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

未来的气候变化预测为遏制人类排放温室气体造成的全球变暖趋势的努力提供了重要的指导。控制气候变化速度的最重要因素之一是海洋吸热(OHU)，它通过吸收海洋温室气体造成的部分过量辐射强迫来限制全球变暖。不幸的是，控制OHU的物理过程仍然缺乏约束和了解，因为它们都与小尺度过程有关，这些过程与热和盐的湍流混合、中尺度海洋涡旋、深水形成有关，我们不知道如何准确地表示这些过程，以及很难准确观察和测量的地表热量和淡水通量。因此，气候预测中的很大不确定性仍然存在，这直接归因于我们缺乏关于海洋热量吸收的准确知识。为了理解如何取得进展，对与OHU相关的垂直热传输物理的坚定的理论理解似乎是必不可少的。不幸的是，水平平均温度的标准垂直/平流扩散模式的有效性和实用性一直是考虑垂直热传递的主要理论工具，但由于它未能考虑地形变化、等温混合和密度补偿温度异常的存在等影响，多年来一直受到反复质疑。为了解决上述困难，我们小组最近开发了一个新的基于过程的垂直平流/扩散模式，该模式利用了过去50年左右积累的海洋水团理论的进步。新模型比以前的模型有了很大的改进，因为它自然地解释了变化的地形、密度补偿的温度异常、等中混合和表面差异加热对垂直传热的精确作用，这些在标准模型中一直是模糊的。在这项提案中，我们的第一个目标将是证明这一新的基于过程的模型的有用性，以解释和合理模拟各种气候变化情景下的海洋热吸收，包括二氧化碳增加、稳定化、辐射强迫超调以及大西洋经向翻转环流的崩溃。我们的第二个目标将是证明，由于我们对海洋热吸收的新的基于过程的理解而取得的重大进展，可以转化为使用简单的气候模型在气候变化预测精度方面的重大改进，该模型特别适用于MAGICC模型，并由英国气象局哈德利中心开发。事实上，尽管我们目前理解气候变化的主要物理基础依赖于耦合的大气-海洋大气环流模型(AOGCM)，但这些模型在计算上运行起来非常昂贵。因此，已经开发了简单的气候模型(SCM)，它能够模拟AOGCM中看到的气候响应，但计算成本大大降低。SCMS是研究气候变化的一个关键工具，正被用于几个目的，例如模拟预测如何取决于关键的气候参数，或用于解释AOGCM预测。SCM经常用于政策建议，并在构成最新国际气候变化专门委员会报告第二和第三工作组基础的科学中发挥核心作用，该报告是最近旨在将全球总体变暖控制在2摄氏度以下的巴黎协议的主要文件。对海洋吸热的物理理解的改进将大大有助于改进气候预测和减少相关的不确定性。

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.