On regional model simulations of climate change over New Zealand
Regional model simulations of New Zealand climate have been performed for present-day conditions and for an equilibrium doubling of atmospheric CO2 concentration. The regional model was nested within ten years output of a nine-level atmospheric general circulation model with a mixed-layer ocean. Statistics of the regional model's representation of present-day surface climate generally compare well with observations. However, it is difficult to assess the accuracy of model simulations of radiatively important clouds, since there is little reliable observational data to compare with. Moreover, although the modelled present-day surface climate is acceptable, results are sensitive to specifications of land surface parameters that are not well known at high resolution. To advance regional climate modelling for New Zealand, there is a need to better describe the characteristics of the New Zealand land surface and the climatology of clouds in the New Zealand/Tasman Sea region. Climate change simulations show a decrease in the strength of the westerly winds over New Zealand, as is typical of equilibrium simulations of CO2 doubling. Mean temperatures over New Zealand rise at around 80% of the global mean rate. Largely through orographic influences, the decrease in the westerlies leads to precipitation and low-level cloudiness increases in eastern regions, combined with a decreased diurnal temperature range (DTR). Wintertime changes in low-level cloudiness and DTR are well correlated, as has been found in the observational record. However, changes in both model fields may be more strongly related to modelled circulation changes than to each other. There is a need to apply regional climate modelling to the output of a number of global models, particularly to coupled ocean-atmosphere transient simulations, to assess the likely range of regional outcomes under future global climate change.