Atlantic overturning: new observations and challenges

The Atlantic meridional overturning circulation (AMOC), one of Earth’s major ocean circulation systems, redistributes heat on our planet and has a major impact on climate. Its past and future strength are therefore of major interest, but the system is not easily measured. In this talk, the author will present what we know about the evolution of the AMOC over the last decades to millennia based on various sources: from the about 15 years of direct continuous observational data, to reanalysis products and ship measurements, to observational temperature and salinity records, and ocean sediment proxy data. This overview shows that while the AMOC displays a large amount of internal variability over the different time scales, its behaviour over the last one to two hundred years appears to be unprecedented compared to the last 1,600 years and various records suggest that the AMOC over the last decades has been weaker than ever before during this period.

Professor Liu will briefly review several advances and issues in paleo modelling of AMOC, with the focus on the glacial-interglacial changes, and their relevance to the present and future climate. (a) Water masses have changed dramatically between glacial and interglacial period in the Atlantic, with the AABW as the sole dominant water mass in the glacial period, caused primarily by the sea ice expansion and the brine rejection. (b) The responses of AMOC to changing greenhouse gases are likely opposite between fast (centennial) and slow (millennial and longer) time scales, partly because of the involvement of the Southern Ocean adjustment.  (c) AMOC instability remains a challenge issue to study from both observational and modelling perspective. Most models seem to be over stable, but there are exceptional CGCMs that exhibits strong bi-stability. (d) The evolution of AMOC transport can be studied qualitatively using certain proxies, notably d18Oc and Ph/Th, while other proxies, such as ∆C¹⁴ and δC¹³, only constrain water masses. (e) The 3-D circulation of AMOC may have also changed during the past, with different mechanisms from the present. 

The ocean carbon sink currently mitigates the continuing build-up of carbon dioxide in the atmosphere by absorbing approximately 30% of all additional CO₂ derived from human activities. Within the Atlantic, the overturning circulation is thought to play a key role, through its effect on biological productivity (by transporting nutrients to productive regions), the physical carbon pump (where heat fluxes change the solubility of surface waters to CO₂), and the removal of surface waters replete with high levels of anthropogenic carbon to depth on climatically important timescales. While decadal variation in the overturning circulation has been linked to changing global carbon uptake patterns, knowledge about their correspondence over shorter timescales, and smaller spatial scales, is less well characterised. Here Dr Brown explores our current understanding of the relationship between AMOC variability and the biogeochemical response, and how projected AMOC changes are likely to impact the continuing ability of Atlantic surface air-sea CO₂ fluxes to mitigate anthropogenic climate change.

The Working Group I Sixth Assessment Report of the Intergovernmental Panel on Climate Change (AR6) assessed past and future changes in the Atlantic meridional overturning circulation (AMOC). The report assessed that there was low agreement between modelled changes and both observations and reconstructions. The low agreement findings meant that confidence in past and future changes was reduced relative to previous reports. In particular, although it was assessed that the AMOC was very likely to decline over the 21st century under all emissions scenarios, there was low confidence in the magnitude of the decline. Similar discrepancies have been seen in the UK HadGEM3 climate model at different resolutions, with the resolution changing model biases, which in turn affect the magnitude of AMOC weakening. The strength of Labrador Sea convection is identified as an important factor in the resolution dependence and is linked to the salinity of the western subpolar gyre. Understanding and reducing the saline bias is a key goal for future climate models and we describe some of the possible causes. 

Over the last century or so, sea surface temperatures in the North Atlantic have undergone multi-decadal periods of relative warmth and relative cold - a phenomenon known as Atlantic Multi-decadal Variability (AMV). These slow ocean changes have been linked to a wide range of climate impacts globally including heat waves, floods and hurricanes. However, understanding of the processes that drive AMV remains very incomplete. One of the key hypotheses is that AMV is an expression of natural multi-decadal changes in the AMOC. However, over the past decade, other hypotheses have been proposed: in particular, that AMV can be largely explained as a response to human activities, especially anthropogenic aerosol emissions, and might not require a role for the AMOC. However, climate models are not perfect and so there is still large uncertainty in how they represent AMV. Therefore, this talk will review the latest understanding of the processes and drivers that shape AMV and discuss the implications for decadal time-scale predictions. Additionally, it will present new efforts to understand the processes shaping the externally forced component of AMV and AMOC through analysis of state-of-the-art climate simulations.

In this presentation Dr Buckley will discuss the role of buoyancy forcing in the mean and time variable Atlantic meridional overturning circulation (AMOC). The higher salinity of the Atlantic compared to the Pacific leads to deep convection being restricted to the Atlantic and thus a deep overturning in the Atlantic basin.  The energy needed to sustain a deep reaching circulation is thought to be provided by wind-driven upwelling over the Southern Ocean. Yet, recent work has shown that is it possible to sustain a vigorous overturning circulation in the absence of wind forcing, provided that there is a zonally unblocked channel, suggesting that the role of buoyancy forcing in maintaining the mean overturning circulation may be underestimated. Large-scale, decadal variability of the overturning circulation is thought to be mostly due to buoyancy forcing over the subpolar North Atlantic.  Winds lead to higher frequency, gyre-specific AMOC variations, which reduce the meridional correlation of the AMOC.

Whether AMOC observations should continue is a provocative question - for whom and to what end? Unlike Argo float data, the AMOC observations cannot be directly assimilated into forecasting models and the resultant improvement in the forecast quantified. AMOC observations provide a benchmark for climate models, but agreement between observations and models is not required nor expected between a single instance of the AMOC in the real ocean vs the many simulations of the AMOC in a freely running coupled system. Yet it is clear that observing the AMOC has fundamentally changed our view of how the ocean circulation varies, how it moves heat and freshwater and interacts with the deep ocean, and presented a raft of new questions about how the ocean and climate system function. From the extant array observations, there is no apparent relationship between the overturning measured at one latitude to that at another. In considering the future of AMOC observing, we must address how individual array observations relate to the concept of an 'Atlantic' meridional overturning circulation. What are we measuring? What is the AMOC? A meridionally-connected circulation or a set of latitudinally-disparate circulation patterns with stochastic connectivity? This will require examining how differences in methodology between arrays influence the measured overturning. A future observing system will need to retain the potential to develop new understanding, to fill identified gaps in our current understanding, and also assess whether advances in ocean observing technology should be integrated into an observing approach. This talk will outline a perspective on future AMOC observing and steps that the community should consider in order to move forward. It will acknowledge but not fully address the practical challenges of sustained observing.