Day 2 - March 13, 2025

Abstract: 

This talk is a tribute to my scientific hero, Klaus Wyrtki.  It is a small repayment of a great personal debt. At a time when ideas about El Niño pointed all over the place, he told me that Bjerknes’ hypothesis was the way forward. He thus set me on the correct path. But Bjerknes stopped short of explaining the oscillatory nature of ENSO, and it took Klaus’s tide gauge data to show that ocean dynamics is the answer.  His analysis was masterful, but there would have been nothing much to analyze without his heroic efforts to deploy those tide gauges in atolls and islands throughout the tropical Pacific.  The Zebiak-Cane model may fairly be described as translating Bjerknes plus Wyrtki into a numerical model. 

His observational work, the key to unlocking ENSO’s secrets, was guided by his almost tactile understanding of ocean physics, which led him to a correct interpretation of what others (like me) could only state as clearly by resorting to equations, which we then interpreted imperfectly.  I will explain why I feel that while the  “delayed oscillator” and the “recharge-discharge” are both insightful paradigms, neither comes as close to capturing the essence of ENSO as Wyrtki did in his underappreciated 1985 paper. 

Abstract: 

Since Wyrtki's day, climate scientists have understood that ENSO acts through the Bjerknes feedback between the Walker Cell and the zonal SST gradient.

But we have often been imprecise about how that gradient is modulated, underlain by assumptions that cool eastern SST reflects a shallow thermocline. In fact the cold tongue extends further west than a direct effect of thermocline depth on SST could explain. Here, we argue that the equatorial cold tongue systematically supports larger - and deeper-extending - vertical mixing than in other parts of the world ocean; thus mechanisms of this mixing are required elements of a theory of ENSO. In the cold tongue, persistent vertical shear in the strongly-sheared, weakly-stratified layer between the surface westward South Equatorial Current and the eastward Equatorial Undercurrent creates an unstable regime below the surface mixed layer that is primed to mix vigorously. Surface-trapped currents in the afternoon warm layer set off diurnal downward-propagating turbulence that reaches well below the surface mixed layer into the upper thermocline. The resulting deep vertical mixing transports surface heat downward and thermocline water properties upward, cooling the surface and allowing the cold tongue to extend further west than simple flow along the upward-sloping thermocline would imply.

Thus, mixing drives efficient surface-thermocline communication even where wind stirring does not reach the depth of the thermocline. Mixing carries ocean memory to the atmosphere, enabling large-scale ocean-atmosphere coupling (Bjerknes feedbacks), and providing a path for carbon-rich water to reach the surface, and sustaining the cold tongue’s productive fishery.

Abstract:

This study analyses the El Niño-Southern Oscillation (ENSO) phase space as simulated by the Coupled Model Intercomparison Projects (CMIP) 5 and 6 models. In the framework of the ENSO phase space, the ENSO cycle is described as a two-dimensional representation of the sea surface temperature anomaly in the eastern equatorial Pacific (T) and the equatorial mean thermocline depth anomaly (h). We find that the characteristics out-of-phase cross-correlation between T and h is shifted to negative values in CMIP models, suggesting that the coupling between T and h is regionally shifted to the east compared to the observed central Pacific. If we consider the CMIP models with an eastward shifted h, then the models have better agreements with the observed characteristics. While the models can capture some of the observed asymmetries with high correlations, they do largely underestimate the strength of non-linear ENSO aspects. They underestimate the likelihood of extreme El Niño and discharge states, they cannot capture the enhanced growth rates during the recharge state, the enhanced decay after the discharge state nor the reduced phase transitions after the La Niña phases. Further, we found no indication of significant improvements from the CMIP 5 to 6 ensemble, suggesting that the two ensembles are essentially the same in terms of their ENSO dynamics. There is, however, a large spread within the model ensembles, with only a few CMIP models accurately simulating the observed asymmetries of the ENSO phase space, leading to models with quite different ENSO dynamics.

Abstract:

Stratification of the upper equatorial Pacific Ocean is regulated by interactions between heat, momentum, and freshwater fluxes at the ocean surface and the horizontal and vertical transport of those properties. The strength of this stratification within the thermocline and within near-surface barrier layers regulate the ocean response to equatorial wind forcing as seen in ocean Kelvin waves and their interactions with ENSO cycles. Previous work has shown that most surface flux parameterizations overestimate surface latent and sensible heat fluxes when compared to direct measurements and to the COARE3.5 bulk flux algorithm. Thus, biases in model fluxes can lead to biases in ENSO.

In this study, we use a 1D ocean mixing model, atmosphere-only, ocean-only, and fully coupled global circulation models (AGCMs, OGCMs, and CGCMs) to show that parameterizations of flux bulk transfer coefficients, diurnal warm layers, the ocean cool skin, and rain layers collectively alter ocean temperature and salinity profiles that govern equatorial ocean stratification and ENSO variability. 1D ocean simulations with 10 cm and 10 m vertical resolution highlight the need for DWL parameterization even with sub-daily ocean-atmosphere coupling and demonstrate the effects of each flux adjustment on subsurface temperature profiles. Compared to control simulations, COARE3.6 fluxes reduce ENSO amplitude and period in OGCM simulations forced with the JRA-55 atmosphere, and improve ITCZ simulation AGCM simulations. CGCM experiments are underway to allow full assessment of the representation of surface fluxes on the ITCZ and ENSO cycles and other modes of climate variability.

Abstract:

The El Niño Southern Oscillation (ENSO) continues its dominant influence on global climate, marked by emergence of strong ENSO events in recent decades.  As the planet keeps warming up, it is critical to assess and improve our understanding of the dynamics and impacts of ENSO under greenhouse warming.  Continued development and analysis of climate models around the world coordinated by the Climate Model Intercomparison Project under the auspices of the World Climate Research Programme has driven progress in this area, along with sustained observations.  This has led to realisations on the importance of nonlinear processes underpinning ENSO in understanding the effect of greenhouse warming on ENSO in the recent, near, and far future, as well as its impacts on climate, cryosphere, and global economy – all of which will be covered in this talk.  

Abstract:

Despite successful achievements in understanding dynamics of El Niño-Southern Oscillation (ENSO) over the past decades, projection of future change in the tropical Pacific sea surface temperature (SST) still has uncertainty. Specifically, a question of whether the zonal SST gradient weakens or strengthens in response to greenhouse gas forcing, a measure of the change in the tropical Pacific mean climate, has been a long-standing issue. The latest CMIP6 model ensemble tends to project the weakening of the zonal SST gradient but the degree varies across models and even a few models reveal an opposite change. This is potentially because several mechanisms acting to change the SST pattern, in addition to the Bjerknes feedback which is the heart of the ENSO dynamics, are at work with different efficacy in different climate models. To clarify the source of model uncertainty about this aspect of climate change, we develop a simple atmosphere-ocean coupled framework based on a recharge model that solves both mean state and ENSO as equilibrium solution and perturbations. By incorporating mechanisms to explain the forced SST pattern change proposed so far into the model, we reproduced a weakening of the zonal SST gradient in response to the uniform radiative forcing. We then explored the sensitivity of the forced solution to changes in parameters that control efficacy of the individual mechanisms. Plotting the parameter values estimated from CMIP6 models can provide a hint to understand the model diversity in the forced SST pattern change.

Abstract:

El Niño-Southern Oscillation (ENSO) is the most significant interannual climate variability, with extensive socioeconomic impacts. While many studies have examined ENSO changes under future greenhouse warming, its response to mitigation efforts remains less understood. Utilizing large ensemble CO₂ removal simulations, we reveal that ENSO variability and global teleconnection patterns exhibit strong hysteresis. Specifically, during the CO₂ ramp-down phase, the variance of eastern Pacific SST anomalies increases significantly compared to the modest increase during ramp-up. This hysteresis is primarily driven by the delayed response of the tropical Pacific ITCZ to CO₂ removal, supported by selected CMIP6 model simulations. In terms of ENSO teleconnections, during CO₂ ramp-up, Pacific-North American and Pacific-South American teleconnection patterns intensify and shift eastward, with further enhancement during ramp-down. This hysteresis is connected to tropical-origin processes, where the prevalence of eastern-Pacific El Niño events during ramp-down leads to hysteresis in eastern Hemispheric teleconnections, increased ENSO skewness, and eastward shifts in tropical atmospheric convection influenced by western Hemispheric factors. These changes are associated with a stronger southward migration of the ITCZ and intensified El Niño-like warming trends. Additionally, mid-latitude changes in the North Pacific can independently induce ENSO teleconnection hysteresis without relying on tropical origins. Our findings highlight the complex, hysteretic behavior of ENSO under CO₂ mitigation, underscoring the need for nuanced climate strategies.

Abstract:

The diverse nature of El Niño-Southern Oscillation (ENSO) events complicates the understanding of tropical ocean-atmosphere interactions. Transitions between different ENSO types—those peaking in the Eastern Pacific (EP) and Central Pacific (CP)—are shaped by various factors, many of which are expected to evolve under climate change. In this study, we evaluate ENSO transitions using observational data and climate model projections, focusing on how the starting ENSO phase influences transition probabilities. Our findings reveal that transitions to CP El Niño events are more common than those to EP events, except from a neutral state. Additionally, El Niño events generally occur as isolated events, while La Niña events can persist in successive years. Notably, EP-type ENSO events do not repeat in back-to-back years. Several transitions appear to be driven by internal dynamics, such as neutral conditions to El Niño, CP El Niño to another El Niño, and CP La Niña to CP El Niño or La Niña. Projections under a high-emission scenario suggest a shift in ENSO dynamics, with CP El Niño events becoming more frequent, while EP La Niña events decline. These changes are expected to emerge by mid-century, with some patterns intensifying by the end of the 21st century. 

Abstract:

Linear Inverse Models (LIMs) are among the most commonly used data-driven models for studying ENSO. Although LIMs possess many desirable properties, in their standard form, they fail to accurately simulate the observed asymmetry and diversity of ENSO events. Observational data shows that strong Central Pacific La Niñas and extreme Eastern Pacific El Niños occur more frequently than their respective counterparts, a feature that standard LIMs do not capture. We propose a simple modification to the standard LIM, referred to here as the Non-Gaussian LIM (NG-LIM), which effectively simulates the key aspects of ENSO asymmetry and diversity. Specifically, it replicates the spatial pattern of sea surface temperature (SST) skewness and the inverted U-shaped relationship between the second and first principal components of Tropical Pacific SSTs. Using the NG-LIM, we explore the differences in the evolution of Central and Eastern Pacific El Niños and La Niñas, finding that, as observed, El Niños tend to exhibit stronger anomalies and grow and decay more rapidly. Given the improvement in simulating ENSO characteristics, we generate an extensive library of synthetic events and demonstrate how these can be used to supplement the limited observational record.

Abstract:

In addition to his pioneering work on the ocean dynamical response to atmospheric forcing during ENSO events, Wyrtki made seminal contributions to ocean biogeochemistry, offering a theoretical explanation for the existence and spatial structure of oxygen minima in the ocean’s interior. The connection between these two themes has only been recently probed thanks to long-term observations of the tropical Pacific and advances in coupled Earth system models. This talk will explore this intersection by reviewing recent modeling and observational studies on how ENSO events reshape ocean biogeochemistry in the tropical Pacific. Particular attention will be given to the oxygen response in the eastern and central tropical Pacific where the oxygen minimum zones have been reported to expand in recent decades without a clear mechanistic explanation. This variability will be further discussed in the context of the readiness and potential of the tropical Pacific observing system to monitor ocean biogeochemistry across critical scales.