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Overview

Scientific discussion meeting organised by Professor Meric Srokosz, Professor Penny Holliday and Professor Harry Bryden.

The Atlantic Meridional Overturning Circulation (AMOC) modulates our climate, including being a potential cause of rapid change. New observations over the last decade or so have revealed surprising aspects of the AMOC and led to a re-consideration of its stability and variability, its role in physical and biogeochemical components of the climate system, and its representation in climate models.

Recordings of the presentations will be available on this page once the meeting has taken place. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

Poster session

There will be an in person poster session on Monday 5 December at the meeting venue and an online poster gallery for the duration of the meeting.

The poster submission deadline has passed. 

Attending this event

This meeting is intended for researchers in relevant fields.

  • Free to attend
  • Both in-person and online attendance will be available
  • Limited places, advance registration essential (more information about registration will be available soon)

Enquiries: contact the Scientific Programmes team

Image credit: Dr Ben Moat

Organisers

Schedule


Chair

09:05-09:30
RAPID AMOC observations: from theory to an international standard in circulation monitoring

Abstract

In this talk Professor Marotzke will relate the history of ideas that led to the establishment of the RAPID AMOC monitoring array at 26.5°N. The development started with largely unsuccessful attempts to find dynamically consistent formulations for zonally averaged models of the AMOC and to construct simplified yet quantitative pictures of the three-dimensional, geostrophic AMOC. A solution arose from plausible assumptions about the hydrographic structure near the boundaries; they could not be rigorously derived but permitted a theoretical estimate of AMOC strength as a function of latitude. A crucial ingredient was the assumption of geostrophic balance also across the western boundary current. This assumption could be turned into a monitoring strategy for the AMOC, using a limited number of full-depth dynamic-height moorings. A remarkable confluence of personal and professional events then allowed the implementation of this strategy through the RAPID monitoring array, creating the reference point for observing systems at other latitudes in the Atlantic. 

Speakers

09:30-09:45
Discussion
09:45-10:15
The big AMOC question stemming from IPCC WG1

Abstract

Has the Atlantic Meridional Overturning Circulation (AMOC) or Gulf Stream System of ocean currents collapsed or slowed due to climate change? The answer to this question is important; a slowdown is widely predicted by climate models and with it the expectation of a cooler, stormier climate, drier summers and wetter winters, to millions of people, especially in Europe. In the most extreme scenario of a complete collapse, a cascade of climate tipping points could ensue including, for example, an end of arable farming in Britain. So, what’s going on with the AMOC? 

In August 2021, the Intergovernmental Panel for Climate Change (IPCC) published its 6th Assessment Report stating there was low confidence in a 20th century AMOC slowdown. While that was consistent with the preceding 2013 report, it is in contrast with the 2019 IPCC Special Report on the Ocean and Cryosphere that stated with medium confidence that the AMOC had weakened relative to 1850–1900. One cause of this reversion in confidence assessment was conflicting observational and model-based estimates of AMOC trends. 

Here, Dr McCarthy takes a perspective on the divergence between climate models and observations of the AMOC and question whether it throws into doubt model projections of future weakening.

Speakers

10:15-10:30
Discussion
10:30-11:00
Break
11:00-11:30
The meridional overturning circulation in the subpolar North Atlantic: new results and new questions

Abstract

The Overturning in the Subpolar North Atlantic Program (OSNAP) is a partnership among oceanographers from the US, UK, Germany, the Netherlands and Canada, whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (AMOC) and its variability. The OSNAP observing system, in place since 2014, provides a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. The first six-years of this time series have revealed surprising results on the mean and variability of the overturning circulation in the subpolar North Atlantic and have shed light on the AMOC forcing mechanisms on seasonal and interannual time scales. Additionally, the six-year time series of the meridional heat and freshwater fluxes reveal the complicated roles that both overturning and gyre dynamics play in the distribution of heat and freshwater across this basin. Complementing the moored array results is the OSNAP float program, which has elucidated the spreading pathways of Nordic Seas overflow waters in the subpolar North Atlantic.  Collectively these Eulerian and Lagrangian measurements paint a complex picture of the overturning structure and variability in the North Atlantic and yet offer suggestions for its continued measurement.

Speakers

11:30-11:45
Discussion
11:45-12:15
Labrador Sea Water spreading and the Atlantic Meridional Overturning Circulation

Abstract

Labrador Sea Water (LSW) is formed by dramatic wintertime and spreads southward in the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). In 1982, Talley and McCartney used the low potential vorticity signature of LSW to make the first North Atlantic maps of its properties. Forty years later, our understanding of LSW variability, spreading time scales, and importance has deepened. The southward spreading of this oxygen- and carbon-rich water mass is thought to prevent large-scale hypoxia in the North Atlantic and store anthropogenic carbon away from the atmosphere for hundreds of years. Motivated by these impacts, Dr Le Bras will highlight observations of LSW export from its formation regions and its subsequent southward spreading in the Deep Western Boundary Current. She will connect these findings to the hypothesized link between changes in LSW production and decadal subtropical AMOC variability and end with thoughts on relevant observing priorities for the next forty years.

Speakers

12:15-12:30
Discussion
12:30-13:30
Lunch

Chair

13:30-14:00
Towards two decades of Atlantic Ocean mass and heat transports at 26.5N

Abstract

Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport at 26.5N is carried by the overturning circulation, and an even larger fraction of the heat transport variability (~94%) is explained by the variability of the AMOC. The authors review the major interannual changes in the AMOC and meridional heat transport that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Consistent with modeling results, it is believed that most of the short-term and interannual variability of the AMOC at 26.5N is related to local and regional wind forcing. However, there is emerging evidence that the secular decline in the AMOC and associated meridional heat transport since 2010 is linked to deep density changes along the western boundary, resulting from deep water mass changes at higher latitudes that are advected southward within the deep limb of the AMOC.

Speakers

14:00-14:15
Discussion
14:15-14:45
The AMOC and ocean salinity

Abstract

The distribution of ocean salinity is expected to change as a result of increasing input of freshwater from the Arctic, a changing hydrological cycle and due to dynamics of the ocean itself. The field has come a long way since the first simple box models that showed the importance of the salt advection feedback on the AMOC. Models of increasing complexity have shown that there are multiple feedbacks that involve freshwater and salinity, some of which counteract each other. However, measuring salinity in the ocean and deriving transports of freshwater or salt from these measurements is not as straightforward as it sounds. In this talk Dr Femke de Jong will discuss advances made in uncovering the complexity of the relation between AMOC and salinity, observed trends in salinity, as well as remaining challenges in modelling and observations.

Speakers

14:45-15:00
Discussion
15:00-15:30
Break
15:30-16:00
Arctic freshwater impact on the AMOC: status and prospects

Abstract

Arguably, the most conspicuous evidence for anthropogenic climate change lies in the Arctic Ocean.  For example, the summer-time Arctic sea ice extent has declined over the last 40 years and the Arctic Ocean freshwater storage has increased over the last 30 years. Coupled climate models project that this extra freshwater will pass Greenland to enter the subpolar North Atlantic Ocean in the coming decades.  Coupled climate models also project that the AMOC will weaken in the 21st century, associated with subpolar North Atlantic buoyancy increases. Yet, it remains unclear when the Arctic anthropogenic freshening signal will be detected in the subpolar North Atlantic, or what form the signal will take.  Therefore, this talk will review the state of knowledge on Arctic Ocean freshening and freshwater export rates and pathways to the subpolar North Atlantic. The talk will focus on the export processes in data-constrained ocean circulation model hindcasts. It will also discuss the prospects to detect the emergence of Arctic anthropogenic freshening and the likely impacts on the AMOC. The challenge to distinguish anthropogenic signals from natural variability will be highlighted.

Speakers

16:00-16:15
Discussion
16:15-16:45
Subpolar AMOC between Greenland and Scotland: large-scale mechanisms and linkages

Abstract

The overturning circulation of the subpolar North Atlantic (SPNA) plays a fundamental role in Earth's climate variability and change. In this talk, Dr Chafik will discuss the recent warming period since 2016 in the eastern SPNA in the context of recent overturning changes from observations. He will also discuss the involved mechanisms and large-scale linkages that may have induced this recent warming in the SPNA. Finally, Dr Chafik will discuss the implications of these results for high-latitude ocean climate predictability.

Speakers

16:45-17:00
Discussion
17:00-18:00
Poster session

Chair

09:00-09:30
Evolution of the AMOC: From decades to Millenia

Abstract

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.

Speakers

09:30-09:45
Discussion
09:45-10:15
AMOC Modeling for Past Climate and Its Relevance to Present and Future

Abstract

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. 

Speakers

10:15-10:30
Discussion
10:30-11:00
Break
11:00-11:30
AMOC and carbon across the Atlantic

Abstract

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.

Speakers

11:30-11:45
Discussion
11:45-12:15
Challenges simulating the Atlantic meridional overturning circulation in climate models

Abstract

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. 

Speakers

12:15-12:30
Discussion
12:30-13:30
Lunch

Chair

13:30-14:00
AMOC, AMV and climate

Abstract

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.

Speakers

14:00-14:15
Discussion
14:15-14:45
The role of buoyancy forcing in the Atlantic Overturning Circulation

Abstract

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.

Speakers

14:45-15:00
Discussion
15:00-15:30
Break
15:30-16:00
Should AMOC observations continue - how and why?

Abstract

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.

Speakers

16:00-16:15
Discussion
16:15-16:45
Implications of Atlantic circulation on UK policy

Abstract

Abstract will be available soon
 

Speakers

Professor Gideon Henderson FRS

Chief Scientific Adviser, Defra, and University of Oxford, UK

16:45-17:00
Discussion