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.
KeywordsClimate change Planetary wave structure Atmospheric dynamics Walker circulation
We thank Xueli Wang for her help in making the Taylor Diagrams. Critical remarks of Wilco Hazeleger and Geert Jan van Oldenborgh helped us to improve the manuscript. The remarks of two anonymous reviewers further substantially improved the manuscript. The ESSENCE project, lead by Wilco Hazeleger (KNMI) and Henk Dijkstra (UU/IMAU), was carried out with support of DEISA, HLRS, SARA and NCF (through NCF projects NRG-2006.06, CAVE-06-023 and SG-06-267). We thank the DEISA Consortium (co-funded by the EU, FP6 projects 508830/031513) for support within the DEISA Extreme Computing Initiative (http://www.deisa.org). The authors thank Andreas Sterl (KNMI), Camiel Severijns (KNMI), and HLRS and SARA staff for technical support.
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