Two new sea surface temperature anomalies indices for capturing the eastern and central equatorial Pacific type El Nino-Southern Oscillation events during boreal summer

Using monthly mean data from the Hadley Center and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, we have constructed two new indices I-CP and I-EP of boreal summer sea surface temperature anomalies (SSTA) in equatorial Pacific for period of 1963-2016 by employing methods including the six-order polynomial fitting and pattern correlations. The new indices I-CP and I-EP are almost independent of each other. Using these two indices, we are able to successfully capture the main features of central Pacific (CP) and eastern Pacific (EP) type SSTA variations in tropical Pacific in both spatial and temporal. The EP-type SSTA events vary on inter-annual timescales whereas CP-type events vary on both inter-annual and inter-decadal. For the CP-type events, stronger positive (negative) SST anomalies are observed in central tropical Pacific while weaker SSTA in the Maritime Continent (MC) and eastern equatorial Pacific during positive (negative) phase of index I-CP. However, For EP-type events, the stronger SSTAs appear in both the equatorial Pacific region east of dateline and Peruvian coastal ocean while weaker SSTAs in MC and western equatorial Pacific regions. Both the anomalous heating induced by CP- and EP-type SSTA may force the atmosphere to respond, resulting in different types of anomalous patterns of circulations, rainfall, and near-surface air temperatures. These results indicate that the new indexes are useful for our better investigating the CP- and EP-type summer SSTA events as well as their climate impacts, thereby providing a clue to monitoring and predicting strong SSTA events in equatorial Pacific.