Variability of the Southern Annular Mode and Southern Ocean Surface
Westerly Winds in E3SM
Abstract
Climate variability and change in the Southern Hemisphere (SH) is
influenced by the southern annual mode (SAM) and is closely related to
changes in the kinematic properties of the SH surface zonal winds. The
SAM and SH surface zonal winds have strong effects on the atmospheric
and oceanic circulation system. In this study we investigate the
variability and trend in the SAM and position and strength of the
surface zonal wind stress (TAUX), using two ensembles of simulations
covering the historical record from the Energy Exascale Earth System
Model (E3SM-HIST and AMIP) for 1979-2014. In addition, performance of
two CO2 forcing simulations from the E3SM (E3SM-1pctCO2 and 4xCO2) is
assessed to examine the sensitivity of the variability and changes in
the SAM and SH surface TAUX to climate forcing. In general, all E3SM
simulations tend to capture the dominant feature of the SAM pattern
reasonably well. The annual SAM index in the E3SM-HIST simulation shows
a significant increasing trend. These features are similar to the trends
in the strength (along with poleward shift in the position) of the
annual surface TAUX. For the climatological surface TAUX position and
strength, the two CO2 forcing simulations show slightly poleward
movement and stronger intensity, while the E3SM-HIST is equatorward and
weaker than observations. In the relationship between the SAM and
surface TAUX, we show that the SAM index exhibits a positive (negative)
relationship with the strength (position) of the surface TAUX in the
variability for all seasons and annual mean.