“…On one hand, tropical processes, including the positive airsea interaction [1] and oceanic dynamics [2], play an important role for the ENSO occurrence. On the other hand, more and more recent studies have demonstrated that atmospheric and oceanic forcings outside the tropical Pacific also play crucial role in modulating onset, development, and amplitude of the ENSO events, such as the North Pacific Oscillation (NPO; an intrinsic atmospheric variability over the extratropical North Pacific) [3,4], Aleutian Low [5], Arctic Oscillation (AO; the dominant atmospheric variability over the extratropical Northern Hemisphere) [6,7,8], North Atlantic SST [9], East Asian monsoon variability, the atmospheric circulation anomalies in the Southern Hemisphere, and the SST variability over the Indian and Atlantic Oceans [10]. For instance, Vimont et al [3] and Chen et al [4] showed that, via seasonal footprinting mechanism, NPO can exert significant impacts on the outbreak of ENSO events.…”