Subsurface Modes Of Tropical Pacific And Relations With Two Types Of El Nino

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,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.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 but the propagation is from the western to the central Pacific which is 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.Three subsurface modes are closely related with two types of El Nino,Eastern Pacific(EP)El Nino and Central Pacific(CP)El Nino.The first mode is related with EP El Nino while the second and third modes are both related with CP El Nino.The second and third modes are independent during the evolution of the CP El Nino which leads to the phenomenon that the CP El Nino tends to have two peaks of strength at different periods.We also find that the EP El Nino may cause the anti-phase modes after eight months of its mature period which implies the EP El Nino may cause CP La Nina to a certain extent.It is widely considered the El Nino has several different types.However,indices proposed so far can only characterize one single type of El Nino.In this paper,we develop a unified index which can characterize different types of El Nino simultaneously.The new index named as unified complex ENSO index(UCEI)is defined in the complex plane whose real part is NIN03+NIN04 and imagine part is NIN03-NIN04.The modulus(r)and quadrants(?)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.