Sources and sinks of variability and predictability in the North Atlantic

• Main Author: Stephenson, Dafydd
• Format: Thesis
• Language: English
• Published: University of Southampton 2021
• Subjects: Start Point; North Atlantic Deep Water; North Atlantic
• Online Access: https://eprints.soton.ac.uk/450584/; https://eprints.soton.ac.uk/450584/1/DafyddStephensonTHESIS_final_29July21.pdf; https://eprints.soton.ac.uk/450584/2/Dafydd_Stephenson_Permission_to_deposit_thesis.docx

The North Atlantic has long been recognised to have a unique role in climate, owing to its ability via deep water formation to sequester large quantities of heat and carbon, and due to the associated meridional overturning circulation (MOC), which permits the northward transport of heat (impacting the climate of neighbouring regions such as Europe). In recent decades, a \data revolution" in both computational power and observational coverage and quality have revealed a significant amount of variability in the North Atlantic, but the origins of these variations and their predictability remain open questions. The simplest explanation is that variations in the North Atlantic are the passive response of the atmospheric forcing to the ocean. In this perspective, surface-borne anomalies are carried into the ocean interior along the ventilation pathways of its water masses. These pathways therefore motivate the start point for this thesis, which begins by presenting a new configuration of an ocean general circulation model (OGCM), and its application to their study. The configuration is used to describe the passive origins and fate of two ocean water masses present in the North Atlantic: North Atlantic Deep Water and North Atlantic Subtropical Mode Water. The configuration repurposes the tangent-linear and adjoint model (TAM)framework used for tracking perturbations and sensitivities, such that feedbacks are nullied and transport is passive. Using the forward and backward modes of the TAM in 400 year simulations, spatiotemporal and thermohaline probability distributions of water mass origins and fate are constructed. This highlights a disparity between the forward and backward modes suggesting an important role for water mass transformation (changes in thermohaline properties). Following this, the passive configuration is applied to exploring the passive and active nature of atmospherically forced heat content variability, and the North Atlantic is compared with other major ocean basins. A stochastic representation of ...