A Study on the Atmospheric, Cryospheric, and Hydrologic Processes Governing the Evolution of Regional Hydroclimates

Abstract

Regional hydroclimates constitute the interplay between climate, weather, and water resources at sub-continental scales. They continually evolve by responding to changes and perturbations in global climate, water cycle, and terrestrial surface and subsurface processes. This dissertation is on region-specific and process-based assessments of the atmosphere, cryosphere, and hydrosphere interactions to understand the mechanisms driving variability in hydroclimates. The overarching motivation is to quantify the availability of water within the Earth’s components and understand how water systems evolve in space and time. This research combines data analysis and modeling tools with fundamental physical laws to study two distinct systems – large inland lakes and mountain glaciers – both critical water resources for societies, economies, and ecosystems.

In the first study, we discuss how large inland water bodies regulate the water cycle for three geographical regions: the African Great Lakes, the Laurentian Great Lakes, and Lake Baikal. We found that these lakes control regional micro- and meso-scale weather and climate, and different modeled lake representations can simulate markedly different coupled lake-atmosphere processes.

The second study analyses the atmospheric moisture budget in the Laurentian Great Lakes region. We developed process-level understanding of the precipitation seasonality and established the role of lakes in inducing differences in the water cycle seasonality. These lakes are a source of moisture through evaporation and generate localized moisture flux convergence/divergence patterns. We further quantified the future changes in the budget using Coupled Model Intercomparison Project (CMIP6) data. There are common patterns of change in the mid-century (2041 – 2070) projections of climate variables, specifically, an increase in evapotranspiration throughout the year and intensification of winter/spring precipitation, indicating a shift in the precipitation seasonal cycle towards the colder months.

The next chapter delves into land surface hydrology, specifically looking at the controls of variability in the terrestrial water budget using a high-resolution model for the Laurentian Great Lakes domain. This is a hydrologically heterogeneous region, with various regulators of the water budget at different timescales. At higher latitudes, snowpack and soil moisture are the principal drivers of variability, while at lower latitudes precipitation, evapotranspiration, and runoff are the dominant controls of budget variability.

The subsequent work centers on mountain cryosphere to study the evolution of Karakoram glaciers using a numerical model. We explored the relation between climate, mass balance, and ice dynamics in driving glacier geometry (length, thickness, area, and volume) changes. We found that similar climate forcing can trigger radically different responses, even in neighboring glaciers, and changes in area and length do not always correspond to a similar change in the glacial volume. We also applied a new approach to calibrate ice dynamics parameters and introduced a novel scheme for dynamic spin-up to match geodetic mass balance observations while accounting for changing glacier area. In the final chapter, the dissertation lays the groundwork to simulate three-dimensional dynamics of mountain glaciers using the Community Ice Sheet Model to advance the field of glaciology as a coupled process within the larger Earth system modeling framework.

Throughout this dissertation, we highlight the significance of understanding the governing processes modulating regional hydroclimates before assessing their future evolution, and the importance of effectively representing water/ice reservoirs in numerical models. This is critical to improve accuracy and reliability of future climate assessments using models which currently have limitations in simulating regional-scale climate.