The Southern Hemisphere semiannual oscillation and circulation variability during the Mid-Holocene

Abstract

Abstract. 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. Despite the seasonal changes, the overall month by month and interannual variability was simulated to have changed little from the Mid-Holocene to present.

Using synoptic type analysis to understand New Zealand climate during the Mid-Holocene

“…Application of the same anomalies to the Mid-Holocene follows uniformitarian principles that suggest similar changes would result in climate regime shifts over longer time periods. Ackerley and Renwick (2010) found that the strongest changes in the SH circulation occurred in SON and MAM, and were associated with the sea-surface temperature response to the changes in insolation shown in Fig. 2.…”

“…The aims of this study, therefore, are to provide a first step for assessing SH circulation patterns as represented by a set of GCMs that cover the Mid-Holocene, using the synoptic climate regime classification method of Kidson (2000) and then comparing those results to palaeoclimate proxy data within the New Zealand sector to see if the models and prox-ies agree. Ackerley and Renwick (2010) have already shown that the seasonal SH mid-latitude circulation is sensitive to differences in the Earth's orbital paramaters during the Mid-Holocene, which cause changes in seasonal insolation (see Fig. 2).…”

“…Many global modelling groups have produced valuable GCM simulations for representations of the Mid-Holocene (6000 yr before present, ybp) and the Last Glacial Maximum (21 000 ybp), which are stored as part of the Palaeoclimate Modelling Intercomparison Project (PMIP, see Joussaume and Taylor, 2000). Despite the large archive of model data (see Braconnot et al, 2007, for an overview of the PMIP2 model results), there have been relatively few studies of the Southern Hemisphere (SH) climate (for examples see Ackerley and Renwick, 2010;Rojas et al, 2009;Rojas and Moreno, 2011) from the PMIP2 data. There have, however, been some studies of the SH westerlies from other model simulations (Wagner et al, 2007;Wardle, 2003).…”

Accelerated greenhouse gases versus slow insolation forcing induced climate changes in southern South America since the Mid-Holocene

“…Analyses of PMIP2 simulations suggest the occurrence of significant changes in global and regional atmospheric circulation, precipitation and temperature, including the Southern Hemisphere. In fact, Ackerley and Renwick (2010) describe decreased (increased) sea level pressure during the MH compared to PI times over polar-subpolar regions and inverse conditions at middle latitudes of the Southern Hemisphere during austral spring (autumn) with a spatial pattern of anomalies similar to the structure of the Southern Annular Mode. These anomalies of atmospheric circulation were also detected by Rojas and Moreno (2011).…”

The role of the Southern Hemisphere semiannual oscillation in the development of a precursor to central and eastern Pacific Southern Oscillation warm events

“…The sea level pressure (SLP) in high and lower latitudes of the Southern Hemisphere has long been known to be dominated by a second harmonic because the circumpolar trough moves meridionally twice a year in the three ocean basins [ van Loon, 1967; van Loon and Jenne, 1972;Raphael, 2004;Ackerley and Renwick, 2010]. The SLP north of 50°S thus has lowest values in May-June-July (MJJ) and November-December-January (NDJ) when the trough of low pressure moves northward, while farther south around Antarctica the minima are in February-March-April (FMA) and August-September-October (ASO) when the trough contracts and intensifies [ van Loon, 1967van Loon, , 1972; van Loon and Rogers, 1984].…”