Periodic Reporting for period 1 - EPOC (Explaining and Predicting the Ocean Conveyor)
Période du rapport: 2022-07-01 au 2023-12-31
The Atlantic meridional overturning circulation (AMOC) is a large-scale system of currents which redistribute heat and other quantities in the ocean, playing a role in removing carbon from the atmosphere and storing it in the deep ocean. In the past, scientists have viewed the AMOC as a “great ocean conveyor”, an idea developed to explain very long timescale (glacial-interglacial) fluctuations or tipping points in past climate. The conveyor belt schematic, however, conflates millennial timescales with present-day and shorter timescales (days to 100 years), leading to misconceptions by the community of scientists investigating the AMOC. Recent scientific advances have called into question this ‘conveyor-like’ conceptual view of the AMOC current system, and how well we understand its past variability and can predict its future evolution. EPOC will capitalise on new understanding about the AMOC and its variability—from two decades of intensive measurement campaigns and advances in technology and numerical simulations—to develop new tools and approaches to quantify past AMOC change and its connectivity across ocean basins. Through joined-up observational and model experiments, focussing on next-generation high resolution coupled models, machine learning techniques and critical re-assessment of paleo proxies, EPOC will generate a new conceptual framework for the AMOC, its meridional connectivity, AMOC-related climate feedbacks and the relationship between deep ocean ventilation and overturning on near-term timescales. This will lead to better predictions of the AMOC and related climate evolution, including the risk of rapid change.
The AMOC is a key component of the climate system, responsible for ocean heat and freshwater transport, associated with the ventilation of anthropogenic carbon, and anticipated to experience or drive climate tipping points. Due to its identification as a potential tipping element in the climate system–a process which, through incremental perturbation, can be forced into a qualitatively different state–it has received substantial attention. From paleo-records, it has been deduced that the AMOC has the potential to rapidly switch between an “on” state with a strong overturning circulation, and an “off” state with a weak or absent circulation, where these switches between on and off states have been used to explain glacial-interglacial cycles. One such example includes the collapse of an ice sheet in the northern hemisphere, where the rapid input and spread of a large volume of freshwater is hypothesised to have shutdown the AMOC, switching from an on to an off state. The concern now is that in a warming climate, the remaining northern hemisphere ice sheet (e.g. the Greenland ice sheet) or the intensification of the hydrological cycle, could input sufficient freshwater to the North Atlantic to slow or shutdown the AMOC. The subsequent reorganisation of the climate system following a shutdown of the AMOC could be dramatic, resulting in changing distributions of mean and variable temperatures and weather patterns. While the IPCC report in 2021 (chapter 9) indicated that there is high confidence that the AMOC will decline in the 21st century, there is low confidence in the magnitude of the decline and only medium confidence that it will not collapse by 2100. At the same time, the IPCC assessment reduced confidence (from medium to low confidence) in whether the AMOC is already declining, where one of the reasons given for this change in confidence (relative to the SROCC report in 2019) was the identification of missing key processes in the numerical simulations used to predict AMOC future evolution. This identification was enabled by the extensive AMOC observing efforts currently ongoing in the Atlantic.
It is against this backdrop that the EPOC project is formed. Outcomes from EPOC are anticipated to firstly deepen our process-level understanding of this complex circulation system, and through these results, to contribute to assessment of past AMOC change and its future evolution. Specifically, we will use multi-observational consistent approaches to diagnose AMOC variability in the instrumental period (since 1993), and re-evaluation and tests of the robustness of various paleo-proxies for AMOC variability since 1850. In addition, EPOC will use high resolution coupled climate models, with grid spacing roughly 10 x finer than the models used in the coupled model intercomparison project (CMIP). These modelling efforts will test and capitalise on the expectation that finer horizontal resolution can reduce model biases, leading to higher fidelity representation of the AMOC and thus more confidence in its representation of key processes.
In addition to contributing to assessments, EPOC will further work towards a more sustainable next generation observing system of the AMOC. Many of the recent advances in our understanding of the AMOC system and what is missing in climate models are due to the recent (since 2000) observing efforts in the Atlantic; however the system has not necessarily been optimised and currently represents substantial investments by multiple national and international consortia. Through both modelling and observing tests to be carried out within EPOC, we will make a recommendation for a future AMOC observing system, considering the AMOC across the whole Atlantic and incorporating next generation technologies where appropriate.
The AMOC is a key component of the climate system, responsible for ocean heat and freshwater transport, associated with the ventilation of anthropogenic carbon, and anticipated to experience or drive climate tipping points. Due to its identification as a potential tipping element in the climate system–a process which, through incremental perturbation, can be forced into a qualitatively different state–it has received substantial attention. From paleo-records, it has been deduced that the AMOC has the potential to rapidly switch between an “on” state with a strong overturning circulation, and an “off” state with a weak or absent circulation, where these switches between on and off states have been used to explain glacial-interglacial cycles. One such example includes the collapse of an ice sheet in the northern hemisphere, where the rapid input and spread of a large volume of freshwater is hypothesised to have shutdown the AMOC, switching from an on to an off state. The concern now is that in a warming climate, the remaining northern hemisphere ice sheet (e.g. the Greenland ice sheet) or the intensification of the hydrological cycle, could input sufficient freshwater to the North Atlantic to slow or shutdown the AMOC. The subsequent reorganisation of the climate system following a shutdown of the AMOC could be dramatic, resulting in changing distributions of mean and variable temperatures and weather patterns. While the IPCC report in 2021 (chapter 9) indicated that there is high confidence that the AMOC will decline in the 21st century, there is low confidence in the magnitude of the decline and only medium confidence that it will not collapse by 2100. At the same time, the IPCC assessment reduced confidence (from medium to low confidence) in whether the AMOC is already declining, where one of the reasons given for this change in confidence (relative to the SROCC report in 2019) was the identification of missing key processes in the numerical simulations used to predict AMOC future evolution. This identification was enabled by the extensive AMOC observing efforts currently ongoing in the Atlantic.
It is against this backdrop that the EPOC project is formed. Outcomes from EPOC are anticipated to firstly deepen our process-level understanding of this complex circulation system, and through these results, to contribute to assessment of past AMOC change and its future evolution. Specifically, we will use multi-observational consistent approaches to diagnose AMOC variability in the instrumental period (since 1993), and re-evaluation and tests of the robustness of various paleo-proxies for AMOC variability since 1850. In addition, EPOC will use high resolution coupled climate models, with grid spacing roughly 10 x finer than the models used in the coupled model intercomparison project (CMIP). These modelling efforts will test and capitalise on the expectation that finer horizontal resolution can reduce model biases, leading to higher fidelity representation of the AMOC and thus more confidence in its representation of key processes.
In addition to contributing to assessments, EPOC will further work towards a more sustainable next generation observing system of the AMOC. Many of the recent advances in our understanding of the AMOC system and what is missing in climate models are due to the recent (since 2000) observing efforts in the Atlantic; however the system has not necessarily been optimised and currently represents substantial investments by multiple national and international consortia. Through both modelling and observing tests to be carried out within EPOC, we will make a recommendation for a future AMOC observing system, considering the AMOC across the whole Atlantic and incorporating next generation technologies where appropriate.
In the period July 2022 - December 2023, the EPOC consortium advanced on multiple fronts, though a substantial fraction of the effort was towards preparation. These preparatory activities included planning and executing fieldwork to deploy instrumentation which will be analysed after their collection in 1 or 2 years time, and the development of and running high resolution coupled model simulations for later analyses. Fieldwork completed within EPOC includes the deployment of new drift-free bottom pressure recorders (at 26°N and 47°N) and the deployment of in situ mooring arrays at 47°N to capture transport variability in the transition zone between the subpolar and subtropical regions. In addition to these more preparatory activities, EPOC is producing multi-observational transport records for the AMOC by developing and applying a machine learning approach–Bayesian hierarchical modelling. Here, data sources include satellite altimetry, gravimetry and in situ oceanographic observations. Initial validation against in situ transport estimates from existing observing arrays show good agreement, but sensitivity tests for the method are still ongoing. In parallel, tools to compute time-varying, synoptic, observationally-based transports into and out of the Arctic for the period 2004–2020 are being prepared. Towards designing a next generation AMOC observing system, a community workshop was held in Hamburg in July 2023 to discuss achievements of the current observing system and a roadmap for the future.
Initial results are still pending – in the case of re-analysis of existing observations, we expect the new transport product to be available within the next reporting period of the project. Deployed beyond-state-of-the-art observing technologies are still pending recovery and the evaluation of the data retrieved.