Abstract:
Understanding variation in streamflow is of paramount importance to water resource managers, because producing accurate short-term to medium-term forecasts will benefit society at large. To accomplish this goal we need to investigate climatic drivers that govern interannual streamflow variability. The Southern Annular Mode (SAM), regional climate and weather regimes are important drivers in New Zealand. The analysis presented here of these drivers is conducted for the seasons with the largest climatological contrasts (i.e., summer versus winter). Since the study mainly concerns the influence of SAM on streamflow, influences from ENSO are controlled for by employing a stepwise multiple linear regression technique. By using Principal Component Analysis and Cluster Analysis, six regions of comparable flow dynamics are identified. This hydrological regionalization provides a spatial framework for exploration of streamflow-SAM associations. Subsequent analysis of the regions suggests there are regional and seasonal differences in response to SAM. Southland and Canterbury, for example are negatively and positively correlated with SAM, respectively, highlighting the importance of orographic effects. Bay of Plenty streamflow demonstrates a negative relationship to SAM in summer and a positive relationship in winter. In terms of regional climate, streamflow parallels rainfall totals and shows a negative relationship to temperature, with an exception in the Tasman/Marlborough region in winter where anomalously warm air temperature triggers snowmelt thus elevating flow volume. Of three general weather regimes, the trough type tends to produce higher streamflow; the zonal and the blocking types tend to suppress streamflow, although noticeable exceptions exist. The streamflow responses to Kidson’s weather regime are consistent to an extent with the regime induced climatological patterns described by others. Notwithstanding this, an important finding is that the association is highly dependent on the phase of SAM.
