Ocean-atmosphere mechanisms explaining the heat buildup leading to El Niño events

Abstract:

In the past decades, steady progress has been made in the understanding and seasonal prediction of El Niño-Southern Oscillation (ENSO), however questions remain on the exact dynamical mechanisms leading to the onset of El Niño (EN) events. Several authors have highlighted how the subsurface heat buildup in the western tropical Pacific and the recharged phase in zonally-averaged heat content are intrinsic elements of ENSO variability, leading to those changes in zonal wind stress, sea surface temperature and thermocline tilt that characterize the growing and mature phases of EN.
Here we use an ensemble of ocean and atmosphere reanalyses to expose the dynamical mechanisms that contribute to the heat buildup and the recharged phase. The relative roles of (i) horizontal upper-ocean convergence and downwelling motion in the western equatorial Pacific, (ii) zonal heat advection by the Equatorial Undercurrent along the thermocline and (iii) subsurface meridional Sverdrup convergence in the central tropical Pacific are described and discussed. Our analyses suggest that the interannual eastward propagation of subsurface warm anomalies along the thermocline results from a combination of all these mechanisms, but their relative contribution varies with space and time. Our results highlight the importance of the three dimensional dynamic and thermodynamic structure of the tropical Pacific on the mechanisms explaining the subsurface heat buildup and subsequent development of EN events.