Modelling Arctic Sea Ice and Ocean Processes in a Changing Climate

Abstract

Warming four times faster than the rest of the globe, the Arctic region is experiencing rapid changes. At the heart of this amplification of anthropogenic climate change lies a strong coupling between the different components of the climate system, including the ocean and sea ice. The numerous non-linear processes dictating the physical and biogeochemical interactions between the ocean and sea ice influence the local and global climate. Those interactions are investigated by combining theoretical approaches and a hierarchy of numerical models. The processes regulating sea ice thermodynamics are non-linear and obfuscate an intuitive understanding of the evolution of sea ice in a changing climate. A simple thermodynamic sea ice model is used to highlight that sea ice acts as a rectifier for atmospheric forcing and integrates external forcing to generate low frequency internal variability. The ice--ocean heat flux, despite being poorly constrained by observations, dictates the future of sea ice in a warming world. The Arctic Ocean is experiencing longer and more frequent marine heatwaves in recent decades. A heat budget applied to a regional numerical model provides an overview of the driving mechanisms triggering and dissipating marine heatwaves in the Arctic. Notably, sea ice melt lengthens marine heatwaves by shoaling the surface mixed layer. Marine heatwaves act to provide a pathway for heat from the atmosphere to the subsurface ocean. The Arctic Ocean is a carbon sink, regulating atmospheric concentrations of greenhouse gases. Importantly, sea ice is not a simple physical lid on the ocean, but plays a role as a pump for carbon into the ocean. By combining a theoretical framework and a one-dimensional numerical model, the drivers of this pump are identified and the underestimation of the ocean carbon sink in global climate models is quantified for present and future states of the Arctic. In this thesis, interactions between ice and ocean in the current and future state of the Arctic Ocean are investigated and the role of sea ice as a climate component is highlighted, regulating heat and carbon exchanges between the atmosphere and the ocean and dampening weather noise to generate climate variability.