Climate Variability: changing weather patterns over New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The original intention of this thesis was to investigate Climate Change (CC), in particular the meteorological impacts of CC on New Zealand (NZ). Succinctly, “to understand what NZ’s future weather may entail”. However, as the research progressed it has led to the larger circulation and has highlighted the teleconnections that are present and the importance of the wider circulation and to NZ . It is apparent that the larger scale circulation needs to be considered in conjunction with, if not before, the synoptic scale. Thus, in order to understand NZ’s future weather first we must understand the Southern Hemisphere and the circulation within it.
CC is often described in a broad global scale and it is difficult to translate and relate these mechanisms into day to day weather terms, which have the advantage of being commonly understood. Synoptic Climatology (SC) can bridge this gap by simplifying the wide variety of weather into a small grouping of types, and thus can provide an understandable alternative.
To undertake this research an existing SC scheme known as the Kidson Types (KTs) was extended with the use of General Circulation Model (GCM) output. The KTs have been widely used in NZ, thus work detailing their future would be advantageous. The GCMs were able to reproduce the observed frequencies of occurrence of the KTs during the late 20th century. Future projections for the late 21st century surprisingly showed little change in annual type frequencies. To investigate this further a sensitivity study was undertaken, which revealed that the methodology was insensitive to annual type frequency change. The range of response from the GCM projections also inhibited determining significant changes in KT frequencies. Additionally, trend analysis using four realisations from one GCM noted both positive and negative trends in some of the types. This also highlights the difficulty in using GCM output, as a larger ensemble can diffuse results and in a small ensemble individual GCMs can unduly bias the results.
Further scrutiny of the KT was then undertaken. An investigation of the KTs to ascertain their influence in the wider circulation using the ERA Interim (ERA-I) reanalysis and trends within the KT using a long term reanalysis data set, the Twentieth Century Reanalysis (20CR). Due to the high year to year variability in the KTs, significant trends were only determined in the 20CR with a reduction in the Zonal Regime representing the occurrence of strong westerly flows over NZ. A composite analysis was also undertaken to evaluate the KTs within the Southern Hemisphere (SH). A positive pressure anomaly was detected far from the Kidson domain, which is defined over NZ, during the SW type. This motivated another study on SH Blocking.
Blocking is a large scale phenomena that can influence the paths of synoptic systems and thus potentially cause or exacerbate adverse weather events. Blocking is an area of climate research that requires further work, as there is a deficit of GCM studies in the SH. This study utilised a Persistent Positive Anomaly (PPA) methodology which is advantageous as the spatial pattern, latitude and longitude, of the Blocking Events (BEs) is determined. To our knowledge, this is the first study to use GCM output using the PPA methodology in the SH and this is also the first blocking study using Coupled Model Inter-comparison Project (CMIP)5 GCM output in the SH. A reduction of BEs was observed over the South Pacific Ocean (SPO) region during summer and spring, in the GCM projections between 2041-2070 and 2071- 2100. The Southern Annular Mode (SAM) has been suggested as an influence on blocking frequency in previous work and this relationship was studied. A high negative correlation between SAM + and BEs was observed in summer with the reanalysis and GCM historical output. This correlation was reduced in 21st century. However, further work is needed in this study in order to gain an understanding of the mechanisms and linkages between SAM and the BEs.