Nonlinear Rectification of Quaternary Climate Drivers at High and Low Latitudes

Abstract

Variations in earth’s climate are externally driven by changes in earth’s orbit around the sun – orbital forcing, namely precession, eccentricity and obliquity – on timescales of 104–105 years. During the Quaternary, from 2.6 million years ago to present, these orbital drivers caused large oscillations of earth’s climate system between cold glacials and relatively warm interglacials. The evolution of earth’s climate however is not directly proportional to this orbital insolation forcing. For instance, reconstructions of (annual mean) paleoclimate show substantial variability on precessional time scales, even though there is no change in annual mean insolation associated with the precessional forcing. This means that nonlinear mechanisms rectify the zero-annual-mean precessional forcing into an annual mean climate response. Further, during the Late Quaternary (beginning roughly 800,000 years ago (800 ka)), glacial cycles have had a dominant period of ~100 ka, a period not associated with a dominant orbital parameter. Feedbacks internal to the earth system thus amplify and modulate the external drivers. The Northern Hemisphere ice sheets are thought to be most directly sensitive to orbital insolation forcing, while other parts of the climate system – such as the tropics and the southern high latitudes – might respond more indirectly to the 100 ka earth system response. This dissertation explores the nonlinearities of precessional rectification and 100 ka modulation throughout the Late Quaternary at low and high latitudes. Many records of tropical hydroclimate show substantial variability on precessional timescales. Part I of this dissertation aims to identify the nonlinear mechanisms responsible for rectifying the seasonal precessional forcing into an annual mean precipitation response. The traditional view of precessionally-forced precipitation changes is that tropical precipitation increases with summer insolation. By comparing two simulations with an earth system model (CESM1.0.3), this paradigm is found to be true for continental but not for oceanic changes in precipitation. Focusing on the Atlantic Intertropical Convergence Zone (ITCZ), it is found that the continental temperature and precipitation response to precessional forcing are key rectifiers of annual mean precipitation over the ocean. A boundary layer response to temperature changes over northern Africa affects the meridional position of the ITCZ over the North Atlantic in boreal spring and summer, but not in fall and winter. Over the equatorial and South Atlantic, the intensity of precipitation is strongly impacted by diabatic forcing from the continents through an adjustment of the full troposphere. Although the top of atmosphere insolation forcing is seasonally symmetric, continental precipitation changes are largest in boreal summer, thus skewing the annual mean response. While the precessional forcing has only meridional gradients, the climatic response has strong zonal components. An important implication of this work is therefore that traditional zonal mean frameworks for assessing the ITCZ response to external forcing do not apply in the case of strong tropical insolation forcing. The response of tropical precipitation to external forcing thus depends on the ratio of tropical (i.e., precessional) to extratropical (i.e., 100 ka) forcing. The Antarctic ice sheet (AIS) has varied substantially during the Late Quaternary, contributing more than 10 m to glacial sea level drop, and an estimated 3–6 m to interglacial sea level highstands. With its large marine margins, the AIS is sensitive to oceanic as well as atmospheric forcing, but the relative contributions of Quaternary climate forcings remain poorly constrained, with previous modeling studies relying heavily on parameterizations of past climate evolution. The evolution of northern and southern polar ice sheets appears to be synchronous on orbital timescales, which is somewhat unexpected given that precession – essential for Northern Hemisphere glacial terminations through its impact on summer insolation – is anti-phased between the hemispheres. Part II of this dissertation studies the drivers of AIS evolution over the last 800 ka by forcing an Antarctic ice sheet model with spatially and temporally varying climate anomalies from a transient simulation with an earth system model (LOVECLIM), in addition to reconstructions of global sea level change. The simulated AIS evolution has a glacial-interglacial amplitude of 10–12 m sea level equivalent. Sensitivity experiments in which atmospheric, ocean temperature and sea level forcing are applied individually show that the full ice sheet response is a non-linear superposition of the individual drivers. The Northern Hemisphere sea level forcing impacts Antarctic ice volume by driving changes in the grounding line position. This grounding line migration modulates the Antarctic response to other climatic drivers: for both accumulation and oceanic melt rates the changes in configuration of the grounded ice sheet dominate over the glacial-interglacial climate forcing. Surface melt rates peak when austral summers are long, especially during periods of high annual mean temperature corresponding to high CO2. These melt peaks provide a critical contribution to Antarctic deglaciation and are in phase with Northern Hemisphere summer insolation. Thus, on glacial timescales, Antarctica and the Northern Hemisphere ice sheets vary in unison through their respective orbital forcings, changes in global sea level, and CO2.