Hydroclimatic extreme events such as droughts and heatwaves can produce significant impacts on environmental, socioeconomic, and public health sectors. Evidence shows that extreme events are becoming more common, severe, and costly as a result of anthropogenic climate change; therefore, the study of hydroclimatic extremes is necessary to combat the rising costs associated with these extremes. In general, previous studies of climate extremes have focused on one variable or feature at a time. In this dissertation, we provide novel investigations of the interactions between different climatic extremes and different features of extremes. Here, we present observational and model-based evidence that climate change has influenced 1) temperature shifts conditioned on drought conditions, 2) changes in the likelihood and magnitude of drought duration, frequency, and severity, and 3) changes in the likelihood of concurrent warm and dry events in response to anthropogenic warming.Chapter 1 first provides a broad overview of the literature framing this dissertation and identifies key research opportunities that following chapters pursue. Chapter 2 presents a conditional perspective of how droughts have been warming in the 20th century and how they are projected to change in a warming climate. This chapter concluded that temperatures during dry meteorological conditions are warming faster than average climate conditions, which have strong implications for future risks of concurrent warm and dry extremes under changes in background climate. This chapter also investigates the physical mechanisms causing substantial warming of droughts. Chapter 3 examines the influence of anthropogenic forcing on general drought characteristics, including drought frequency, duration, and intensity, to illustrate the contribution of human activities to our current hydroclimatic state. Using model simulations with and without anthropogenic forcing, this study found substantial regional shifts in drought frequency, maximum drought duration, and maximum drought intensity, especially in the wetter regions of the globe. Chapter 4 presents a study of the impacts of anthropogenic climate change on compound warm and dry extremes. Previous attribution studies have ignored dependencies between climate variables. Thus, in this chapter, we focus on attribution of changes in compound water and dry extremes in response to anthropogenic emissions. This study showed that most of the global land area has experienced significant increases in meteorological warm and dry months that can be attributed to human activities. We also introduce a new conditional indicator that demonstrates the impact of climate change on high temperature exceedances under dry conditions (conditional warm spells). The research presented in this dissertation provides insight into the drivers and feedback mechanisms that influence warm and dry conditions and quantify how climate change has impacted the nature of hydroclimatic extremes. From the results of these studies, we have provided a novel perspective on temperature change conditioned on droughts and created a foundation for understanding the impacts of climate change on droughts and concurrent warm and dry events. By better understanding these hydroclimatic extremes, local and national decision-makers can better prepare for future extreme events in the near and distant future.

Global temperatures have risen 0.6 degrees Celsius in the 20th century and have been projected to rise an additional 1.0-3.7 degrees Celsius in the 21st century depending on the emissions scenario. Climate records also show that drought events have been occurring more frequently during high temperature anomalies. Previous studies show strong feedbacks between drought conditions and surface temperatures, which prompted the question of whether drought conditions are experiencing larger temperature increases in comparison to the average climate. The objective of this study was to investigate whether droughts have been warming faster than average climate conditions in the contiguous United States. Using gridded observations and climate models, we compared temperatures during different categories of drought severity on a monthly scale and mapped areas displaying an escalation of temperature with stricter definitions of drought. We observed a historical shift of warming temperatures during dry months in Southern and Eastern regions between the early and late halves of the 20th century. Future projections also showed a larger warming shift during dry months in the Southern US between the 20th and 21st centuries. In the climate projections, the higher temperature shift was mostly attributed to the summer months. The summer associated temperature shift is rooted in preceding winter and spring precipitation, which influence the surface energy balance in regions with moderate climate. These mid-latitude temperature shifts associated with dry conditions are an important piece in understanding and deconstructing climate conditions in a rapidly changing environment.