Abstract
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.
Highlights
The generation of palaeoclimate-proxy data reconstructions has allowed us to understand how the climate of the past has differed from the present day
Proxy-model intercomparison establishes the ability of climate models to simulate past climatic change, which is an essential step in evaluating the General Circulation Models (GCMs) used to simulate future climate
Having identified how the synoptic regimes differed seasonally in the MH compared to the PI in the GCMs, we can infer how the seasonal climate may have been different in New Zealand during the Mid-Holocene
Summary
The generation of palaeoclimate-proxy data reconstructions has allowed us to understand how the climate of the past has differed from the present day. Changes in the synoptic type frequency are associated with distinct regimes that generate regionally disparate precipitation and temperature anomalies in New Zealand climate over months, seasons, and longer periods (demonstrated by Lorrey et al, 2007). The aims of this study, 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 comparing those results to palaeoclimate proxy data within the New Zealand sector to see if the models and proxies agree. A further test of the synoptic type frequency changes during the Mid-Holocene, as determined by the model ensemble, is to compare the results to past conditions established by palaeoclimate regime reconstructions for the same time period. While these differences are an important consideration, they are unlikely to affect the results of the analysis presented (see Renssen et al, 2006)
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