Structure and variances of equatorial zonal circulation in a multimodel ensemble

The structure and variance of the equatorial zonal circulation, as characterized by the atmospheric mass flux in the equatorial zonal plane, is examined and intercompared in simulations from 9 CMIP3 coupled climate models with multiple ensemble members and the NCEPNCAR and ERA-40 reanalyses. The climate model simulations analyzed here include twentieth century (20C3M) and twenty-first century (SRES A1B) simulations. We evaluate the 20C3M modeled zonal circulations by comparing them with those in the reanalyses. We then examine the variability of the circulation, its changes with global warming, and the associated thermodynamic maintenance. The tropical zonal circulation involves three major components situated over the Pacific, Indian, and Atlantic oceans. The three cells are supported by the corresponding diabatic heating extending deeply throughout the troposphere, with heating centers apparent in the mid-troposphere. Seasonal features appear in the zonal circulation, including variations in its intensity and longitudinal migration. Most models, and hence the multi-model mean, represent the annual and seasonal features of the circulation and the associated heating reasonably well. The multimodel mean reproduces the observed climatology better than any individual model, as indicated by the spatial pattern correlation and mean square difference of the mass flux and the diabatic heating compared to the reanalysis based values. Projected changes in the zonal circulation under A1B forcing are dominated by mass flux changes over the Pacific and Indian oceans. An eastward shift of the Pacific Walker circulation is clearly evident with global warming, with anomalous rising motion apparent over the equatorial central Pacific and anomalous sinking motions in the west and east, which favors an overall strengthening of the Walker circulation. The zonal circulation weakens and shifts westwards over the Indian Ocean under external forcing, whereas it strengthens and shifts slightly westwards over the Atlantic Ocean. The forced circulation changes are associated with broad SST and atmospheric diabatic heating changes in the tropics. Linear trends of these forced circulation changes, as characterized by regional spatial maximum amplitudes of mass fluxes and their longitudes over the three oceans, are statistically significant at the 5 % level for 2000–2099 for the multimodel mean. However, wide differences of the trends are apparent across the models, because of both deficiencies in the simulation of the circulations in different models and the high internal variability of the circulations.