-tropics (Hoskins and Karoly 1981; Hoskins and Ambrizzi 1993). It is therefore natural to investigate the role of the tropics in explaining the anthropogenic change of the planetary wave structure. One of the more robust changes in the tropical circulation induced by anthropogenic warming in CMIP3 models is a weakening of the Walker circulation. The physical cause of this weakening has been discussed by Vecchi and Soden (2007) and Held and Soden (2006). The weakening has also already been observed from data (Vecchi et al. 2006). Figure 1 shows for a set of twelve