The Southern Annular Mode (SAM) in PMIP2 Simulations of the Last Glacial Maximum

Abstract:

The increasing trend of the Southern Annular Mode (SAM) in recent decades has influenced climate change in the Southern Hemisphere (SH). How the SAM will respond increased greenhouse gas concentrations in the future remains uncertain. Understanding the variability of the SAM in the past under a colder climate such as during the Last Glacial Maximum (LGM) might provide some understanding of the response of the SAM under a future warmer climate. We analyzed the changes in the SAM during the LGM in comparison to pre-industrial (PI) simulations using five coupled ocean–atmosphere models (CCSM, FGOALS, IPSL, MIROC, HadCM) from the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2). In CCSM, MIROC, IPSL, and FGOALS, the variability of the simulated SAM appears to be reduced in the LGM compared to the PI simulations, with a decrease in the standard deviation of the SAM index. Overall, four out of the five models suggest a weaker SAM amplitude in the LGM, consistent with a weaker SH polar vortex and westerly winds found in some proxy records and model analyses. The weakening of the SAM in the LGM was associated with an increase in the vertical propagation of Rossby waves in southern high latitudes.