Reinspecting two types of El Nio: a new pair of Nio indices for improving real-time ENSO monitoring

This study reinspects the dominant modes of different types of El Nio from the perspectives of monthly mean and seasonality using a combined technique referred as RC-REOF. Several features have been revealed. (1) The explained variances of eastern Pacific (EP) El Nio and central Pacific (CP) El Nio are comparable, in the ranges of 33-43 and 23-28 %, respectively. (2) This feature is more in line with the frequent occurrence of CP El Nio compared to the result from orthogonal EOF analysis in which El Nio Modoki explains a smaller variance of 11-12 %. (3) Both special patterns of EP El Nio and CP El Nio are of equatorial asymmetry that is often overlooked previously by the traditional Nio indices. Based on the features captured by the two leading RC-REOF modes, the authors propose a new pair of Nio indices referred to as Nio3b and Nio4b that have the following advantages: (1) simple calculation, (2) robust and stable relationship with the EP/CP El Nio modes, (3) more representative for Pacific decadal signals, (4) easier to distinguish ENSO types, and (5) without restriction by orthogonality, and others. The Nio3b and Nio4b indices are potentially useful for both scientific research and real-time monitoring of the two types of El Nio. Besides, an index namely Nio3.4b is also introduced to describe the hybrid ENSO events, with relatively equal covariance to EP El Nio and CP El Nio and with larger covariance at decadal time scale than index Nio3.4.