Periodic Reporting for period 1 - EPOC (Explaining and Predicting the Ocean Conveyor)
Período documentado: 2022-07-01 hasta 2023-12-31
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.