The atmospheric seasonal cycle of the North Atlantic region is dominated by meridional movements of the circulation systems: from the tropics, where the West African Monsoon and extreme tropical weather events take place, to the extratropics, where the circulation is dominated by seasonal changes in the jetstream and extratropical cyclones. Climate variability over the North Atlantic is controlled by various mechanisms. Atmospheric internal variability plays a crucial role in the mid-latitudes.

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... However, El Niño-Southern Oscillation (ENSO) is still the main source of predictability in this region situated far away from the Pacific. Although the ENSO influence over tropical and extra-tropical areas is related to different physical mechanisms, in both regions this teleconnection seems to be non-stationary in time and modulated by multidecadal changes of the mean flow. Nowadays, long observational records (greater than 100 years) and modeling projects (e.g., CMIP) permit detecting non-stationarities in the influence of ENSO over the Atlantic basin, and further analyzing its potential mechanisms. The present article reviews the ENSO influence over the Atlantic region, paying special attention to the stability of this teleconnection over time and the possible modulators. Evidence is given that the ENSO-Atlantic teleconnection is weak over the North Atlantic. In this regard, the multidecadal ocean variability seems to modulate the presence of teleconnections, which can lead to important impacts of ENSO and to open windows of opportunity for seasonal predictability. Atmosphere 2016, 7, 87 2 of 19 the energy balance in the tropics and therefore the global atmospheric circulation. This alteration triggers the appearance of atmospheric teleconnections in regions far away from the tropical Pacific (e.g., the North Atlantic). The present article is mainly focused on the North Atlantic eastern sector, a region in which the ENSO teleconnection is still poorly understood and controversial. This connection, when established, takes place through different mechanisms that involve the divergent and non-divergent circulation over the troposphere and the stratosphere [1]. In this regard, the atmospheric seasonal cycle of the North Atlantic acts as a modulator of the ENSO teleconnection, whose strength is sensitive to the time of year. The North Atlantic Oscillation (NAO) dominates climate variability over the extra-tropical North Atlantic. This oscillation is defined as an exchange of mass between subtropical and subpolar regions and is related to changes in the position of the eddy-driven jet-stream ([2], and papers therein). The NAO exhibits different global signatures, being related to a Tropical Northern Hemisphere pattern (TNH) in winter [3] and to a more internal and northward shifted configuration in summer [4] . In winter, the NAO is also highly influenced by the polar stratosphere [5] . The NAO has profound impacts on a variety of ecological processes and, consequently, on species' abundance and dynamics [6] . Although most of the variability associated with the NAO is explained by internal and non-linear processes [7-9], some predictability has been found from ENSO [1] and the tropical North Atlantic. The importance of tropical versus extratropical ocean-atmosphere interactions on the NAO has not been fully determined yet ([2]; and papers therein). Although traditionally the scientific community has focused on the ENSO influence over the North Pacific and the American Continent [10] , several studies have also suggested a consistent and statistically significant impact of ENSO over the North Atlantic and Europe (NAE; [11] [12] [13] [14] [15] [16] ). The ENSO signature over the NAE is stronger during late winter and early spring, when El Niño (La Niña) is typically accompanied by a weakening (strengthening) of the subtropical high surface pressure system associated with a negative (positive) NAO-like pattern. In winter, ENSO influences the polar stratosphere, by weakening (strengthening) the polar vortex during El Niño (La Niña) conditions [11, 17] , a result confirmed by model simulations and long reanalysis data records (e.g., [18, 19] ). Although the anomalous circulation associated with ENSO reaches Europe in a stationary way regardless of the period considered [3], recent observation-and model-based studies suggest that the impact on rainfall is not stationary in time and can be modulated by multidecadal changes in the oceanic conditions [20] [21] [22] . The time-varying influence of ENSO over Europe could also be due to multidecadal changes in the NAO structure and strength [23] [24] [25] [26] . In the Tropical North Atlantic (TNA), the so-called West African Monsoon (WAM) is also affected by ENSO during boreal summer (June to September). The WAM affects mainly the sub-Saharan semi-arid region of Sahel, whose economy and sustainability are particularly vulnerable to fluctuations in the amount of rainfall. This region experiences a strong seasonal rainfall regime between the months of July and September, which is related to a semi-annual shift of the Intertropical Convergence Zone (ITCZ) and the temperature contrast between the Sahara and the Gulf of Guinea. Although General Circulation Model (GCM) experiments show that ENSO produces a decrease (increase) of rainfall under its positive (negative) phase, its impact has been found to be non-stationary in time [27, 28] , being the relationship stronger in some decades and weaker in others [29] . Also in the tropics, and in relation to the African Easterly Waves (which also modulate rainfall during the WAM), hurricanes have ENSO as the primary source of predictability at interannual scales [10, 30] . These extraordinary energetic weather systems often produce very strong winds, heavy precipitation, and storm surge events over the tropical areas they affect [31] [32] [33] [34] . Nevertheless, the stability in time of ENSO impact on Atlantic hurricane activity has been little analyzed in the literature so far. Thus, a common feature for European and tropical North Atlantic regions is that ENSO teleconnection seems to be non-stationary in time. In the present paper, we provide further insight into the ENSO impact on the anomalous rainfall over these regions under a non-stationary approach. This article discusses the available literature, trying to conciliate different results that have emerged