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It is well known that El Nino events can induce worldwide impacts. However, the fact that strong El Nino events do not necessarily induce strong impacts raises a new research question: how to estimate the impacts of El Nino events in advance? To address this question, we studied the El Nino impacts from the perspective of complex network. By comparing the results from five El Nino events with distinct impacts, we found that the phase transition of the surface air temperature network over tropical Pacific is closely related to the El Nino impacts. This phenomenon was used to explain the less-than-expected impacts of the strong 2015/2016 El Nino, which is suggested more like a Central Pacific-Eastern Pacific mixed El Nino. To monitor the impacts objectively, we further proposed an index, which can be used in real-time operations.

Plain Language Summary It is well known that El Nino event has substantial impacts on climate, which can induce extreme events or even natural disasters. There are a variety of indices (e.g., Nino3.4 index) to measure the strength of the El Nino, but the fact that strong El Nino does not necessarily mean strong impacts calls for appropriate approaches to quantify the El Nino impacts. Here we proved a close relation between the El Nino impacts and the state of the surface air temperature field over the tropical Pacific. That is, if an El Nino event is not strong enough to significantly alter the state of the upper surface air temperature field, then its influences will not be able to be remarkably transported to remote regions via atmospheric bridges. Using complex network analysis, we quantified the state changes of the surface air temperature field and proposed a new index to measure the El Nino impacts. The new index well distinguished the Eastern Pacific and the Central Pacific El Nino and explained the less-than-expected impacts of the 2015/2016 El Nino. Since the calculations are based on past observations, the approach proposed here can be used in operations for objective estimation of the El Nino impacts.