Linking Preconditioning to Extreme El Niño and ENSO Predictability

The contribution of the subsurface precursor, defined as the buildup of heat content in equatorial subsurface prior to El Niño – Southern Oscillation (ENSO), to ENSO amplitude and predictability has been unclear for some time. To address the issue, this study implements a careful experimental design to construct three March-initialized precursor ensembles using CCSM4, one ensemble with ENSO-neutral initial conditions, one with a warm precursor in the subsurface, and one with a cold precursor. The initialized warm, cold, or neutral precursors of each respective ensemble, although identically wind-forced, differ slightly due to intrinsic sources of “noise” in the ocean and atmosphere. The ensembles are then integrated fully-coupled to produce a distribution of outcomes per each type of initial condition. Results show that a precursor is not essential to produce moderate El Niño and the full range of La Niña events, whereas a warm precursor is mechanistically vital to generate extreme El Niño. The findings imply that extreme El Niño and the coldest La Niña events are fundamentally different. The full distribution of CCSM4 ENSO variability, extreme El Niño to the coldest La Niña, can be reproduced via two processes, the warm precursor and the coupled instability mechanism.

Presence of a warm (cold) precursor in the initial condition results in a warm (cold) shift and narrowing of the distribution of outcomes, thus increasing the predictability of El Niño (La Niña). Although the cold precursor is not necessary to produce La Niña, its presence in the initial condition increases La Niña predictability more than the warm precursor increases El Niño predictability. In other words, La Niña may be more predictable than El Niño.