The Role of Anthropogenic Climate Change in the 2013 Drought over North Island, New Zealand

In the latter part of the 2012/13 austral summer season (January - March), the North Island of New Zealand endured its most severe drought in 41 years of instrumental records. When the 2013 drought is defined by potential evapotranspiration deficit, 34.2% of the North Island land surface experienced its highest recorded cumulative deficits, significantly greater than the 14.3% recorded for the previously severest drought (1997/98). The New Zealand Treasury estimates reduced agricultural production attributed to the drought cost the national economy at least US$1.3 billion, with continued impacts expected for another two years. To examine the role of anthropogenic climate change, we chose to consider the drought in terms of monthly mean sea level pressure (MSLP) anomalies across the North Island, and the maximum three month accumulation of dry days in each extended austral summer (November - April). Those models in the CMIP5 ensemble for which historical simulations compare well against observations are then quantitatively compared to simulations which excluded the impact of anthropogenic changes, such as greenhouse gases and ozone depletion. Results show the monthly pressure anomalies associated with the 2013 drought were on average 0.4 hPa higher as a result of anthropogenic climate change. However, no model was able to capture the extremity of the 3-month dry day index, likely a combination of bias in the models and the severity of the event relative to the size of the model ensemble. Using the observed 90th percentile of the dry day index as a more general threshold for drought, a 72% increase in the number of events was found, suggesting a change in drought risk. The human-induced shift in monthly dry days and MSLP were found to be very well correlated (R = 0.95), underlining that, in these models, the human-induced contribution to drought over New Zealand occurs through increased high pressure. Since the 2013 drought was not associated with record high pressures, natural internal variability in the climate system must have also played a strong role in the underlying causes of its severity.