Title: Understanding how irrigation, plant physiology, and the Madden-Julian Oscillation shape regional water cycles and their extremes
Abstract: The water cycle is one of the most fundamental building blocks of the earth system; without it, life would cease to exist, but its extremes pose a threat to both economies and ecosystems. It is thus especially important to understand the hydrologic cycle in as much depth as possible, including how human actions are already shaping it and how it could change in the future. In that vein, this dissertation addresses three distinct topics – how irrigation has altered precipitation; how plant-physiological changes in response to rising CO2 can alter future flooding and streamflow; and how the Madden-Julian Oscillation (MJO) modulates tropical cyclones (TCs) in the West Pacific.
The first main chapter of this dissertation (Chapter 2) assesses non-local hydroclimate responses to irrigation in India, using an ensemble hindcast approach aimed at clarifying the ongoing debate in the literature about the robustness of forced water cycle responses relative to high levels of atmospheric internal variability. The results suggest a strong sensitivity to the initial synoptic condition, with separate non-local hotspots responding to irrigation differently (but robustly) under different initial conditions. This argues that chaos plays a major role such that even heavy irrigation such as that over India has difficulty manifesting as strong, robust non-local water cycle responses. On longer time scales and across ensembles, the Meiyu-Baiu rainband region is highlighted as having a potentially robust non-local irrigation-induced precipitation signal, opening new questions.
In my second study (Chapter 3), I investigate streamflow changes as driven by (1) atmospheric responses and (2) plant-physiological responses to rising CO2. A series of four modeling experiments help isolate these two response pathways and their combined effects, revealing that the plant physiological driver is actually of first order importance to projections of future flooding and streamflow. This is especially true in the tropics, where river discharge increases are controlled almost exclusively by the plant response in the Amazon, Parana, Congo, and Yangtze. This adds to a growing recognition in our field that profound changes in regional water cycles can occur even without warming-induced changes of precipitation as a result of how the terrestrial biosphere adapts to increased CO2.
The final project (Chapter 4) seeks to better understand the mechanisms through which the MJO, a slow-moving tropical weather pattern, modulates West Pacific TCs. How, where, and even if the MJO modulates tropical cyclones is unresolved in this region, despite the fact that it is especially prone to human vulnerability from both high levels of current TC activity and future MJO amplification. Through a novel downscaling strategy that creates thousands of synthetic cyclone tracks for each phase of the MJO, I reveal two previously un-emphasized but distinct stationary modes in opposite portions of the West Pacific basin that are modulated out of phase with one another. The South China Sea region is particularly responsive to the oscillation, driven by a transient combination of dynamic and thermodynamic factors. This adds to a decades long debate about what mechanisms mediate MJO-TC modulation in nature and identifies new subregions that will be particularly important to focus on in the coming decade, towards advancing understanding of how MJO amplification may affect TC hazards in a future climate.