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University of Edinburgh (2020)

Mechanisms of climate variation in the broader Mexican-United States region : the role of atmospheric circulation and anthropogenic forcing

García Martínez, Ivonne Mariela

Titre : Mechanisms of climate variation in the broader Mexican-United States region : the role of atmospheric circulation and anthropogenic forcing

Auteur García Martínez, Ivonne Mariela

Université de soutenance : University of Edinburgh

Grade : PhD Doctor of Philosophy 2020

Résumé partiel
The climate of the region comprising Central America, Mexico and the United States (US) has undergone important temperature and precipitation changes in the recent decades. For instance, a rapid warming has been identified, in line with the current global trend. Annual precipitation has decreased over central and southern Mexico with increased precipitation variability and more severe droughts, while a positive trend has been observed in northern Mexico and most of the US. This has had profound impacts on society, water resources, and the local economy. Along with greenhouse gases, anthropogenic aerosols –in particular sulphate– are thought to be the main contributors to these changes. Yet, the relative contribution from aerosols represents the largest uncertainty in current estimates of this human-driven climate change. This PhD research is therefore aimed at contributing in the understanding of the mechanisms that modulate the climate variations in the region, in particular the role of dynamical features and anthropogenic forcing in the mean and extreme climate states. This is achieved by using a range of observational and remote-sensing datasets, atmospheric reanalyses and advanced modelling experiments. We apply both simple and more sophisticated statistical techniques to create an integral mechanistic picture of the pathways linking large-scale dynamics with regional changes. The thesis is organised as follows : after a general introduction (Chapter 1), I present three independent but related core chapters (2-4), followed by Conclusions (Chapter 5). In chapter 2, I analyse the summer spatial structure and sub-monthly temporal evolution of one of the key dynamical features of Central American climate, the Caribbean Low-Level Jet (CLLJ), by means of extended empirical orthogonal functions (EEOFs) and lead and lag linear regressions using reanalysis data (1979-2010). This approach reveals new insights into the dynamical processes and spatio-temporal evolution of the CLLJ summer intensification and allows to identify significant climate links in the broader Caribbean region. The results show that the CLLJ generates significant precipitation and temperature responses with a distinct temporal evolution over the Caribbean-Atlantic and the tropical Pacific, which hints at different underlying controlling mechanisms over these two regions. An influence of extratropical hemispheric-wide waves on the CLLJ intensification is identified, which along with the weakening of a thermal low in northeast Mexico-central US trigger the summer intensification of the CLLJ. Additionally, two leading modes of tropical variability, El Niño Southern Oscillation and the Madden-Julian Oscillation, are found to further intensify the CLLJ and to extend its life cycle during summer. The full nature of these relationships was only partially appreciated, if not overlooked, in previous works.

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