Abstract
It is widely considered the El Nino-South Oscillation (ENSO) has several different types which can be simply classified as eastern Pacific (EP) type and central Pacific (CP) type. However, indices proposed so far can only characterize one single type of ENSO. In this paper, we develop a unified index which can characterize two types of ENSO simultaneously. The new index named as unified complex ENSO index (UCEI) is defined in the complex plane whose real part is NINO3 + NINO4 and imagine part is NINO3-NINO4. The modulus (r) and quadrants (θ) represent the ENSO strength and the ENSO types, respectively. Apart from the EP and CP types, the UCEI could further distinguish the MIX type of ENSO. Besides, the UCEI can capture the type-transforming processes within one ENSO event. Applying UCEI on historical events from 1950 to 2017 demonstrates the new index could be a very useful tool for the research of different types of ENSO.
Highlights
We develop a unified complex ENSO index (UCEI) which can characterize and distinguish EP and CP types of ENSO simultaneously
The El Nino type could be determined by the sign of N3 + N4 and N3 − N4
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Summary
To better descript how we construct the new index, we demonstrate the averaged SSTA distributions of 1997– 1998, 2004–2005 and 1991–1992 El Nino events as the examples that represent the EP, CP and MIX types, respectively (Fig. 1). Even though the classification is based on only N3 and N4 regions, the composite distributions show the complete and typical EP, CP and MIX patterns in the whole tropical Pacific including the secondary feature regions such as the far eastern Pacific, off-equatorial regions and western Pacific which are consistent with the observation results This demonstrates that the classification method of UCEI is very effective in distinguish different types of ENSO. We noticed CP ENSO only appeared 3 times during 1950–1977 but 9 times during 1977–2016 which demonstrates the tendency that CP type of ENSO appeared more frequently[3,6]
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