Linking interannual 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 timescales, 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 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 El Niño Southern Oscillation 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 intraseasonal 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.

Climate and rivers

“…Other modes of climate variability revealed as important for moderating hydrological processes include the Southern Annular Mode (SAM), the Indian Ocean Dipole (IOD), the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and the Atlantic Meridional Oscillation (AMO). In the case of the SAM, influences are mainly restricted to the Southern Hemisphere where for New Zealand, Li and McGregor () demonstrate both geographical and seasonal dependence of river flow variability on the SAM. Using proxies of river flow, the hydrological importance of SAM for drainage basin processes has also been demonstrated for the southern half of South America and Tasmania Australia (Allen et al, ; Araneo & Villalba, ; Lara et al, ; Munoz et al, ).…”

Hydroclimatology, modes of climatic variability and stream flow, lake and groundwater level variability

“…Coleman and Budlikov, 2013;Sheldon and Burd, 2014;Dery et al 2013;Gobena et al, 2015;Li and McGregor, 2017) point to asymmetric or non-linear associations between teleconnection indices and stream flow variability. This characteristic has been addressed explicitly by Frauen et al (2014) who conducted a number of atmospheric general circulation model simulations using idealised SST patterns representing eastern Pacific and central Pacific El Nino events of varying intensity in order to establish climate response.…”

“…A teleconnection index largely conspicuous by its absence in stream flow variability analyses is the SAM, as highlighted by Li and McGregor (2017). Having controlled in their analyses for an increasing positive trend in the SAM index and the confounding influence of ENSO, they describe a complex relationship between stream flow variability across New Zealand and the SAM, dependent on season and hydrological region.…”

“…Having controlled in their analyses for an increasing positive trend in the SAM index and the confounding influence of ENSO, they describe a complex relationship between stream flow variability across New Zealand and the SAM, dependent on season and hydrological region. In contrast to Li and McGregor (2017), who used measured stream flow, a number of workers have used tree ring chronologies as proxies of stream flow to analyse the influence of SAM on southern hemisphere stream flow beyond instrumental timescales. These studies have shown varying influences of SAM on constructed time series of streamflow for drainage basins across the southern half of South America and Tasmania, Australia (Allen et al, 2015;Araneo and Villalba, 2015;Lara et al, 2015;Munoz et al, 2016).…”

“…These indicate that high (low) inflows and lake levels are coherent with high (low) north west to south east pressure gradients and thus strong north westerly (weak south westerly) atmospheric flows and moisture transport over the South Island of NZ. With respect to understanding the possible one season ahead impact of SAM on these lakes, through SAM stream flow links, the work of Li and McGregor (2017) is relevant.…”