Possible reasons for low frequency modulation of ENSO: A quantitative analysis
The reasons for low frequency modulation of ENSO are not entirely clear. It could be due to extratropical influences, but it also could be a consequence of internal modes of the coupled ocean-atmosphere system in the Pacific. In this presentation we run a set of quantitative analyses to identify or to rule out different candidates. The strategy is to find significant quantitative relationships between measures to characterize the dominant modes of ENSO and potential predictors that may explain ENSO modulation. The average wavelet power in the interannual band (2 to 7 years) of the leading EOF of tropical Pacific SST anomalies is used as an index to characterize the ENSO modulation. A non-linear, non-parametric statistical modeling method is applied, to identify significant relationships between this index and potential predictor candidates. Comprehensive analyses are carried out using uni- and multivariate analyses. This approach allows identifying simultaneously complex non-linear multivariate interactions between ENSO variability and climate phenomena like NAO, PNO, QBO, equatorial and South-Western thermocline depth variability and the transport changes within the tropical cells. Indices are computed from observations and assimilation products (SODA). The same analyses are also applied to a global coupled simulation and results are compared to the ones derived from observations.
Nonlinearity of ENSO and its interdecadal changes
So far, there is no consensus as to whether El Nino is linear or nonlinear. In this study, we evidently showed that the ENSO undergoes a nonlinear cyclic process; the weak heat accumulation in the whole equatorial Pacific is followed by the strong El Nino and the subsequent strong drain of equatorial heat content toward the off-equator precedes a weak La Nina. This asymmetric ENSO evolution implies that the nonlinear instability enhances the growth of El Nino, and it dwarfs the growth of La Nina. Especially, a strong scatter from the natural cycle during the transition phase from La Nina to El Nino implies that the stochastic forcing agitates the nonlinear cycle. Since the late 1970s the nonlinearity has been changed. For the pre-1980s, the ENSO cycle associated with the thermocline is less asymmetrical then that during the post-1980s so that the nonlinearity of the ENSO cycle has become stronger.
A Numerical Investigation of Decadal ENSO Variability
There have been extensive studies on the decadal variability of the North Pacific and its relation to ENSO, with several different mechanisms proposed to explain the oceanic connections between the tropics and the extra-tropics. Relatively less attention has been focused on South Pacific ocean teleconnections. We use a primitive equation, reduced-gravity ocean model to explore the oceanic linkages between South Pacific subtropical subduction sites and the tropics on decadal time scales. Simulations from a 49-year, realistically forced experiment indicate that decadal variability of temperature along the equator originates from subsurface anomalies of temperature in the tropical eastern South Pacific. Through western boundary and interior pathways in the thermocline, the subsurface anomalies that are subducted in the eastern tropical South Pacific are first transferred westward and then northward, eventually appearing along the equator. The anomalies then propagate eastward along the equator in the Equatorial Undercurrent, and then upwell to the surface in the eastern equatorial ocean. We find only very weak temperature anomalies along the equator that originate from subtropical subduction sites in the South Pacific, although those same subduction sites can generate significant subsurface variability locally in the subtropical gyre. The reasons for these decadal temperature anomalies are also investigated
Weathering Of ENSO Supercriticality And Decadal ENSO Bursting
A simple conceptual nonlinear recharge model for ENSO is used in order to study the bifurcation characteristics of the El Ni\~no-Southern Oscillation phenomenon for slowly varying coupling parameters. Even for very slow forcing (order 10$^3$ years) the stability properties do not correspond to the stationary bifurcation diagram. This discrepancy originates from the so-called ``critical slowing down'' effect and the delayed occurence of Hopf bifurcations in slowly forced systems. In the parameter vicinity of a Hopf bifurcation, transient perturbations cannot be damped out efficiently since one real eigenvalue becomes very small. Furthermore, we develop a theory for the stochastic weathering of ENSO supercriticality. The results indicate that the question as to whether ENSO is a stable system or a limit cycle is an academic questions which is based on the non-verifyable assumption of a stationary background state and no-noise. Furthermore, we propose a new mechanism that explains two key features of the observed El Ni\~no-Southern Oscillation (ENSO) phenomenon -- its irregularity and decadal amplitude changes. Using a low order ENSO model we show that the nonlinearities in the tropical heat budget can lead to bursting behaviour that is characterized by decadal occurences of strong El Ni\~no events. La Ni\~na events are not affected, a feature that is also seen in ENSO observations. One key result of our analysis is that decadal variability in the tropics can be generated without invoking extratropical processes or stochastic forcing. The El Ni\~no bursting behaviour simulated by the low order ENSO model can be understood in terms of the concept of homoclinic and heteroclinic connections. We show that this new model for ENSO amplitude modulations and irregularity, although difficult to prove, might explain some features of ENSO dynamics seen in more complex climate models and the observations.
Rectification of the ENSO Variability by Interdecadal Changes in the Equatorial Background Mean State in a CGCM Simulation
A 260-yr long coupled general circulation model (CGCM) simulation is used to investigate the characteristics and impact of the low frequency variability of the background stratification on the ENSO modulation. The model produces quite realistic mean state characteristics, despite a sea surface temperature (SST) climatology too cold and a thermocline shallower than observations in the western Pacific. The periodicity and spatial patterns of the modelled ENSO compare well with those observed over the last 100 years although the quasi-biennial timescale is dominant. A vertical mode decomposition of the model mean vertical structure is performed, which indicates a larger contribution of the high-order baroclinic modes than observed consistently with a too diffuse simulated thermocline. The characteristics of the baroclinic mode alone do not allow the explanation of the timescale of the variability of the simulated ENSO cycle which is mostly in the quasi-biannual frequency band. Using an anomaly intermediate coupled model of the tropical Pacific tuned with the baroclinic mode parameters and seasonal cycles as derived from the CGCM, it is demonstrated that the relatively high-frequency of the model variability is due to the presence of a fast coupled basin mode associated with over-energetic zonal advection in the western equatorial Pacific and weak climatological equatorial upwelling. This 8-month-period mode shares many characteristics with the so called "mobile mode" as described by Mantua and Battisti (1995) and participates to some extent to the ENSO modulation in the model. Decadal thermocline variability is then analysed: Enhanced variance over the western tropical South Pacific (~7$^\circ$S) are found. The associated subsurface temperature variability is primarily due to adiabatic displacements of the thermocline as a whole due to Ekman pumping anomalies located in the central Pacific south of the equator. These wind anomalies appear to be caused by SST anomalies in the eastern equatorial Pacific. Further analysis shows the existence of a coupled mechanism similar to the one proposed in the observational study by Luo and Yamagata (2001): namely, the off-equatorial sub-surface temperature anomalies move northwestward to the western equatorial Pacific and modify the equatorial coupled dynamics in such a way that the original SST anomaly are reversed in the east. This quasi-decadal variability has a period of ~11 years. Decadal variations of the derived baroclinic mode parameters are then used to force the intermediate coupled model. Results of sensitivity experiments indicate that a rectification of the interannual variability (ENSO timescales) by the interdecadal variability associated to changes in the oceanic mean states does take place. However, the magnitude and characteristics of this rectification highly depends on the strength of the interaction between the ENSO mode and the fast coupled basin mode. A mechanism by which the tropical decadal mode feedbacks on the ENSO modulation through the modulation of the mobile mode is proposed.
Connections between the tropical Pacific oceans and ENSO modulation on decadal timescales
Connections between tropical Pacific decadal variability and low frequency modulation of El Nino and Southern Oscillation (ENSO) amplitude are analyzed from the results of a 400-year integration with the COLA coupled ocean-atmosphere model. The model reasonably simulates the principal mode of tropical Pacific decadal variability. Both SST and heat content anomalies for this mode are characterized by a standing oscillation with a triangular shape in the tropical Pacific basin. This low frequency mode is largely independent of ENSO modulation. There is, however, low frequency variability of the tropical Pacific that is associated with ENSO amplitude modulation. The mechanism of this variability associated with ENSO decadal modulation involves the tropical western Pacific oceans with advection of heat content anomalies off equator. In order to understand the mechanism associated with two different tropical Pacific variability, simple coupled model experiments were conducted.