Linking Inter-annual River Flow River Variability Across New Zealand to the Southern Annular Mode, 1979-2011

Abstract

River flow constitutes an important element of the terrestrial branch of the hydrological cycle, yet knowledge regarding the extent to which its variability, at a range of time scales, is linked to a number of modes of atmospheric circulation is meagre. This is especially so in the Southern Hemisphere where strong candidates, such as El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM), for influencing climate and thus river flow variability can be found. This paper presents the results of an analysis of the impact of the SAM on winter and summer river flow variability across New Zealand, purposefully controlling for the influence of ENSO and the tendency for the SAM to adopt a positive phase over the last 10 – 20 years. Study results, based on identifying hydrological regions and applying circulation-to-environment and environment-to-circulation approaches commonly used in synoptic climatology, reveal a seasonal asymmetry of the response of river flow variability to the SAM; winter flows demonstrate a higher degree of statistical association with the SAM compared to summer flows. Further, because of the complex orography of New Zealand and its general disposition normal to zonal flows of moisture bearing winds, there are intra-seasonal spatial variations in river flow SAM associations with clear rain shadow effects playing out in resultant river flow volumes. The complexity of SAM river flow associations found in this study warns against using indices of large scale modes of atmospheric circulation as blunt tools for hydroclimatological prediction at scales beyond hydroclimatological regions or areas with internal hydrological consistency.