Ocean heat uptake: drivers, variability and impacts

Thesis

Attributions of causes in the fully coupled atmosphere-ocean system are difficult due to a wide range of interactions in the climate system. One way to address this is to break down the fully coupled closed system into two open systems. Such studies often depend on the specification of sea surface temperature to link ocean and atmosphere. Instead, in this thesis we focus on understanding the impacts of perturbations to surface heat fluxes as this gives a better handle on imposing changing processes and improves the realism of air-sea interactions.

We identify (Chapter 2) the local and non-local response of ocean heat content to idealised surface heat flux perturbation patterns, demonstrating the importance of circulation change in communicating the non-local response. The global heat content response pattern is shown to be a linear combination of the responses to flux perturbations in individual ocean basins, primarily the North Atlantic and the Southern Ocean.

We further investigate (Chapter 3) processes and forcings that regulate the ocean heat content change in the South Pacific Gyre, finding an important cooling contribution from a weakening of the ocean circulation as a response to high-latitude Southern Ocean heat flux forcing. This mechanism is not robust in CMIP5 but is identified in the MPI-GE fully-coupled large ensemble.

In order to investigate the impact of changing ocean circulation on atmospheric extreme events, we introduce, develop and validate (Chapter 4) the novel HadSM4 slab ocean-atmosphere model configuration. Using large ensembles of HadSM4, we quantify the impact of the ocean heat convergence anomaly that occurred in the North Atlantic in the 2013-14 winter season. Our results indicate that the 2013-14 anomaly induced a warming of around 1 K in the North Atlantic, 0.5 K in Western Europe, and an increase in winter precipitation that up to halves the return times of extremely wet winters over parts of Europe.

To isolate the impact of air-sea feedbacks on mean state, variability and extreme events, sea surface temperature realisations from this slab ensemble are used to force an atmosphere-only sister ensemble (Chapter 5). Disabling the feedback of heat fluxes onto SST strongly increases the ensemble spread of seasonal mean air-sea heat flux components, as the first-order impact of coupling is a negative feedback. Regionally important changes in extreme events indicate the importance of representing atmosphere-ocean coupling for studies exploring the impact of external forcing on weather extremes.

This work improves our understanding on how ocean and atmosphere force each other’s dynamics and variability through surface energy exchange, by the way of the centennial impact of atmospheric forcing on ocean dynamics, the seasonal impact of ocean forcing on atmospheric dynamics, and the atmospheric impact of mixed-layer coupling.

Authors: Aengenheyster, M

• DOI: 10.5287/ora-koevywarj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: air-sea interactions; climate dynamics; ocean dynamics; large ensemble modelling; ocean heat uptake; climate modelling; extreme value analysis; Mixed-layer feedbacks; slab ocean
• Subjects: Ocean dynamics; Oceanography; Physics; Atmospheric physics; Climate physics