A unified complex index to characterize two types of ENSO simultaneously.

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.

Leading modes of tropical Pacific subsurface ocean temperature and associations with two types of El Niño.

Using empirical orthogonal function (EOF) analysis of the monthly tropical Pacific subsurface ocean temperature anomalies (SOTA) from 1979 to 2014, we detected three leading modes in the tropical Pacific subsurface temperature. The first mode has a dipole pattern, with warming in the eastern Pacific and cooling in the western Pacific, and is closely related to traditional El Niño. The second mode has a monopole pattern, with only warming in the central Pacific subsurface. The third mode has a zonal tripole pattern, with warming in the off-equatorial central Pacific and cooling in the far eastern Pacific and western Pacific. The second and third modes are both related to El Niño Modoki. Mode 1 is linked with a Kelvin wave that propagates from the central to the eastern Pacific and is induced by the anomalous westerlies that propagate from the western to the central Pacific. Mode 2 is also linked with a Kelvin wave that propagates from the western to the central Pacific induced by the enhancement of westerlies over the western Pacific. Mode 3 is linked with a Rossby wave that propagates from the central to the western Pacific driven by the anomalous easterlies over the eastern Pacific.