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Fluctuations in groundwater level in the Ria Formosa coastal aquifers, southern Portugal, owe 80% of the variability to climate-induced oscillations. Wavelet coherences computed between hydraulic heads and the North Atlantic Oscillation (NAO) and East Atlantic (EA) atmospheric teleconnections show nonstationary and spatially varying relationships. The NAO is the most important teleconnection and the main driver of long-term variability, inducing cycle periods of 6–10 years. The NAO fingerprint is ubiquitous and it accounts for nearly 50% of the total variance of groundwater levels. The influence of EA emerges coupled to NAO and is mainly associated with oscillations in the 2–4-year band. These cycles contribute to less than 5% of the variance in groundwater levels and are more evident further from the coast, in the northern part of the system near the main recharge area. Inversely, the power of the annual cycle increases towards the shoreline. The weight of the annual cycle (related to direct recharge) is greatest in the Campina de Faro aquifer, where it is responsible for 20–50% of the variance of piezometric levels. There, signals linked to atmospheric teleconnections (related to regional recharge) are low-pass filtered and have periods >8 years. This behavior (lack of power in the 2–8-year band) emphasizes the vulnerability of coastal groundwater levels to multi-year droughts, particularly in the already stressed Quinta do Lago region, where hydraulic heads are persistently below sea level. [ABSTRACT FROM AUTHOR]

Highlights • Climate forcing explains 80% of the piezometric level variability in Portugal. • NAO is the leading pattern (40–60% of explained variability) with 6–10 year cycles. • Higher frequency fluctuations (2.5, 3.3 and 4.2 year periods) are linked to EA and SCAND. • EA is more influent in the north and SCAND in the south of the country. • Coupled NAO and EA phases are associated with extreme piezometric levels. Abstract Under the increasing risk of water scarcity, aquifer management strategies can take advantage of a deeper knowledge about the natural long-term fluctuations driven by climate patterns. This study examines the links between major large-scale atmospheric circulation modes and inter-annual to decadal oscillations in groundwater levels in Portugal. Precipitation and piezometric records (1987–2016) from two aquifer systems, Leirosa-Monte Real in the north and Querença-Silves in the south, are analyzed using wavelet transform methods and singular spectral analysis. Wavelet coherences computed between hydrologic time series and the North Atlantic Oscillation (NAO), East Atlantic (EA) and Scandinavia (SCAND) climate patterns show non-stationary relationships that are nonetheless consistent in distinct period bands. 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 (mainly after 2005). NAO is the mode playing the most relevant role with a stronger influence in the south (60% on average) than in the north (40% on average). The relatively higher frequency (<5 year period) contributions of EA and SCAND are difficult to set apart but their joint impact accounts for approximately 20% and 40% of the total variance of groundwater levels in the south and north of the country, respectively. Wavelet coherence patterns also expose transitive couplings between NAO, EA and SCAND. Often, coupled phases between climate modes mark abrupt transitions in synchronization patterns and shifts in the time-frequency domain. Extremes in groundwater storage coincide with anti-phase NAO and EA combinations: maximum piezometric levels occur during NAO-EA+ (coincidentally also SCAND+) phases while minimum levels occur during NAO+EA− phases. [ABSTRACT FROM AUTHOR]