Mixing in the open ocean from the Last Glacial Maximum to present date: tidal dissipation and ocean resonance

Lead Research Organisation: Bangor University
Department Name: Sch of Ocean Sciences

Abstract

Summary Vertical mixing in the ocean causes a vertical transport of water, and with it nutrients, salt, heat and pollutants. It is also one of the main mechanisms behind the large scale ocean circulation, and therefore of immense importance for the climate of our planet. The driving force of the vertical mixing in the deep parts of the open ocean, far away from coasts and the wind-mixed upper kilometre of the water column, is the tide. In order to understand and be able to predict changes in climate, we must look back in time and try to understand, explain and quantify previous changes in climate. In the past 21 000 years, since the Last Glacial Maximum (LGM), the sea level has risen by 115-135 m. A major consequence of this sea level rise has been the flooding of the continental shelf seas. Although the massive expansion of the shelf seas has been described as the most important geological event of recent time, there has been no integrated study of the impact of this event on global ocean circulation, and only a few investigations of its impact on ocean mixing. Whilst the contemporary shelf seas only account for 7% of the surface area of the global ocean, over 70% of the tidal energy dissipation takes place there. In consequence the historical rise in sea level has had a profound effect on the dissipation of tidal energy in shelf seas. This will in turn be reflected in a change in the dissipation of tidal energy in the deep ocean. As the tide provides a major, if not the major, component of the mechanical energy which ventilates the deep ocean through turbulent mixing, the rise in sea level has had a substantial impact on the marine system. Tides therefore do not only have the potential to affect the global climate, by influencing the strength of the oceanic circulation, but they can also influence key global biogeochemical cycles and atmospheric carbon dioxide levels. Within the project we will therefore aim to improve the accuracy of the estimates of the tidally driven open ocean mixing in the historical ocean. This will be done using two computer models of the ocean tides. We will simulate the changes in the global ocean mixing rates caused by the ocean tides from the peak of the last ice-age until today, and try to explain and quantify the changes which occur. This will open for a further understanding of the forcing of our ocean circulation system in the past and today. It will also provide insight into the sensitivity of the ocean mixing system, and how it relates to climate dynamics and climate change.

Publications

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Wilmes S (2017) Global Tidal Impacts of Large-Scale Ice Sheet Collapses in Journal of Geophysical Research: Oceans

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Wilmes S (2014) The evolution of tides and tidal dissipation over the past 21,000 years in Journal of Geophysical Research: Oceans

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Stammer D (2014) Accuracy assessment of global barotropic ocean tide models in Reviews of Geophysics

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Simpson J (2009) The structure of dissipation in the western Irish Sea front in Journal of Marine Systems

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Schindelegger M (2018) Can We Model the Effect of Observed Sea Level Rise on Tides? in Journal of Geophysical Research: Oceans

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Rosier S (2014) Modeling Antarctic tides in response to ice shelf thinning and retreat in Journal of Geophysical Research: Oceans

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R. Stephenson G (2015) Baroclinic energy flux at the continental shelf edge modified by wind-mixing in Geophysical Research Letters

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Porter M (2016) Drifter observations in the summer time Bay of Biscay slope current in Journal of Marine Systems

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Pickering M (2012) The impact of future sea-level rise on the European Shelf tides in Continental Shelf Research

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Pelling H (2013) Sea level rise and tidal power plants in the Gulf of Maine in Journal of Geophysical Research: Oceans

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Herold N (2014) A suite of early Eocene (~ 55 Ma) climate model boundary conditions in Geoscientific Model Development

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Green J (2013) Non-assimilated tidal modeling of the South China Sea in Deep Sea Research Part I: Oceanographic Research Papers

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Green J (2013) A Comparison of Tidal Conversion Parameterizations for Tidal Models in Journal of Physical Oceanography

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Green J (2015) Climatic Consequences of a Pine Island Glacier Collapse in Journal of Climate

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Green J (2013) Tidal dissipation in the early Eocene and implications for ocean mixing in Geophysical Research Letters

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Green J (2010) Ocean tides and resonance in Ocean Dynamics

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Green J (2018) Is There a Tectonically Driven Supertidal Cycle? in Geophysical Research Letters

 
Description We have shown that the tides during the Eocene (some 50 million year ago) were far weaker than they are today, except in the southwest Pacific Ocean. This lends support to data from sediment cores in the Pacific which suggest a very different ocean circulation during the Eocene - a circulation which can only have existed if the deep Pacific Ocean received more tidal energy.

This work was then expanded further back in time, to 600 million years ago, and 250 million years into the future, and the results still hold: the tides have indeed been weaker that at present throughout this period. We also identified what is now known as a supertidal cycle associated with the supercontinent cycle. This means that when Earth's crust comes together in a supercontinent, the tides are weak. As the continent breaks up and teh plate fragments scatter aroudn the globe, the tides can become more energetic and reach a maximum about halfway into the supercontinent cycle. That's where Earth is now - the tides are very energetic at the moment. They will then decrease as the next supercontinent slowly forms.

These results also supports the "old Moon" theory. Due to the tides the moon is moving away from Earth, but with the present rate the moon must be younger than 1500 million years. However, a lot of data suggest that the moon is over 4000 million years, which suggests that the tides must have been far smaller than today during long periods in the past. Our results supports this, and provides a clue to the evolution of the Earth-Moon system.

We have also continued the work into the effects of sea-level rise on tides, tidal dissipation, and wider earth system consequences. We now have a firm handle on the sensitivity of the tidal dissipation during the LGM, and we can now accurately model the effects of realistic levels of future SLR on tides.
Exploitation Route Work is continuing to simulate the past 600Ma and the future 300Ma in more detail, and to link the cahnges in with other Earth ssytem events.
This has also highlighted the importance of including tidally driven mixing in simulations of past, present and future climates, especially on time scales when the tides may change.
Sectors Aerospace, Defence and Marine,Environment

 
Description Research built on outcomes from this project has been extensively reported on by popular science media, most noteably two The Conversation articles, which generated a significant interest in deep time tides. This has been pursued to generate an interest in STEM subjects, and it has been used in a number of outreach events by the PI and collaborators.
First Year Of Impact 2017
Sector Environment
Impact Types Cultural

 
Description Deeming Summer Bursary
Amount £1,500 (GBP)
Organisation Bangor University 
Sector Academic/University
Country United Kingdom
Start 05/2014 
End 09/2014
 
Description Modelling the abyssal mixing during the Cenozoic 
Organisation Purdue University
Country United States 
Sector Academic/University 
PI Contribution The Eocene (some 50 million years ago) was a period with a very warm climate and different continental configuration compared to today. For example, there was a land bridge between South America and Antarctica, but not between south and north America. Australia was also located further south and was connected to Antarctica, and India had yet to collide with Asia. Later on during the Cenozoic, during the Miocene (25 million years ago), things had changed quite a bit - we now had an Antarctic Ice sheet, for example - but the Atlantic was still quite small, and the continents had not quite reached their present positions. The climate had changed, though, and became more similar of today's climate. In this collaboration we evlauate the impact of tidally driven mixing during these two periods. In publications about to be submitted we show that the tides were far weaker than at present on a global scale. However, the dissipation of tidal energy in the deep ocean was far stronger than at present, and that this had an impact on the ocean circulation, especially during the Eocene. The results also suggest that the present day dissipation rates are unusually large. This has a wider importance in that the present age model of the earth-moon system does not fit the current recession rate of the moon (which largely is controlled by the tidal dissipation), but requires far weaker tides for long geological time periods - something our results support.
Start Year 2012
 
Description The sensitivity of the ocean circulation to tidal mixing 
Organisation Oregon State University
Country United States 
Sector Academic/University 
PI Contribution The climate controlling ocean circulation if partly sustained by an input of mechanical energy, a significant amount of which comes from the tides. However, most climate models do not specifically include tides in their formulation. Here, we we use an established intermediate resolution coupled cliamte model to investigate how significant it is to have tidally driven mixing in a model of this type. We will also investigate how the enhanced tides during the last ice age modified the ocean circulation and may have helped kick-start the ocean after the collapse of the large ice-sheets.
Start Year 2013
 
Description Tides during the Devonian and their implication for the evolution of terrestrial vertebrates 
Organisation Uppsala University
Country Sweden 
Sector Academic/University 
PI Contribution Initiating tidal model simulations of the Devonian (400Ma) to investigate if mega-tides were present. This has implications for the evolution of terrestrial vertebrates.
Collaborator Contribution Initiating the collaboration, providing bathymetric dtabases, leading on a second publication.
Impact Publication now out: H. A. M. Byrne*, J. A. M. Green, S. A. Balbus, and P. E. Ahlberg, 2020: Tides: A key environmental driver of osteichthyan evolution and the fish-tetrapod transition? Proceedings of the Royal Society, Series A, 476, 20200355.2
Start Year 2015
 
Description Were tides resonsible for the low oxygen levels during the Turonian mass extinction? 
Organisation European Centre for Research and Teaching of Environmental Geosciences (CEREGE)
Country France 
Sector Academic/University 
PI Contribution Tidal model simulations to be used in a climate model of the Turonian (95Ma).
Collaborator Contribution The French team has provided bathymetry and stratification data for the Turonian, to be used in the tidal model i base my work on.
Impact none to date.
Start Year 2016
 
Description AGU OS 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Gave three presentations at the 2018 AGU Ocean Sciences meeting, including work on deep-time tides, tides and sea-level rise, and mixing in the Arctic.
Year(s) Of Engagement Activity 2018
 
Description AGU Ocean Sciences 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Science conference presentations, including convening sessions

collaborations spawned; exchange of information
Year(s) Of Engagement Activity 2010,2012,2014
 
Description Forschung aktuel 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact German State Radio 4 has a daily 25-minute show on recent research highlights, and decided to interview me about the work on Eocene tides. Focus was on the age of the moon aspect, and the program aired on June 5.

None so far
Year(s) Of Engagement Activity 2013
 
Description New Scientist article 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Jeff Hecht, writing for New Scientist, published an article based on the work on Eocene tides with special focus on the aspect of the age of the moon.

The publication of this work lead to an interview by German Radio. i also include this work in other outreach activities.
Year(s) Of Engagement Activity 2013
 
Description Popular science articles 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Popular science articles in Nature (https://www.nature.com/articles/d41586-018-02034-w) and Science (http://www.sciencemag.org/news/2018/02/strong-tides-may-have-pushed-ancient-fish-evolve-limbs) about the Devonian results based on a presentation at AGU Ocean Sciences 2018.
Year(s) Of Engagement Activity 2018
URL http://www.sciencemag.org/news/2018/02/strong-tides-may-have-pushed-ancient-fish-evolve-limbs
 
Description SOS Sea food festival 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact my display introduced the public to the concept of tides and how they help control climate, including discussions about the subject.

None yet.
Year(s) Of Engagement Activity 2014