Abstract:ENSO is the strongest interannual variability in our climate system.Recently,it has been found to vary as two types in spatial terms;the Central Pacific (CP) type,whose SST anomalies (SSTAs) are strongest in the central Pacific;and the Eastern Pacific (EP) type,characterized by SSTAs being strongest in the eastern equatorial Pacific.These two types of El Niño involve different teleconnections and climatic impacts,as the intensity and location of their associated SST-induced heating are different.Many studies have pointed out that CP El Niño events have increased in frequency in recent decades.Several kinds of indices have been designed for different types of El Niño events.For instance,the El Niño Modoki index (EMI) captures the second EOF pattern of equatorial Pacific SST anomalies.Another somewhat complicated index pair called the Central and Eastern Pacific Index (CPI/EPI),which is also an EOF-based index but with the SSTA related to the Niño1+2 or Niño4 index removed,has also been put forward.The most striking difference between the two types of El Niño is the longitudinal displacement of maximum anomalous SST along the equator in their mature stages.Motivated by this method of classification,we redefined two indices of SST anomalies,i.e.,ICP and IEP,for describing CP and EP El Niño/La Niña events and their interactions during boreal winter,by using both NCEP/NCAR reanalysis data and SST data from the Hadley Centre,for the period 1963-2013.This index pair was reconstructed based on the function for the CP pattern that changes with longitude and is perpendicular to the function for the EP pattern.Compared to other El Niño indices proposed previously,we found that the new indices can be used for better separating EP-type ENSO events from CP-type events,both spatially and temporally.The major features of air-sea coupling in the tropical Pacific related to CP-and EP-type events were investigated using the two new indices.It was found that these two distinct types of ENSO events demonstrate different periodic variation.Specifically,although they both exhibit 2-7- and 10-15-year oscillations in the tropical SSTA field,the decadal (10-15-year) variations for CP-type ENSO events are more dominant.It was also found that the anomalous warming center of CP-type El Niño appears in the central equatorial Pacific as expected,which is quite different from canonical EP-type El Niño events with their largest warm centers located in the eastern equatorial Pacific.Consequently,during CP-type events,the ascending branch of Walker circulation shifts farther west than normal,resulting in rainfall intensification mainly over the central tropical Pacific.However,when EP-type El Niño events occur,anomalous ascending motion is observed in the eastern equatorial Pacific,along with increased rainfall.Interestingly,during boreal winter,both types of El Niño can affect the Maritime Continent,but the CP type more strongly.Specifically,the Maritime Continent receives less than normal rainfall.Both of the reconstructed SSTA indices developed in this study are expected to be highly useful for monitoring ENSO events,and helpful in improving our understanding of the similarities and differences in the influence of CP-and EP-type ENSO events on climate variation at the global and regional (particularly East Asia) scale.