Secular changes in New Zealand rainfall characteristics 1950-2009

Current climate model projections suggest a warming climate will correspond to higher precipitation rates, as well as more frequent extreme rainfall events. Using daily rainfall from eight stations around New Zealand, we compare changes across two thirty year periods (1950-1979 and 1980-2009). We focus on changes in total annual precipitation, numbers of days with recorded rainfall, the intensity of these rainfall events and the mean seasonal cycle at each station. Our preliminary results show a pattern of rainfall decrease in the north and rainfall increase in the southwest, with matching changes to the number of rain days at some stations. While changes to overall daily rainfall total distributions are generally small, there is a strong correlation between changes in total annual rainfall and the frequency of heavy rain days. Further work is needed to understand the internal characteristics of future precipitation patterns with continued climate change. However, these results support current scenarios for New Zealand of rainfall decreases in the northern half of the North Island and increases in the west and south of the South Island. 1. Introduction Increased moisture content in the lower atmosphere is a well-understood physical response to a warming climate, following the Clausius-Clapeyron relation between temperature and saturation specific humidity. For every degree of air temperature increase, there is maximum increase of about 7% in specific humidity (Mullan et al. 2008). Global climate model projections suggest that, for many regions, this will correspond to an increase in mean precipitation (Dai, 2006). Precipitation distributions are also expected to change, with increases in the frequency of extreme precipitation likely in most locations (IPCC 2012), as has already been observed (Alexander et al. 2006). However, under a changing climate, increases in the intensity of both heavy rainfall and long dry spells are expected in different regions and in different seasons, with contrasts between wet and dry regions, and wet and dry seasons expected to increase (IPCC, 2013). This is summarised by the rule of thumb that “the wet get wetter and the dry get drier”. Hence, understanding the characteristics of changing precipitation patterns requires analysis that extends beyond the evaluation of traditional measures such as annual rainfall totals (Sansom & Renwick, 2007). New Zealand lies at an intersection of multiple large-scale atmospheric influences, including the Southern Annular Mode (SAM, Renwick and Thompson 2006, Renwick 2011, Thompson et al. 2011) and the El Niño Southern Oscillation (ENSO, Mullan 1995, Salinger and Mullan 1999). In addition, complex topographic features and prevailing westerly winds fundamentally determine the country’s mean climate (Sturman and Tapper 2006). It is therefore difficult to apply to New Zealand a simple translation of projected global changes with a warming climate, especially for rainfall Harrington & Renwick: NZ Rainfall 1950-­‐2009 Weather and Climate,34,50-­‐59 51 patterns which are so easily influenced by the effects of orography and atmospheric