A new index for identifying different types of El Niño Modoki events

Abstract

El Nino Modoki events can be further classified into El Nino Modoki I and II in terms of their opposite impacts on southern China rainfall (Wang and Wang, J Clim 26:1322–1338, 2013) and the Indian Ocean dipole mode (Wang and Wang, Clim Dyn 42:991–1005, 2014). The present paper develops an index to identify the types of El Nino events. The El Nino Modoki II (MII) index is defined as the leading principle component of multivariate empirical orthogonal function analysis of the normalized El Nino Modoki index, Nino4 index and 850 hPa relative vorticity anomalies averaged near the Philippine Sea during autumn. The MII index exhibits dominant variations on interannual (2–3 and 4–5 years) and decadal (10–20 years) timescales. El Nino Modoki II events can be well identified by using the MII index value being larger than 1 standard deviation. Further analyses and numerical model experiments confirm that the MII index can portray the major oceanic and atmospheric features of El Nino Modoki II events. The constructed MII index along with previous ENSO indices can be used for classifying and identifying all types of El Nino events. Because of distinct impacts induced by different types of El Nino events, the implication of the present study is that climate prediction and future climate projection under global warming can be improved by using the MII index and other indices to identify the types of El Nino events.