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

This study reinspects the dominant modes of different types of El Niño 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 Niño and central Pacific (CP) El Niño 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 Niño compared to the result from orthogonal EOF analysis in which El Niño Modoki explains a smaller variance of 11–12 %. (3) Both special patterns of EP El Niño and CP El Niño are of equatorial asymmetry that is often overlooked previously by the traditional Niño indices. Based on the features captured by the two leading RC-REOF modes, the authors propose a new pair of Niño indices referred to as Niño3b and Niño4b that have the following advantages: (1) simple calculation, (2) robust and stable relationship with the EP/CP El Niño modes, (3) more representative for Pacific decadal signals, (4) easier to distinguish ENSO types, and (5) without restriction by orthogonality, and others. The Niño3b and Niño4b indices are potentially useful for both scientific research and real-time monitoring of the two types of El Niño. Besides, an index namely Niño3.4b is also introduced to describe the hybrid ENSO events, with relatively equal covariance to EP El Niño and CP El Niño and with larger covariance at decadal time scale than index Niño3.4.