The Influence of Large-Scale Climate Drivers on Synoptic-Scale Precipitation Variability in the Snowy Mountains, South-East Australia

Substantial variability of precipitation in the Snowy Mountains region of south-east Australia exists on intra- and inter annual timescales, with periods of flood and drought a classical feature of the region's climate. Precipitation variability places stress on the region's water resources which are critical for environmental flows along some of Australia's major river systems, irrigated agriculture, as well as hydro-electric power generation. Improved understanding of the causes of this variability is necessary to assess impacts of future climate on the region's water resources and accordingly their management.

We present a 112 year (1900 – 2012) precipitation climatology for the Snowy Mountains utilising long-term observational precipitation records and ECMWF's ERA-Interim, ERA-40 and ERA-20C reanalysis datasets. This multi-decadal approach has allowed variability of the dominant synoptic types responsible for precipitation in the region to be placed in context of long-term climate and the emerging global warming trend. The influence of large-scale climate drivers (“teleconnections”) on the frequency of synoptic types, and their precipitation is presented. Attention is paid to precipitation amounts responsible for generating inflow to rivers in the Snowy Mountains. The combined effects of key teleconnection phases on the region's hydroclimate through change in synoptic circulation patterns is also investigated, and highlights the influence of for example, the Pacific Decadal Oscillation (PDO) in modulating the higher frequency impacts of El Niño and La Niña.

Linear correlation and wavelet analyses of teleconnections and synoptic types confirm the tropical Pacific Ocean plays a major role in influencing precipitation bearing synoptic types to the region. For example, results show that the Southern Oscillation Index (SOI) is positively correlated with synoptic types that have moisture sources in both the tropical Pacific and Indian Oceans (r² between 0.19 and 0.32). Mean precipitation totals for each synoptic type vary significantly between La Niña and El Niño phases of the SOI.

The Indian Ocean is also shown to exert considerable influence as a major source of tropical-sourced moisture to southeast Australia. Statistically significant correlations exist between the Indian Ocean Dipole (IOD) and several synoptic types with dominant moisture pathways originating north-west of Australia (r² between -0.20 and -0.34). During the cool-season, the extra-tropical Southern Annular Mode (SAM) exerts more influence on synoptic types associated with the mid-latitude westerly wind belt, with moisture pathways originating in the Southern Ocean. Strong statistically significant negative correlations exist between embedded cold fronts and SAM during cooler months (r² = -0.43), with nearly three times as many embedded cold fronts occurring during negative SAM phases. The dominant warm-season synoptic type is significantly correlated with the positive phase of the SAM (r² = 0.24), occurring approximately 1.4 times more frequently, as well as with above average SSTs in the Tasman Sea (r² = 0.30). These relationships with SAM are likely linked to the southward movement of the subtropical ridge, possibly contributing to greater influx of tropical moisture.

Our analyses reveal that individual teleconnections account for a maximum of 43% (SAM influence on winter-time embedded cold fronts) and usually no more than 30% of frequency and precipitation variance according to season. Partial correlations between teleconnections are presented and reiterate that they rarely act independently. For example, lagged correlations between the SOI and IOD suggest that a positive SOI phase co-occurs with negative IOD phase at zero-lag. In addition, the SOI is shown to lead (lag) the IOD by 18 (17) months, with positive correlations in both instances. This is in agreement with the results of Izumo et al (2010) where a positive IOD precedes a La Niña event 18 months later, and demonstrates the interactions between the tropical Pacific and Indian oceans.

Wavelet analyses show the modulation of high frequency ENSO events by the PDO, whereby wet (dry) conditions are reinforced during the negative (positive) PDO periods that have occurred since 1900. These wetter and drier periods in the PDO and ENSO records are apparent in the precipitation and frequency time series of all synoptic types combined. In addition, an anti-phase relationship between the SOI and IOD (i.e. the concurrent co-occurrence of positive SOI and negative IOD phases) has occurred during (approximately) 1900-1930 and post 1970s. Both of these ‘wet' phases, and in particular a positive SOI, reinforce moisture transport along tropical north-east and north-west moisture pathways towards the Snowy Mountains region.

Warming trends in Tasman Sea surface temperatures, and increasingly positive trends in SAM – apparent in our trend analysis over recent decades, are projected to continue in the future, enhancing warm season precipitation in the Snowy Mountains. Conversely, embedded cold fronts associated with the mid-latitude westerly wind belt in winter are likely to undergo continued decline due to positive trends in SAM and southward migration of the subtropical ridge. In addition, increasing precipitation intensity – a widely expected consequence of the global warming signal - is apparent in our results.

Our results demonstrate the high multi-decadal variability of precipitation-bearing synoptic systems in the Snowy Mountains region of Australia, placing recent declines in precipitation in context of long-term climate fluctuations. The improved understanding of how teleconnections influence synoptic types that deliver precipitation to the Snowy Mountains provides knowledge essential for inter-annual water management along some of Australia's most important river systems.