Tracing the Effects of NAO and AO Signals on Specific Runoff, Snow Depth, Precipitation and Air Temperature in the western Carpathians

Long-distance relationships between climate phenomena, known as teleconnections, provide a useful framework for linking pressure anomalies over the North Atlantic and the Arctic to regional variability of hydroclimatic series. Among these, the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) are widely recognized as dominant modes influencing winter climate in Europe, yet their impacts on hydrological variables are known to be temporally non-stationary. We examined the influence of the NAO and AO on the intra-annual variability of monthly specific runoff, snow depth, precipitation, and air temperature. The analysis draws on long-term observations from 26 small to medium-sized catchments spanning the western Carpathians and the adjacent Pannonian Plain, covering a wide range of hydroclimatic and physiographic conditions.

To capture scale-dependent relationships, the Continuous Wavelet Transform (CWT) with a Morlet basis was applied to the hydroclimatic time series and the AO/NAO indices. Wavelet coherence was used to identify statistically significant time-frequency regions of co-variability, which are subsequently employed as spectral filters to reconstruct the oscillation-related components of the hydroclimatic signals. The relative contribution of each climate mode is quantified using a signal-to-noise ratio (SNR), defined as the ratio b between the variance of the coherent, climate-related component and the residual background variability.

The results reveal pronounced temporal intermittency in the influence of both NAO and AO, with the strongest impacts occurring during winter and spring high-flow periods. The NAO generally exhibits a stronger and more spatially coherent imprint, particularly during winter and early spring, whereas the AO contribution is weaker and more episodic. The identified non-stationary fingerprints of NAO and AO highlight the scale-dependent and time-varying nature of teleconnection controls on runoff generation and snow accumulation and may have direct implications for runoff predictability, water-resources management, and the interpretation of long-term hydroclimatic variability in Central Europe under a changing climate.

Acknowledgements

This work was supported by the Slovak Research and Development Agency, under the contract No. APVV-23-0332; VV-MVP-24-0208, and the VEGA grant agency under contract No. VEGA 2/0115/25, VEGA 1/0657/25.