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

Abstract

Despite extensive ongoing efforts on improving the long-term prediction of El Niño-Southern Oscillation, the predictability in state-of-the-art operational schemes remains limited by factors such as the spring barrier and the influence of atmospheric winds. Recent research suggests that the 2014/15 El Niño (EN) event was stalled as a result of an unusually strong basin-wide easterly wind burst in June, which led to the discharge of a large fraction of the subsurface ocean heat. Here we use observational records and numerical experiments to explore the sensitivity of EN to the magnitude of the heat buildup occurring in the ocean subsurface 21 months in advance. Our simulations suggest that a large increase in heat content during this phase can lead to basin-wide uniform warm conditions in the equatorial Pacific the winter before the occurrence of a very strong EN event. In our model configuration, the system compensates any initial decrease in heat content and naturally evolves towards a new recharge, resulting in a delay of up to one year in the occurrence of an EN event. Both scenarios substantiate the non-linear dependency between the intensity of the subsurface heat buildup and the magnitude and timing of subsequent EN episodes.

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

“…A similar immediate release of the excess heat was described in Ballester et al [2016c] for the TM + 80% experiment 10 months earlier, but in that case, the surface warming at the end of year À1 was found to be uniformly distributed along the equatorial Pacific. Under these conditions, the Bjerknes feedback cannot be activated, and therefore, the initial warming only represents a transition step toward a very strong EN episode of similar magnitude 1 year later.…”

“…We describe a new family of ensemble experiments with initial conditions corresponding to a lead time of 11 months before the December peak of this event (i.e., 1 January of year 0) and compare the results with a family initialized 10 months earlier (i.e., 1 March of year À1, simulations reported in Ballester et al [2016c]). We note that these stages of the episode correspond to an early phase of the basin-wide deepening of the thermocline (referred here to as RP, standing for Recharge Phase) and the maximum in the slope of the equatorial thermocline (TM for Tilting Mode), respectively.…”

“…10.1002/2017GL074557 Ballester et al [2016c] recently studied the sensitivity of EN to a rise or fall in ocean heat content 21 months earlier by means of an ensemble of numerical simulations. These experiments showed that basin-wide uniform warm equatorial SSTs follow a major increase in heat content, which favor the occurrence of a very strong EN event 1 year later.…”

“…Ballester et al [2016c] showed that in these runs a weak LN event develops at the end of year À1, which reactivates the generation of the subsurface heat buildup in the western Pacific and leads to a strong EN episode that peaks in December of year +1 (Niño3.4 Index = +2°C). Differences between both cases therefore highlight the importance of the timing of the prescribed decrease in heat content (Figures 1e, 2e, and 3e).…”

Western Pacific Oceanic Heat Content: A Better Predictor of La Niña Than of El Niño

“…The seminal studies of Wyrtki (1985) and Jin (1997a) actually underlined the predictive role of the heat content in the western equatorial Pacific (WWV w ; i.e., as WWV but within 120°E to 155°W, 5°S to 5°N) rather than across the entire equatorial Pacific (i.e., WWV). Recent studies have also renewed interest in WWV w by showing that ENSO events tend to be preceded by anomalous ocean heat content anomalies in the western equatorial Pacific up to 1.5 years before the event (Ballester et al, 2016;Izumo et al, 2018;Larson & Kirtman, 2017;Petrova et al, 2017;Ramesh & Murtugudde, 2013).…”