Heat advection processes leading to El Niño events as depicted by an ensemble of ocean assimilation products

Abstract

The oscillatory nature of El Niño‐Southern Oscillation results from an intricate superposition of near‐equilibrium balances and out‐of‐phase disequilibrium processes between the ocean and the atmosphere. The main objective of the present work is to perform an exhaustive spatiotemporal analysis of the upper ocean heat budget in an ensemble of state‐of‐the‐art ocean assimilation products. We put specific emphasis on the ocean heat advection mechanisms, and their representation in individual ensemble members and in the different stages of the ENSO oscillation leading to EN events. Our analyses consistently show that the initial subsurface warming in the western equatorial Pacific is advected to the central Pacific by the equatorial undercurrent, which, together with the equatorward advection associated with anomalies in both the meridional temperature gradient and circulation at the level of the thermocline, explains the heat buildup in the central Pacific during the recharge phase. We also find that the recharge phase is characterized by an increase of meridional tilting of the thermocline, as well as a southward upper‐ocean cross‐equatorial mass transport resulting from Ekman‐induced anomalous vertical motion in the off‐equatorial regions. Although differences between data sets are generally small, and anomalies tend to have the same sign, the differences in the magnitude of the meridional term are seen to be key for explaining the different propagation speed of the subsurface warming tendency along the thermocline. The only exception is GECCO, which does not produce the patterns of meridional surface Ekman divergence (subsurface Sverdrup convergence) in the western and central equatorial Pacific.

Sensitivity of El Niño intensity and timing to preceding subsurface heat magnitude

“…This feature corresponds to the easternmost edge of the western Pacific warm pool, the maximum in zonal salinity gradient and the area of surface ocean convergence, downward motion and advection and subsurface divergence26484950. The correct simulation of these dynamical features is seen to be key for the reproduction of the ENSO oscillation and the transition between events of opposite sign, including the generation of the heat buildup in the western Pacific and the eastward propagation of the accumulated heat2147.…”

“…a lead time of 21 months with regard to the December maximum), chosen from a reference 100 year spin-up simulation. This stage of the ENSO oscillation is characterized by cold LN-like conditions in the tropical Pacific and the generation of a subsurface heat buildup in the western tropical Pacific (Supplementary Figure 1b,e;21,26). In each of these sets of ensemble experiments, the intensity of the subsurface warm anomaly was decreased (negative sign representing a discharge in heat content) or increased (positive sign or recharge) by ±20%, ±40%, ±60%, ±80% and ±100%.…”

“…This theory is based on the delay between anomalies in the longitudinally-averaged depth of the thermocline, and therefore the THC of the basin, and eastern Pacific Sea Surface Temperatures (SST). This paradigm, which has been thoroughly validated in the literature2223, describes a recharge (discharge) phase of Warm Water Volume (WWV) along the equator that leads EN (LN) events by about 2 to 3 seasons2124. This stage is in turn preceded by the tilting mode22526, which is characterized by an anomalous zonal gradient of the depth of the thermocline and is significantly associated with ENSO at long-lead times (e.g.…”

“…Mounting evidence has favored the view of ENSO as a slightly damped periodic oscillation modulated by stochastic noise1214181920, with its long-range potential predictability arising from the dynamics of the ocean subsurface321. The recharge oscillator1011 is currently the leading paradigm for the link between the Tropical Heat Content (THC) in the ocean subsurface and ENSO variability.…”

Timing of subsurface heat magnitude for the growth of El Niño events

“…El Niño-Southern Oscillation (ENSO) is the dominant variability mode at interannual timescales [Wang and Picaut, 2004], a primary source of seasonal-to-interannual climate predictability [Chen et al, 2004;Izumo et al, 2010;Barnston et al, 2012;Petrova et al, 2016], and a key factor explaining large-scale teleconnections [Ballester et al, 2011] with global impacts [Ballester et al, 2013[Ballester et al, , 2016a. The dynamics of ENSO arise from a complex interaction between the atmosphere and the ocean in the tropical Pacific [Bjerknes, 1969;McPhaden et al, 2006], involving upper and lower level atmospheric winds, ocean waves traveling along the equatorial thermocline, zonal redistribution of surface and subsurface ocean temperatures, and meridional recharge or discharge of the tropical ocean heat content [Suarez and Schopf, 1988;Jin, 1997;Picaut et al, 1997;Weisberg and Wang, 1997;Fedorov et al, 2015].…”