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Modulating mechanisms of the onset of El Niño events

Final Report Summary - MEMENTO (Modulating mechanisms of the onset of El Niño events)

The project “Modulating mechanisms of the onset of El Niño events” (MEMENTO) is structured in two parts. Objectives 1 to 3 refer to the analysis of observational records in order to describe the main processes occurring in the tropical and extratropical Pacific during the onset of El Niño (EN) events. Instead, objectives 4 to 6 correspond to the analysis and design of climate simulations and forecasts in order to evaluate, better understand and predict the main physical mechanisms observed in the datasets. In parallel with the implementation of all these objectives, the main results and conclusions of the research have been disseminated to the general public and in specialized circles.
- Aim 1: Origin of the heat buildup in the western equatorial Pacific (WPAC). Research was focused on two particular sources of variability: the relationship between the onset of EN with preceding La Niña (LN) events inducing warm WPAC anomalies, and the role of stochastic forcing and westerly wind bursts in the generation of EN events.
- Aim 2: Interaction between the ocean and the atmosphere. The physical ocean-atmosphere feedback mechanisms explaining the variability of El Niño-Southern Oscillation (ENSO) were studied, with research centered on the relative role of high-frequency stochastic noise and atmospheric winds forcing the underlying ocean.
- Aim 3: Teleconnection between the tropics and the extratropics. Research was focused on the tropical processes modulating the atmospheric variability in the extratropics through atmospheric bridges or teleconnections, such as the ocean-atmosphere energy exchange through latent and sensible heat fluxes or the propagation of Rossby waves from WPAC to the southern and northern extratropics.
- Aim 4: Evaluation of the onset of EN in model simulations. State-of-the-art coupled ocean-atmosphere climate model simulations were analyzed in order to validate the above-mentioned mechanisms. Particularly, the main features that have been considered include the dynamical mechanisms explaining the generation of the subsurface heat buildup in WPAC or the type of dominant direction of propagation of EN events (eastward and/or westward),
- Aim 5: Design of new numerical experiments. The evaluation of these mechanisms in an ensemble of simulations determined the most suitable climate models for the design of new numerical experiments. These experiments shed light on some key questions that cannot be easily addressed through the analysis of observational records, such as the modulation of tropical variability or the role of stochastic forcing.
- Aim 6: Design of a statistical prediction scheme of ENSO events. All these leading processes are used for the design and implementation of a basic prediction scheme of ENSO activity in the tropical Pacific. This operational forecast scheme incorporated other tropical and extratropical premonitory signals previously described in the literature, and predictions were validated against available observational records.
- Aim 7: Dissemination of scientific results and outreach activities. Scientific results derived from the above-mentioned objectives were exposed in specialized circles and discussed in peer-reviewed magazines.

The investigation has shown that the initial subsurface warming in WPAC and the propagation towards the east are clearly larger in the subsurface than in the surface, justifying the use of ocean analyses and reanalyses for the description of the physical processes leading to EN events. In this way, an ensemble of 9 state-of-the-art ocean assimilation products were considered, namely NEMOVAR-COMBINE, GECCO1, GECCO2, ORAS4, ORAS3, ORA-XBTc, ECDA, SODA2.1.6 and SODA2.2.6 in order to identify the mechanisms consistently simulated by all the datasets.

Results have shown that the zonal migration of the Pacific warm pool is associated with the displacement of the areas of deep atmospheric convection, active precipitation, strong trade winds and enhanced evaporation. These relationships explain the intimate link between ENSO variability and the zonal thermohaline structure of the upper equatorial Pacific, with increased (decreased) zonal contrast of temperature and salinity during LN (EN) events, when the area of maximum precipitation is enhanced in the western Pacific (is shifted to the central Pacific) and the trade winds and westward surface currents are strengthened (relaxed) in the central Pacific. These associations in turn determine the intensity and vertical extension of surface horizontal current convergence and downwelling motion in the warm pool, as well as the zonal position and vertical tilt of local isopycnals through simultaneous changes in both the warm pool edge and the salinity front. All these factors were found to contribute to the generation of an initial subsurface warming (cooling) in WPAC.

The initial warm buildup in WPAC was found to be associated with the strengthening of the trades during LN conditions and the subsequent enhancement of the clockwise (counterclockwise) wind stress curl in the central off-equatorial north (south) Pacific, which together with the rise in dynamic height due to the accumulation of warm waters in the western Pacific, drives anomalous geostrophic Sverdrup transport towards the equator. This subsurface-intensified meridional convergence is particularly strong in the western and central Pacific, where the strongest interannual ENSO-like zonal wind stress anomalies are found. This configuration with surface Ekman-induced meridional divergence and subsurface convergence results into an area of anomalous upper-ocean upwelling motion to the east of the dateline. In the warm pool, instead, anomalous zonal currents are strongly convergent in the surface and divergent in the subsurface. These zonal signals are locally stronger than the expected meridional Ekman-induced surface divergence and the meridional Sverdrup subsurface convergence, therefore inducing anomalous local downward motion to the west of a sharp transition at 170E.

Thus, about 18-24 months before the mature phase of EN on average, meridional heat advection due to anomalous equatorward Sverdrup convergence is found to contribute to the subsurface warming at the level of the thermocline near and to the east of the dateline. In the warm pool, instead, surface horizontal convergence and downwelling motion have a leading role in subsurface warming, since equatorward mass convergence is weaker and counterbalanced by subsurface zonal divergence. This initial warming is seen to be advected by the equatorial undercurrent to the central Pacific, 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. Results also show that the subsequent recharge phase in equatorial heat content is characterized by the increase of the meridional tilting of the thermocline and the southward upper-ocean cross-equatorial mass transport as a result of the Ekman-induced anomalous vertical motion in the off-equatorial regions.

In addition, the model output corresponding to the best 5 coupled ocean-atmosphere climate models from the last IPCC report was obtained for further analysis, namely CCSM4, GFDL-CM3, CNRM-CM5, GISS-E2-R and NorESM1-M. Results have shown that CCSM and GFDL are the best models in the simulation of the warm pool and the generation of the initial subsurface warm buildup, and so they were considered for further analyses. GFDL was shown to simulate the westernmost edge of the warm EN (cold LN) signal too close to the core of the warm pool (140E), while this transition edge in CCSM was similar to that in the observations (155E). Given that the simulation of the longitudinal location of the subsurface heat buildup in the western Pacific is key for understanding the dynamic origin of ENSO events, the last available version of this model (CESM v1.2) was installed and used for the design of numerical experiments.

In these experiments, 10 sets of ensemble runs were performed, with initial conditions corresponding to a very early phase of the onset of EN, on March of the year preceding the winter peak of a moderate warm event (i.e. 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. Each of the 10 sets of ensemble experiments corresponds to an initial ocean condition in which the intensity of the subsurface warm anomaly in the western Pacific was decreased or increased by ±20%, ±40%, ±60%, ±80% and ±100%. Each set of experiments is in turn made up of 10 simulations with slightly perturbed initial conditions.

Results show that the larger is the initial subsurface heat buildup, the larger is the magnitude of the subsequent EN event 18 to 24 months later. Tropical heat content is found to increase during the first year at nearly the same rate in all the experiments, when the recharge phase is found in the reference simulation, but then the warming of the ocean subsurface starts to diverge. In general terms, the larger is the initial tropical heat content, the earlier occurs its peak, at an average ratio of 2.7·10^15 J per year (r^2 = 0.81). The delay of the recharge phase in turn affects the onset of the EN event. In those simulations with reduced initial subsurface warming, the weaker is the initial tropical heat content, the larger is the delay of the EN peak (5·10^15 J per year, r^2 = 0.52). This dependency is stepwise given the seasonal locking of EN peak, so that some experiments peak in the same winter than in the reference simulation, while the others peak the following winter. Instead, the peak of EN is not delayed in the simulations with enhanced initial subsurface warming. Nevertheless, in those experiments with largest initial warming, a moderated EN peak is already observed a year before, although it is followed by a much stronger event a year later.

A collaboration with Desislava Petrova was established, who developed a statistical modeling technique of structural (unobserved components) time series applied to EN forecasting. In the state-of-the-art, this advantageous statistical technique with regression parameters obtained by a State Space approach has never been applied to ENSO. Its distinguishing feature is that observations consist of various components - level, seasonality, cycles, disturbance, and regression variables incorporated as explanatory covariates. These components are modeled separately and ultimately combined in a single forecasting scheme. Customary statistical models for EN prediction essentially use SST and wind stress in the equatorial Pacific. In addition to these, a new domain of regression variables accounting for the state of the subsurface ocean temperature in the western and central equatorial Pacific was introduced, motivated by previous analyses, as well as by recent and classical research, showing that subsurface processes and heat accumulation there are fundamental for the genesis of El Niño.

The new model has been tested with the prediction of all warm events that occurred in the period 1996-2015. Retrospective forecasts of these events were made for long lead times of at least two and a half years. Hence, results demonstrate that the theoretical limit of ENSO prediction should be sought much longer than the commonly accepted spring barrier. The high correspondence between the forecasts and observations indicates that the proposed model outperforms all current operational statistical models, and behaves comparably to the best dynamical models used for EN prediction. Thus, the novel way in which the modeling scheme has been structured could also be used for improving other statistical and dynamical modeling systems.