The eastward shift of the Walker Circulation in response to global warming and its relationship to ENSO variability
- 1.2k Downloads
This study investigates the global warming response of the Walker Circulation and the other zonal circulation cells (represented by the zonal stream function), in CMIP3 and CMIP5 climate models. The changes in the mean state are presented as well as the changes in the modes of variability. The mean zonal circulation weakens in the multi model ensembles nearly everywhere along the equator under both the RCP4.5 and SRES A1B scenarios. Over the Pacific the Walker Circulation also shows a significant eastward shift. These changes in the mean circulation are very similar to the leading mode of interannual variability in the tropical zonal circulation cells, which is dominated by El Niño Southern Oscillation variability. During an El Niño event the circulation weakens and the rising branch over the Maritime Continent shifts to the east in comparison to neutral conditions (vice versa for a La Niña event). Two-thirds of the global warming forced trend of the Walker Circulation can be explained by a long-term trend in this interannual variability pattern, i.e. a shift towards more El Niño-like conditions in the multi-model mean under global warming. Further, interannual variability in the zonal circulation exhibits an asymmetry between El Niño and La Niña events. El Niño anomalies are located more to the east compared with La Niña anomalies. Consistent with this asymmetry we find a shift to the east of the dominant mode of variability of zonal stream function under global warming. All these results vary among the individual models, but the multi model ensembles of CMIP3 and CMIP5 show in nearly all aspects very similar results, which underline the robustness of these results. The observed data (ERA Interim reanalysis) from 1979 to 2012 shows a westward shift and strengthening of the Walker Circulation. This is opposite to what the results in the CMIP models reveal. However, 75 % of the trend of the Walker Circulation can again be explained by a shift of the dominant mode of variability, but here towards more La Niña-like conditions. Thus in both climate change projections and observations the long-term trends of the Walker Circulation seem to follow to a large part the pre-existing dominant mode of internal variability.
KeywordsGlobal warming Walker Circulation Zonal atmospheric circutlation ENSO variability Asymmetry of ENSO Changes in the modes of variability
We acknowledge the World Climate Research Program’s Working Group on Coupled Modeling, the individual modeling groups of the Climate Model Intercomparison Project (CMIP3 and CMIP5) and ECMWF for providing the data sets. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through project DO1038/5-1, the ARC Centre of Excellence in Climate System Science (CE110001028), the ARC project “Beyond the linear dynamics of the El Nino Southern Oscillation” (DP120101442), the RACE Project of BMBF, the NACLIM Project of the European Union and the Australian Climate Change Science Program. We thank Hanh Nguyen for providing the code to calculate the zonal stream function and Hardi Bordbar, Sabine Haase and the anonymous reviewers for discussion and useful comments.
- Bayr T, Dommenget D (2013b) Comparing the spatial structure of variability in two datasets against each other on the basis of EOF-modes. Clim Dyn. doi: 10.1007/s00382-013-1708-x
- DiNezio PN, Clement A, Vecchi GA, Soden B, Broccoli AJ, Otto-Bliesner BL, Braconnot P (2011) The response of the Walker circulation to Last Glacial Maximum forcing: Implications for detection in proxies. Paleoceanography 26. doi: 10.1029/2010PA002083
- DiNezio PN, Vecchi GA, Clement AC (2013) Detectability of changes in the Walker circulation in response to global warming. J Clim 130114154537002, doi: 10.1175/JCLI-D-12-00531.1
- Haarsma RJ, Selten F (2012) Anthropogenic changes in the Walker circulation and their impact on the extra-tropical planetary wave structure in the Northern Hemisphere. Clim Dyn. doi: 10.1007/s00382-012-1308-1
- Meng Q, Latif M, Park W, Keenlyside NS, Semenov VA, Martin T (2011) Twentieth century Walker Circulation change: data analysis and model experiments. Clim Dyn 1–17. doi: 10.1007/s00382-011-1047-8
- Philander S (1990) El Niño, La Niña, and the southern oscillation. Academic, San DiegoGoogle Scholar
- Power SB, Kociuba G (2011) What caused the observed 20th century weakening of the Walker circulation? J Clim. 110629145325000. doi: 10.1175/2011JCLI4101.1
- Simmons A, Uppala S, Dee D, Kobayashi S (2007) ERA-Interim: new ECMWF reanalysis products from 1989 onwards. ECMWF Newsl 110:25–35Google Scholar
- Yu J-Y, Kim ST (2011) Reversed spatial asymmetries between El Niño and La Niña and their linkage to decadal ENSO modulation in CMIP3 models. J Clim. 110506133859050. doi: 10.1175/JCLI-D-11-0024.1
- Yu B, Zwiers FW (2010) Changes in equatorial atmospheric zonal circulations in recent decades. Geophys Res Lett 37:L05701Google Scholar
- Yu B, Zwiers FW, Boer GJ, Ting MF (2012) Structure and variances of equatorial zonal circulation in a multimodel ensemble. Clim Dyn. doi: 10.1007/s00382-012-1372-6