On the external forcing of Indian Ocean climate variability across timescales

Abstract

It is imperative to understand the dynamics of external climate forcings and the nature of the climate system’s responses for improved predictability. These forcings include low-probability, high-impact events like explosive volcanic eruptions as well as the continued injection of anthropogenic greenhouse gases into the atmosphere. This thesis explores how external forcings affect Indian Ocean climate in the past, present, and future using paleoclimate archives in conjunction with observational and climate model data. Chapter 2 presents a novel geochemical stalagmite record from northern Madagascar which spans the end of the last glacial period. Stable isotope and trace metal proxies indicate drier conditions in response to North Atlantic cooling events, such as Heinrich stadials, and wetter conditions during North Atlantic warming events, such as the Bølling–Allerød. These responses are opposite what would be expected from north-south shifts in the Intertropical Convergence Zone. Instead, we hypothesize that west-east tropical Indian Ocean temperature gradient variability akin to the modern-day Indian Ocean Dipole explains the consistent hydroclimate response to North Atlantic forcing reconstructed by eastern African sites. Chapter 3 explores the effects of volcanic eruptions on interannual Indo-Pacific climate variability using an ensemble of last millennium simulations. Following the largest tropical eruptions, these simulations demonstrate a consistent negative Indian Ocean Dipole response which leads an El Niño. This response scales with eruption intensity and persists for up to 8 years for the strongest events. We also find that Interdecadal Pacific Oscillation phasing at time of eruption preconditions the initial Indian Ocean Dipole response via low frequency thermocline depth modulation. Finally, in Chapter 4 we use marine sedimentary archives in combination with climate simulations to expand on the Atlantic-Indian Ocean teleconnection hypothesized in Chapter 2. The reconstructed west-east surface temperature gradient responds in lockstep to previous instances of Atlantic Meridional Overturning Circulation (AMOC) variability during the last glacial period, such as Heinrich stadials, the Bølling–Allerød, and the Younger Dryas. An analysis of single-forcing simulations featuring meltwater addition to the North Atlantic under glacial and interglacial boundary conditions further demonstrates this inter-basin connectivity. We find that in simulations of high greenhouse gas emission scenarios, uncertainties in future Indian Ocean temperature and precipitation patterns are attributable to uncertainties in the magnitude of future AMOC weakening. This thesis bridges disparate timescales and data sources to gain insight into how the external forcing of the Earth system works at a fundamental level, from geochemical records of abrupt climate transitions during the last ice age to numerical simulations of the Atlantic overturning slowing by the end of the 21st century.