How Robust Is the Weakening of the Pacific Walker Circulation in CMIP5 Idealized Transient Climate Simulations?

The tropical overturning circulations are likely weakening under increased CO2 forcing. However, insufficient understanding of the circulations' dynamics diminishes the full confidence in such a response. Based on a CMIP5 idealized climate experiment, this study investigates the changes in the Pacific Walker circulation under anthropogenic forcing and the sensitivity of its weakening response to internal variability, general circulation model (GCM) configuration, and indexing method. The sensitivity to internal variability is analyzed by using a 68-member ensemble of the MPI-ESM-LR model, and the influence of model physics is analyzed by using the 28-member CMIP5 ensemble. Three simple circulation indices-based on mean sea level pressure, 500-hPa vertical velocity, and 200-hPa velocity potential-are computed for each member of the two ensembles. The study uses the output of the CMIP5 idealized transient climate simulations with 1% yr(-1) CO2 increase from preindustrial level, and investigates the detected circulation response until the moment of CO2 doubling (70 yr). Depending on the indexing method, it is found that 50%-93% of the MPI-ESM-LR and 54%-75% of the CMIP5 ensemble members project significant negative trends in the circulation's intensity. This large spread in the ensembles reduces the confidence that a weakening circulation is a robust feature of climate change. Furthermore, the similar magnitude of the spread in both ensembles shows that the Walker circulation response is strongly influenced by natural variability, even over a 70-yr period.