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Abstract
Atmospheric variability and ocean circulation in the Southern Hemisphere (SH) are considerably influenced by changes of the Southern Annular Mode (SAM) and SH surface westerly winds. In this study, we examine changes in the annual mean SAM and position and strength of the annual mean SH surface zonal wind stress in two different types of data sets (Historical and AMIP-type data) of Energy Exascale Earth System Model (E3SM) simulations and two reanalyses of ERA_Interim (ERA_Int) and NCEP/NCAR Reanalysis I (NCEP_R1). We also compare the annual variabilities and changes in two CO2 simulations (1pctCO2 and abrupt-4xCO2 data) of E3SM as anthropogenic forcing experiments for long-term periods. For the historical and AMIP data, the period over 1979-2014 is considered and the last 36 years of the entire period for each CO2 data are used. Both ERA_Int and NCEP_R1 show a clear shift toward positive values of the SAM index. The historical, AMIP, and 1pctCO2 simulations also show an increasing trend in the SAM index, while the 4xCO2 simulation indicates a negative SAM trend. The increase of the historical simulation appears higher than that seen in the two reanalyses. The reanalyses and E3SM simulations, except for 4xCO2, show positive (negative) trends in the strength (position) of the annual mean surface zonal wind stress. In general, the simulations of E3SM show a weaker annual mean strengthening trend than the reanalyses. It is shown that the SAM trend is closely related to the strength of the surface westerly wind stress. The climatological zonal-mean positions of westerly wind stresses in all E3SM simulations are similar to those in reanalyses, while the climatological zonal-mean strengths of the simulated wind stresses are overestimated.
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