Clim. Dyn. http://doi.org/r86 (2014)

Climate models have forecast that the Walker atmospheric circulation cell over the Pacific Ocean should weaken and move eastwards, but recent observations show the cell strengthening and shifting to the west. Analyses of numerical model simulations and observational data suggest that this recent behaviour could be the result of the recent predominance of La-Niña-like conditions in the Pacific region.

Tobias Bayr of GEOMAR, Germany, and colleagues used the CMIP3 and CMIP5 climate models to assess past and potential future changes in the Pacific Walker circulation cell from 1950 to 2100. The models project either strengthening or weakening of the Walker circulation over the Pacific, but all simulate an eastward shift relative to the 1950–1979 average, in contrast to observed trends. The difference between the models and observations seems to arise from the projected mean state of the equatorial Pacific Ocean. During periods when on average the eastern Pacific is anomalously cool and the western anomalously warm — analogous to the recent La-Niña-like conditions — the circulation is stronger and shifts westwards.

However, mean conditions more analogous to El Niño events, such as the projected trend for 2070–2100, favour a weakening and eastward shift of the circulation.