Master Thesis defense by Gabriel Jean Guyader

Title : ENSO-driven variability in summer daily temperature range in Arizona and the mediating influence of the North American Monsoon 

Abstract : In the US Southwest, the North American Monsoon (NAM) plays a central role in shaping the summer climate providing a substantial increase in atmospheric mois ture often associated to heavy rainfall. In this semi-arid region, moisture availabil ity and thunderstorms modulate temperatures, with distinct effects on daytime and night-time conditions. The NAM variability is itself influenced by large-scale cli mate drivers. In the Southwest, rising temperature at both day and night represents a growing risk to ecosystems, energy systems, and public health. Therefore, un derstanding how large-scale climate variability influences the summer hydrological cycle, and in turn temperature variability, is essential. This master’s thesis investigates the extent to which the summer hydrological cy cle related to El Ni˜no Southern Oscillation (ENSO) and global warming (GW) contribute to spatial and temporal variations in daytime and night-time tempera tures across Arizona. Using observational and climate model data, the study first examines the spatial influence of ENSO on daily maximum and minimum tem peratures. Then, it investigates whether precipitation and atmospheric moisture mediate the relationships between large-scale climate drivers and near-surface tem perature through a mediation analysis. Results provide little evidence that ENSO drives daily summer temperature extremes. Indeed, the findings suggest that ENSO alone provides an incomplete representation of NAM variability. However, a statis tically and physically consistent relationship is found between atmospheric moisture and temperature: higher water vapor mixing ratio is associated with cooler day time and warmer night-time conditions, and vice versa. These results highlight the importance of the diurnal cycle and atmospheric moisture in understanding temper ature variability in semi-arid regions. Therefore, a more comprehensive framework that accounts for other oceanic climate drivers, intraseasonal monsoon characteris tics, and the processes that control atmospheric moisture during the NAM is needed to better understand summer temperature variability in the US Southwest.

Supervisor: Jens Hesselbjerg Christensen