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- Groundwater response to climate variability in Mediterranean type climate zones with comparisons of California (USA) and PortugalPublication . Malmgren, Katherine; Neves, Maria; Gurdak, Jason J.; Costa, Luis; Monteiro, JoséAquifers are a fundamental source of freshwater, yet they are particularly vulnerable in coastal regions with Mediterranean type climate, due to both climatic and anthropogenic pressures. This comparative study examines the interrelationships between ocean-atmosphere teleconnections, groundwater levels and precipitation in coastal aquifers of California (USA) and Portugal. Piezometric and climate indices (1989-2019) are analyzed using singular spectral analysis and wavelet transform methods. Singular spectral analysis identifies signals consistent with the six dominant climate patterns: the Pacific Decadal Oscillation (PDO), the El Nino-Southern Oscillation (ENSO), and the Pacific/North American Oscillation (PNA) in California, and the North Atlantic Oscillation (NAO), the Eastern Atlantic Oscillation (EA) and the Scandinavian Pattern (SCAND) in Portugal. Lower-frequency oscillations have a greater influence on hydrologic patterns, with PDO (52.75%) and NAO (46.25%) on average accounting for the largest amount of groundwater level variability. Wavelet coherences show nonstationary covariability between climate patterns and groundwater levels in distinct period bands: 4-8 years for PDO, 2-4 years for ENSO, 1-2 years for PNA, 5-8 years for NAO, 2-4 years for EA and 2-8 years for SCAND. Wavelet coherence patterns also show that coupled climate patterns (NAO+ EA- and paired PDO and ENSO phases) are associated with major drought periods in both the Mediterranean climate zones.
- The signature of NAO and EA climate patterns on the vertical structure of the canary current upwelling systemPublication . Georg, Tina; Neves, Maria; Relvas, PauloThe current study aims to analyse the vertical structure of the ocean during upwelling events using in situ and modelled data. Additionally, the influence of climate patterns, namely the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, on the vertical structure and their impact on the upwelling activity are assessed for a period of 25 years (1993-2017). The study focuses on the central part of the Canary Current (25-35 degrees N) with persistent upwelling throughout the year, with an annual cycle and the strongest events from June to September. Upwelling is determined using two different approaches: one index is calculated based on temperature differences between the coastal and the offshore area, and the other is calculated based on wind data and the resulting Ekman transport. Different datasets were chosen according to the indices. Stable coastal upwelling can be observed in the study area for the analysed time span, with differences throughout the latitudes. A deepening of the isothermal layer depth and a cooling of temperatures are observed in the vertical structure of coastal waters, representing a deeper mixing of the ocean and the rise of cooler, denser water towards the surface. During years of a positive NAO, corresponding to a strengthening of the Azores High and the Icelandic Low, stronger winds lead to an intensification of the upwelling activity, an enhanced mixing of the upper ocean, and a deeper (shallower) isothermal layer along the coast (offshore). The opposite is observed in years of negative NAO. Both effects are enhanced in years with a coupled, opposite phase of the EA pattern and are mainly visible during winter months, where the effect of both indices is the greatest. The study therefore suggests that upwelling activities are stronger in winters of positive North Atlantic Oscillation coupled with a negative East Atlantic pattern and emphasizes the importance of interactions between the climate patterns and upwelling.
- The impact of atmospheric teleconnections on the coastal aquifers of Ria Formosa (Algarve, Portugal)Publication . C. Neves, Maria; Costa, Luis; Hugman, Rui; P. Monteiro, J.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.