OUTCROP: New prOcess-based UndersTanding of ocean heat Uptake with an application to improved Climate pRojections for pOlicy and Planning

Future climate change projections provide essential guidance for the efforts to curb the global warming trend caused by human emissions of greenhouse gases. One of the most important factors controlling the rate of climate change is ocean heat uptake (OHU), which is responsible for limiting global warming by absorbing part of the excess radiative forcing due to greenhouse gases by the ocean. Unfortunately, the physical processes controlling OHU remain poorly constrained and understood, as they are all associated with small scale processes related to turbulent mixing of heat and salt, meso-scale ocean eddies, deep water formation, which we do not know how to represent accurately, as well as to the surface fluxes of heat and freshwater, which are difficult to observe and measure precisely. As a result, large uncertainties in climate projections remain that are directly attributable to our lack of precise knowledge about ocean heat uptake. To understand how to make progress, a firm theoretical understanding of the physics of vertical heat transfer associated with OHU appears to be essential. Unfortunately, the validity and usefulness of the standard vertical/advection diffusion model for the horizontally-averaged temperature, which has been the primary theoretical tool to think about the vertical heat transfer, has been repeatedly questioned over the years owing to its failure to account for such effects as a varying topography, isopycnal mixing and the existence of density-compensated temperature anomalies.

To resolve the above difficulties, our group recently developed a new process-based vertical advection/diffusion model for the heat balance that exploits advances from the theory of ocean water masses accumulated over the past 50 years or so. The new model represents a considerable improvement over the previous one, in that it naturally explains the precise role of a varying topography, density-compensated temperature anomalies, isoneutral mixing, and differential surface heating on the vertical heat transfer, which had remained obscure in the standard model. In this proposal, our first objective will be to demonstrate the usefulness of this new process-based model to interpret and rationalise the simulated ocean heat uptake for a wide range of climate change scenarios including increasing CO2, stabilisation, radiative forcing overshoot, and a collapse of the Atlantic meridional overturning circulation. Our second objective will be to demonstrate that the major advances due to our new process-based understanding of ocean heat uptake can be translated into a major improvement in the accuracy of climate change projections using Simple Climate Models, with a particular application to the MAGICC model, and one developed by the Met Office Hadley Centre. Indeed, although the main physical basis for our current understanding of climate change relies on coupled atmosphere-ocean general circulation models (AOGCMs), these models are computationally very expensive to run. Therefore, simple climate models (SCMs) have been developed, which are able to mimic the climate response seen in the AOGCMs, but at a much reduced computational cost. SCMs represent a key tool in the study of climate change, and are being used for several purposes, e.g. simulating how the projections depend on key climate parameters, or for the interpretation of the AOGCM projections. SCMs are often used for policy advice and play a central role in the science forming the basis for Working groups 2 and 3 of the latest International Panel on Climate Change report, the main document at the origin of the recent Paris agreement aimed at limiting the overall global warming below 2C.

The improved physical understanding of ocean heat uptake will significantly contribute to improved climate projections and reductions of associated uncertainties.

The modified simple climate models, with improved parameters and a clarified traceability, will be of great benefit for the interface between climate science and climate policy. MAGICC6 is widely used by the IPCC as a tool to interpolate AOGCM scenarios for impact assessment as well as adaptation and mitigation studies (IPCC AR5). The proposed research will establish alternative simple climate models for these purposes, and will improve MAGICC6 and UK Hadley Centre SCMs. The fact that this is based on physical processes and constraints will lend more credibility and reliability to these models and the scenarios they produce. As a highly relevant impact, this affects policymakers and the wider public alike. The project results will be of great use for the IPCC, and might increase the acceptance of climate models and their scenarios in the public debate. These impacts should materialize already during the project lifetime, as soon as the first publications have appeared.

The career development of the PDRA will benefit from this project. The PDRA will gain knowledge of many different aspects of climate modelling, namely full-fledged, three-dimensional AOGCMs; sequential data assimilation involving AOGCMs; and simple one-dimensional climate models. With climate science becoming more and more interdisciplinary, having worked on this project will equip the PDRA with broad skills for their future career.