Mountains and Earth's Climate: Insights from Two Earth System Model

Earth's land surface topography, including mountains and ice sheets, is known to impact climate on various spatial and timescales through a variety of mechanisms. However, the role of mountains in shaping the most basic, large-scale characteristics of Earth's global climate system is debated, and changes in Earth's mountains have been linked to long-term climate change such as the Cenozoic cooling trend. Here I present analysis of numerical modeling experiments intended to shed light on the impact of mountains on Earth's climate, as well as the mechanisms by which Earth's topography influences the climate system.;Removing all land-surface topography from two state-of-the-art Earth System Models results in large regional changes to Earth's surface temperature and precipitation, but these changes largely cancel each other out on a global scale. Mountains are found to influence key aspects of the climate system, and this dissertation explores the changes to two atmosphere-ocean coupled phenomena in particular: one that occurs in the tropics, and the other at high Northern latitudes. The El Nino Southern Oscillation is less frequent and more regular when mountains are not present, and associated anomalies are much larger without mountains probably because deep atmospheric convection is no longer orographically moored to the Maritime Continent (Indonesia). The Atlantic Meridional Overturning Circulation is much weaker when mountains are not present because of the influence that mountains have on the strength and the shape of the winds in the North Atlantic and the impact on mixing of warm, salty waters from the subtropical gyre into the cooler, fresher waters of the subpolar gyre.;Finally, this work reports the changes in our experiments in a specific region -- East Africa -- and compares the changes to modern observations of climate variables in that region, and to climate changes induced from other perturbation experiments. This region is of particular interest because it contains the majority of hominin fossils that document the evolution of our own species, and also records in paleo-archives concurrent changes in climate and climate variability. Removing mountains has a much larger impact on East African precipitation than quadrupling atmospheric carbon dioxide, opening a Panamanian seaway between the tropical Atlantic and Pacific Oceans, or adding one million cubic meters per second of freshwater to the North Atlantic over the course of a century. The East African precipitation changes imparted by the other perturbation experiments are not large enough to distinguish from noise as estimated by historically poorly constrained observations of precipitation in East African.