1National Institute of Water and Atmospheric Research Ltd, Private Bag 14901, Kilbirnie, Wellington, New Zealand
2National Institute of Water and Atmospheric Research Ltd, Private Bag 99940, Newmarket, Auckland 1149, New Zealand
3Climate Change Research Centre, University of New South Wales, Sydney, Australia
4GKSS Research Center, Institute for Coastal Research, Paleoclimate Group, Max Planck Strasse 1, 21502 Geesthacht, Germany
5Bristol Research Initiative for the Dynamic Global Environment, Geographical Sciences, University of Bristol, Bristol, BS8 1SS, UK
6Center for Climate System Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8568, Japan
7Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ward Yokohama, 236-0001, Japan
8Department of Geography, University of Sheffield, UK
*currently at: Monash Weather and Climate, Monash University, Clayton, Victoria, 3800, Australia
Abstract. Diagnosing the climate of New Zealand from low-resolution General Circulation Models (GCMs) is notoriously difficult due to the interaction of the complex topography and the Southern Hemisphere (SH) mid-latitude westerly winds. Therefore, methods of downscaling synoptic scale model data for New Zealand are useful to help understand past climate. New Zealand also has a wealth of palaeoclimate-proxy data to which the downscaled model output can be compared, and to provide a qualitative method of assessing the capability of GCMs to represent, in this case, the climate 6000 yr ago in the Mid-Holocene.
In this paper, a synoptic weather and climate regime classification system using Empirical Orthogonal Function (EOF) analysis of GCM and reanalysis data was used. The climate regimes are associated with surface air temperature and precipitation anomalies over New Zealand. From the analysis in this study, we find at 6000 BP that increased trough activity in summer and autumn led to increased precipitation, with an increased north-south pressure gradient ("zonal events") in winter and spring leading to drier conditions. Opposing effects of increased (decreased) temperature are also seen in spring (autumn) in the South Island, which are associated with the increased zonal (trough) events; however, the circulation induced changes in temperature are likely to have been of secondary importance to the insolation induced changes. Evidence from the palaeoclimate-proxy data suggests that the Mid-Holocene was characterized by increased westerly wind events in New Zealand, which agrees with the preference for trough and zonal regimes in the models.