Construction of new indices for the two types of ENSO events

Taking into account the different equatorial Pacific sea surface temperature (SST) patterns obtained from a combined EOF-regression analysis for the Eastern Pacific type of ENSO (EP-ENSO) and the Centre Pacific type of ENSO (CP-ENSO), this study constructs IEP (EP-ENSO Index) and ICP (CP-ENSO Index) for the two types of ENSO. The analysis is performed for the period of 1950-2011 using the monthly mean Met Office Hadley Centre SST analyses data. Some conclusions are as follows. Firstly, the two indices are nearly orthogonal and the skewness coefficients for IEP and ICP are 1.56 and -0.43, respectively. In addition, EP-ENSO and CP-ENSO show different periods. The former has two dominant periods, one is near the 2 year band and the other is near the 4-8 year band. The latter has two significant powers around the 2-4 year and 12 year band from 1965 to 1995. Therefore, low correlation, different leading periodicity and skewness are all desirable properties for the above defined indices to separate the EP and CP type of ENSO. Secondly, IEP and ICP indices can describe the location of SST anomalies in the mature period of the two types of El Nio and La Nia events. Thirdly, from the perspective of probability and statistics of random events, two types of El Nio and La Nia events are more strictly defined by using this pair of new indices, which could be applied to monitoring the two types of ENSO effectively and in real-time. Finally, the two new indices can also depict characteristics of the equatorial Pacific SST zonal propagation and evolution during ENSO events, their running lagged correlations capture different ENSO phase propagations and ENSO regime changes associated with the climate shift in 1976/1977. It is also noticed that most strong El Nio events belong to the EP type, but most strong La Nia events are to the CP type, and CP type of La Nia events tend to occur after the EP type of El Nio events.