Global and regional ocean heat content sensitivity to changing water mass properties and circulation processes
Creators: name: Savita, Abhishek
Publisher: name: University Of Tasmania
Year: 2023

description: Anthropogenic warming is the major driver of the global mean sea level (GMSL) rise observed since the Industrial Revolution. Human activities, such as burning of fossil fuels, increase atmospheric concentrations of heat-trapping greenhouse gases, resulting in a positive energy imbalance of the Earth system. More than 90% of the additional energy trapped in the Earth system by human induced greenhouse gases has been stored in the ocean in the form of heat. As seawater warms, its volume expands (because of the thermal expansion effect) and contributes to GMSL rise. Thermal expansion is a major contribution to present-day GMSL rise and will continue to be an important contribution by the end of this 21st century and beyond, as projected by Atmosphere Ocean General Circulation Models (AOGCMs). However, there is large diversity in observational and modelled estimates of ocean heat content (OHC) change. In general, spread in observational estimates arise from methodological choices used to create regular gridded fields from sparse in situ ocean observations in time and space, as well as from technological changes in the observing system measuring the ocean’s dynamic and physical properties. In large part, spread in AOGCMs occurs due to the ocean response to surface forcing, in terms of ocean heat uptake and transport by physical processes. Consequently, improving our understanding of the causes of spread in observations and models will contribute towards reducing uncertainty with respect to present and future changes in sea level and in our climate, at global and regional scales.<br>In Chapter 2, I examined observational OHC spread arising from choices of interpolation techniques (i.e., mapping methods), eXpendable Bathythermograph instrumental bias corrections (XBT correction) and due to differences in the definition of a common ocean mask. The basis for this quantitative analysis is an internationally-coordinated protocol, in which the same global data sets are used to calculate OHC change based on six mapping methods and six XBT bias correction schemes, and then the spread among them. In Chapters 3 and 4, another internationally-coordinated protocol with common surface flux perturbations applied to ocean (OGCM) and AOGCM models was the basis for my quantitative analyses of the key physical processes transporting heat from surface to subsurface, and across ocean basins, including related multi-model spread in OHC change.<br>Building on from Boyer et al. (2016), who used the same internationally-coordinated protocol to report that mapping method was the largest source of global OHC change spread, followed by XBT correction, I find in Chapter 2 that about 2-13% of the spread in global OHC can arise due to differences in the definition of the ocean mask across mapping methods. Furthermore, my regional analyses indicate where the largest contributions to spread originate: (i) due to mapping method is confined to the eddy energetic regions in western boundary currents (Gulf Stream, Kuroshio, Agulhas Retroflection, Brazilian currents) and along the Antarctic Circumpolar Current (ACC); and (ii) due to XBT correction is confined to the tropics, particularly the tropical Pacific Ocean. By basin, the spread in OHC from choice of mapping method is largest in the Indian Ocean, and in the Pacific Ocean, independent of the choice of XBT bias correction. These new regional findings will guide future community efforts to more effectively reduce (or at least more properly quantify) associated uncertainties.<br>In Chapter 3, to quantify the spread in OHC and the role of key physical processes across AOGCMs, I used idealised climate change experiments from the World Climate Research Programme Coupled Model Intercomparison Project Phase 6 (CMIP6) – known as the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) protocol – in which OGCMs and AOGCMs are forced with a common set of surface forcing perturbations. In agreement with similar past studies, my analysis of the CMIP6 AOGCM simulations underpinned the FAFMIP protocol demonstrate that OHC change is found to be large in response to surface heat flux perturbations in the North Atlantic and Southern Ocean, while the response to surface freshwater and wind stress perturbations are of more local importance, and both oppose each other in the Southern Ocean. Spread in OHC change is largest in areas where the magnitude of the change is large across the AOGCMs, mostly in highly energetic eddy and frontal regions. At the process level, also consistent with previous work, I find that resolved mean advection is the largest source of spread in simulated vertical heat transport, followed by parameterized mesoscale eddy advection, for both the steady state (control run) and FAFMIP experiments. Spread in OHC change in response to surface flux perturbations is highest in the regions where spread is highest in control simulations. A new finding is that, although the multi-model spread in OHC change in response to surface heat flux is statistically significant where the multi-model mean is large, the responses to surface freshwater and wind stress perturbations are within the inter-model variability of the unperturbed (control run) experiments in most of the ocean. In other words, while surface heat flux perturbations drive a response that is well outside the range of internal variability (i.e. it is significant), the response to wind stress and freshwater forcings cannot be separated from internal variability. Compared to other physical processes in response to surface heat flux perturbations, the total OHC change spread is smaller in the upper 2000 m than below 2000 m. However, in response to freshwater and wind stress perturbations, total OHC change spread is large in the deep ocean and within the other physical processes in the upper 2000 m. These findings suggest that uncertainty in sea level rise projections could be reduced by focus on improving ocean advection schemes at the eddy energetic and frontal regions.<br>Finally, in Chapter 4, I provide the first ever diagnosis in temperature-depth space of the global thermal overturning cells as well as the super residual circulation (i.e. the sum of resolved mean advection and parameterized mesoscale eddy advection and eddy diffusion) by mathematically implementing individual temperature tendency diagnostics into a novel thermodynamic framework. In steady state simulations from the Australian Community Climate and Earth System Simulator Ocean Model (ACCESS-OM2), forced by JRA55-do atmospheric surface fluxes, the super residual circulation is dominated by a thermally direct cell (i.e. warm water upwells and cold-water sinks) which transports heat upwards and cools the ocean. This upward heat transport is mostly balanced by mixing processes, such as diapycnal diffusion and parameterized surface boundary layer mixing. This newly developed thermodynamic framework can be applied to CMIP6 AOGCM simulations (including FAFMIP experiments) to unravel individual aspects of the ocean circulation and their multi-model spread, complementing the results from traditional budget analyses.<br>This thesis work performed intercomparison analyses for both observed and simulated OHC change, with the objective to understand spread and their causes. These intercomparison analyses were not feasible until recently due to lack of coordinated experiments (i.e. protocols), availability of the relevant diagnostics and involvement of a significant number of members in both communities. These detailed analyses of OHC change in both observations and AOGCMs, along with some similar recently published studies and other underway work, will help to refine our current knowledge of the role of the oceans in Earth’s climate and future changes, and so to provide greater confidence in projections of climate and sea level under a range of future climate change scenarios, and thus ultimately to refine policy-making decisions for mitigation and adaptation.