El Nino without Southern Oscillation: A New View of Tropical Pacific Warming

Evidence is presented to show there are two distinct types of tropical Pacific warming: an eastern Pacific (EP) type that grows out of South American coast and has most of its sea surface temperature (SST) anomalies in the eastern Pacific, and a central-Pacific (CP) type that has most of its SST anomalies in the central equatorial Pacific and extended into the northeastern subtropical Pacific. The EP type is the conventional El Nino and is associated with a basin-wide change in the Walker circulation, i.e. Southern Oscillation. This type has a leading periodicity of 4-5 years and is generated by the thermocline variation described by the delayed oscillatory theory. But the CP type of El Nino does not come with a Southern Oscillation in the atmosphere or a basin-wide variation in the thermocline. Observational analysis and numerical experiments with NCAR CCSM3 are conducted to show that the CP type is generated by atmospheric forcing from Asian-Australian monsoon and subtropical atmosphere. This type is dominated by a biennial (~2 years) periodicity. The CP warming has a close association with the variation of the Hadley circulation. This EP/CP view of El Nino is used to explain why super El Nino may occur every 12-15 years and to project El Nino will shift from the EP type to the CP type in the future warming world.