Eastern Pacific and central Pacific types of ENSO

In this study, I identify and contrast two types of El Nino Southern Oscillation (ENSO): one located in the eastern Pacific near the South American coast (i.e. EP-ENSO), and the other in the central Pacific close to the date line (i.e. CP-ENSO). The EP-ENSO possesses the properties of the canonical ENSO and is related to thermocline variations. It is characterized by basin-wide surface and subsurface evolution, coupled with Southern Oscillation and dominated by 2 to 4 year timescale. In contrast, the CP-ENSO is characterized by in-situ evolution and local atmosphere-ocean coupling, and is likely driven by atmospheric forcing. From an upper-ocean heat budget analysis, the CP-ENSO is found to be related to the trade wind forcing associated with the variations of the northern subtropical high. Wind-induced surface heat flux forcing first warms up the upper ocean temperature in the Northeastern Subtropical Pacific. The SST anomalies later spread toward the central equatorial Pacific through heat-flux forcing and vertical advection processes, are further enhanced by zonal advection, and eventually terminate by surface heat flux. The budget results suggest a possible interaction pathway between the north-eastern subtropics and central equatorial Pacific. The CP-ENSO is dominated by a quasi-biennial (∼2.5 yr) periodicity that is also found in subtropical high and Asian-Australian monsoon variability. The possible linkage between CP-ENSO and monsoon variability is demonstrated by an Indian Ocean-decoupled experiment using a coupled GCM. The biennial CP-ENSO in the model is significantly reduced when the Indian Ocean coupling is turned off to weaken the biennial monsoon variability. This study suggests the existence of a distinct CP-ENSO that is a result of interactions among Asian-Australian monsoon, northern subtropical Pacific and central equatorial Pacific.