Chile Niño/Niña processes across a hierarchy of models

École doctorale: ED173 “Sciences de l’Univers, de l’Environnement et de l’Espace”

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^{The eastern boundary upwelling region of the southeastern Pacific (SEP) is one of the most biologically productive oceanic areas on a global scale, which makes it highly important for food security and socio economic activities. On the other hand, over recent years, marine heat waves (MHW) have attracted the attention of the scientific community due to their deleterious impacts on ecosystems and the expectation that they will increase in frequency in the future. This is a concern for the SEP where a certain type of MHW takes place known as Costal Niño. Coastal Niño events off Central Chile, also coined Chile Niño, are instances of coastal warming that share dynamical similarities with its tropical basin-scale counterpart. They are more regionally confined and much less persistent, but are thought to interact with basin-scale climate variability. Enhancing our ability to predict these events poses a significant challenge with clear societal implications for the countries along the west coast of South America.}

^{In this thesis we focused on the underlying mechanisms of Chile Niño events based on a variety of models, from simple conceptual models and data-driven models, to intermediate complexity and full-physics climate models. As a first step, we show that Hasselman's type of models have a relatively limited skill in accounting for Chile Niño interannual variability, although the consideration of a seasonally varying growth rate allows interpreting the energy in the near-annual frequency band as resulting from a combination tone process between ENSO and the annual cycle. We then analyze 62 global climate models participating to the Coupled Model Intercomparison Project (CMIP, Phases 5 and 6). It reveals that the models are skillful in accounting for many properties of the Chile Niño, including their magnitude, seasonality as well as horizontal and vertical temperature patterns. The analysis also suggests a diversity in the Sea Surface Temperature (SST) patterns of Chile Niño, with events peaking along the coast of Central Chile while others having an open-ocean maximum. Such diversity is difficult to assess due to limitations of the short observational record. Models also tend to simulate too much cold events (Chile Niña) resulting in a weaker positive asymmetry than in the observations. Considering the relative good skill of CMIP models, we further investigate the changes of Chile Niño properties under climate change based on a sub-group of models having skill in simulating ENSO non-linearity. We find a modest but significant increase in Chile Niño variability, which is shown to be due to an increase in amplitude rather than an increase in the number of events. This increase in variability is interpreted as resulting from the increase in ENSO variance and the increase in thermocline feedback off Central Chile. In order to further elucidate drivers of Chile El Niño, a regional atmospheric model (RegCM) coupled to a slab ocean is developed and sensitivity experiments to boundary forcings are carried out. The analysis suggests that Chile Niño can arise from purely internal dynamics and can be interpreted as slightly unstable mode resulting from ocean-atmosphere coupling through heat fluxes and driven by atmospheric noise. The control run experiment also indicates that certain Chile Niño events may need to be preceded by a strong basin-scale El Niño in order to fully develop. The analysis is complemented by mixed-layer heat budgets of the different simulations and a Reanalysis product. Perspectives of this work are proposed at the end}

Jury

Diego NARVÁEZ	University of Concepción, Chili	Rapporteur
Pascal OETTLI	Chiba University, Japon	Rapporteur
Emilia SANCHEZ-GOMEZ	Météo-France, Toulouse	Examinatrice
Oscar PIZARRO	University of Concepción, Chili	Examinateur
Fabien SOLMON	LAERO, Toulouse	Co-directeur de thèse
Boris DEWITTE	CECI/CERFACS/IRD, Toulouse	Directeur de thèse
Cristian MARTINEZ-VILLALOBOS	University Adolfo Ibáñez, Chili	Invité