Teleconnections between NAO, EA, and SCAND and groundwater levels in Portugal

This study examines the teleconnections between the North Atlantic Oscillation (NAO), East Atlantic (EA), and Scandinavia (SCAND) climate patterns and groundwater level fluctuations in Portugal. Precipitation and piezometric records (1987-2016) from two aquifer systems, Leirosa- Monte Real in the north and Querenca-Silves in the south, are analyzed using wavelet transform methods and singular spectral analysis. The capacity of wavelet transform methods to analyze processes at various scales exposes not only the impacts of climate modes (NAO, EA and SCAND) but also the existence of complex transitive couplings among modes. Extreme events coinciding with coupled phases mark sharp boundaries in mode interaction patterns or sudden shifts in the time-frequency space. Groundwater levels over the analyzed time interval display consistent relationships with climate indices in distinct period bands and time windows. The strongest covariability occurs in the 6-10 years band for NAO, in the 2-4 years band for EA (especially after 1999) and in the 4-6 years band for SCAND (especially after 2005). Episodes displaying simultaneously multiple coherent relationships are associated with coupled phases among NAO, EA and SCAND. The results indicate that NAO, EA and SCAND patterns together are responsible for most (80%) of the inter-annual variability of groundwater levels in Portugal. NAO is the first leading pattern of variability, accounting for 40% and 60% of the total variance of groundwater levels in the north and south of the country, respectively. The joint contributions of EA and SCAND account for the remaining 40% of variability in the north and 20% of variability in the south, on average. Monthly groundwater levels averaged over years of positive and negative phases of climate indices provide additional insights on the influence of coupled patterns. Combined winter NAO-EA+ phases are associated with maximum groundwater levels while combined NAO+EA- phases are associated with minimum levels. Further understanding on how coupled climate modes influence groundwater storage can improve future projections of groundwater availability and guide integrated water resource management practices not only in Portugal but around the world. This work is supported by FCT- project UID/GEO/50019/2019 - IDL