Climate of the Past www.clim-past.net 1814-9324 1814-9332 6 4 2010 10.5194/cp-6-415-2010 http://www.clim-past.net/6/415/2010/ http://www.clim-past.net/6/415/2010/cp-6-415-2010.html http://www.clim-past.net/6/415/2010/cp-6-415-2010.pdf 415 430 2010-07-05 The Southern Hemisphere semiannual oscillation and circulation variability during the Mid-Holocene D. Ackerley d.ackerley@niwa.co.nz J. A. Renwick National Institute of Water and Atmospheric Research Ltd, Private Bag 14901, Kilbirnie, Wellington, New Zealand The Paleoclimate Modelling Intercomparison Project (PMIP) was undertaken to assess the climatic effects of the presence of large ice-sheets and changes in the Earth's orbital parameters in fully coupled Atmosphere-Ocean General Circulation Models (AOGCMs). Much of the previous literature has focussed on the tropics and the Northern Hemisphere during the last glacial maximum and Mid-Holocene whereas this study focuses only on the Southern Hemisphere. This study addresses the representation of the Semiannual Oscillation (SAO) in the PMIP2 models and how it may have changed during the Mid-Holocene. The output from the five models suggest a weakening of the (austral) autumn circumpolar trough (CPT) and (in all but one model) a strengthening of the spring CPT. The effects of changing the orbital parameters are to cause warming and drying during spring over New Zealand and a cooling and moistening during autumn. The amount of spring warming/drying and autumn cooling/moistening is variable between the models and depends on the climatological locations of surface pressure anomalies associated with changes in the SAO. This study also undertakes an Empirical Orthogonal Function (EOF) analysis of the leading modes of atmospheric variability during the control and Mid-Holocene phases for each model. 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