Physics of the Ocean Summer School

I’m co-organisiing, with Martin Visbeck and Sabrina Speich, a Summer School on Physics of the Ocean at the Physikzentrum Bad Honnef, Germany, 2-7  July 2023. Deadline: 15 February.

The school runs every 3 years although sadly the 2000 edition was cancelled due to COVID. The previous school in 2017 was one of the most uplifting experiences of my scientific career. Ocean again, we have a fantastic line-up of lecturers, and I’m looking forward to welcoming another fantastic group of students!

Spurious forces dominate the vorticity budgets of ocean gyres in C-grid models

DPhil Student Andrew Styles has published his first paper in the Journal of Advances in Modeling Earth Systemsm showing that spurious forces can dominate the vorticity budgets of ocean gyres in C-grid ocean circulation models.  

Gyres are large scale circulations in the world ocean that often interact with the sea floor. It is important to develop a method to assess how the representation of the sea floor in models affects gyre circulations. By calculating how model forces generate vorticity (the tendency to rotate) in the flow, we are able to determine the forces acting with and against the gyre circulation. In his paper, this method is applied to results from a simplified double gyre model and the Weddell Gyre in a realistic global model. The paper shows that spurious forces which emerge from the layout of the model grid play an important role in the presented gyre circulations.

The spurious forces originate from the calculation of the Coriolis acceleration in the model. In previous studies, it has been argued that gyre circulations interact with the sea floor primarily by forming pressure gradients. However, while the contributions from pressure gradients are significant, the spurious forces are similar in size and also emerge from interactions with the sea floor. The spurious forces can be alleviated by using a B-grid or a terrain-following vertical coordinate.

The work was carried out collaborative with the Met Office through a CASE award.


Acute sensitivity of global ocean to eddy energy dissipation

Our paper led by former post doc Julian Mak has been published in Geophysical Research Letters on the Acute sensitivity of global ocean circulation and heat content to eddy energy dissipation time-scale. 

In this paper, we show that the timescale over which energy is removed from Southern Ocean eddies has a dramatic impact on the following: (a) the strength of the Antarctic Circumpolar Current, the largest current in the global ocean; (b) the strength of the Atlantic meridional overturning circulation, responsible in part for the relatively mild climatic conditions over northwestern Europe; and (c) global ocean heat content, a key parameter in the global climate system. These results have significant implications for both past and future climates, and highlight the importance of combining observational, theoretical and modeling efforts to better understand and constrain the energy balance of ocean eddies.

Symmetric instability in cross-equatorial western boundary currents

Fraser Goldsworth has published the first paper from his DPhil in the Journal of Physical Oceanography, demonstrating how boundary currents become “symmetrically unstable” upon crossing the equator. Using a hierarchy of models, he investigates how symmetric instability might manifest itself if excited in cross-equatorial flows. He finds that when the instability is excited, it generates stacked overturning cells which reorganise the current to make it neutrally stable to symmetric instability. This process is likely to be occurring in the Atlanticcean off the coast of Brazil where, indeed, there is some observational evidence for the “smaoking gun” of symmetric instability (zero potential vorticity).


Geometric interpretation of Southern Ocean eddy form stress

Mads Poulsen, a recent visiting graduate student from the University of Copenhagen, has published an article on “A Geometric interpretation of Southern Ocean eddy form stress“, has been published in the Journal of Physical Oceanography. Complimentary to the use of eddy ellipses to describe eddy Reynolds stresses, the paper shows that eddy form stress, which transfers zonal momentum downward in the Southern Ocean, can be fully described by a vertical ellipse whose size, shape and orientation with respect to the mean-flow shear determine the strength and direction of vertical momentum transfers.

Eddy form stress ellipses
Vertical section showing eddy form stress ellipses as a function of latitude and depth in a 1/6 degree resolution ocean circulation model

Theoretical understanding of Atlantic overturning circulation

Our review paper led by Helen Johnson, “Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation“, has been published in the Journal of Geophysical Research: Oceans. This is part of a special collection of review papers on “Atlantic Meridional Overturning Circulation: observational and modelling advances“.

AMOC mechanisms
Overview of key processes and processes that determine the strength, structure, and variability of the Atlantic Meridional Overturning Circulation

Neutral Surface Topology

I’m super proud that former graduate student, Geoff Stanley‘s outstanding sole-author paper on Neutral Surface Topology has been published in Ocean Modelling. This should redefine how we calculate neutral surfaces in the ocean. A huge bonus is that they are extremely fast to compute: 50s for the surface shown below, versus 12hrs for the equivalent omega surface (the previous gold standard).

Topological regions (colours) for pressure ( ~ depth) on a σ2 potential density surface, used to initialise the algorithm to create a topobaric surface. Within each coloured region, the density is an empirically-fitted single-valued function of pressure. Grey regions indicate the isopycnal has outcropped or grounded, or has disconnected from the main surface.

First OSNAP results published in Science

photo credit: Helen Johnson

A new international study finds that the Atlantic meridional overturning circulation, a deep-ocean process that plays a key role in regulating Earth’s climate, is primarily driven by cooling waters west of Europe.  

In a departure from the prevailing scientific view, the study shows that most of the overturning and variability is occurring not in the Labrador Sea off Canada, as past modelling studies have suggested, but in regions between Greenland and Scotland. There,  warm, salty, shallow waters carried northward from the tropics by currents and wind, sink and convert into colder, fresher, deep waters moving southward through the Irminger and Iceland basins. 

Overturning variability in this eastern section of the ocean was seven times greater than in the Labrador Sea, and it accounted for 88 percent of the total variance documented across the entire North Atlantic over the 21-month study period.

University of Oxford researchers Helen Johnson (Earth Sciences) and David Marshall (Physics) participated in the study, led by Duke University in the United States and the National Oceanography Centre in the UK. 

“The overturning circulation has a major impact on how the Atlantic sector responds to climate change” said Marshall. “Recent work at Oxford has shown that the high latitude North Atlantic is the most important region for understanding how the overturning responds to anthropogenic climate change”. 

“Changes in the rate of overturning affect the transfer of heat to high latitudes and can impact on Arctic sea ice” added Johnson. 

“These findings, unexpected as they may be, can help scientists better predict what changes might occur to the meridional overturning circulation and what the climate impacts of those changes will be”, said project lead Susan Lozier (Duke University, United States). 

“As scientists, it is exciting to learn that there are more pieces to the overturning puzzle than we first thought,” said coauthor Johannes Karstensen (GEOMAR Helmholtz Centre for Ocean Research, Germany). 

The paper, published on 1 February in Science, is the first from the £25 million, five-year initial phase of the OSNAP (Overturning in the Subpolar North Atlantic Program) research project, in which scientists have deployed moored instruments and sub-surface floats across the North Atlantic to measure the ocean’s overturning circulation and shed light on the factors that cause it to vary.

Primary funding came from the US National Science Foundation’s Physical Oceanography Program and the UK Natural Environment Research Council. Additional funding came from the European Union 7th Framework Programme and Horizon 2020. 

CITATION: A Sea Change in Our View of Overturning – First Results from the Overturning in the Subpolar North Atlantic Program, M.S. Lozier, F. Li, S. Bacon, F. Bahr, A.S. Bower, S.A. Cunningham, M.F. de Jong, L. de Steur, B. DeYoung, J. Fischer, S.F. Gary, N.J.W. Greenan, N.P. Holliday, A. Houk, L. Houpert, M.E. Inall, W.E. Johns, H.L. Johnson, C. Johnson, J. Karstensen, G. Koman, I.A. LeBras, X. Lin, N. Mackay, D.P. Marshall, H. Mercier, M. Oltmanns, R.S. Pickart, A.L. Ramsey, D. Rayner, F. Straneo, V. Thierry, D.J. Torres, R.G. Williams, C. Wilson, J. Yang, I. Yashayaev and J. Zhao. Science, Feb. 1, 2019. DOI: 10.1126/science.aau6592.


Climate Scientists Podcast

At the start of October, I cycled over to Cambridge to spend a couple of days at the British Antarctic Survey, to be interviewed by Dan Jones for an episode of his excellent Climate Scientists Podcast and also to work with my former graduate student and post doc Dave Munday.

The podcast can be found to on a number of platforms. Dan and I had an enjoyable and wide-ranging conversation covering creativity in science (and jazz), my group’s efforts to represent the effect of small-scale eddies in ocean climate models through the GEOMETRIC project, the OSNAP international programme to observe and interpret the overturning circulation in the subpolar North Atlantic, my pathway into science, and the importance of finding what it is that you love doing in your work.

If you do enjoy listening to this and/or other episodes, please do leave Dan with some positive feedback!

A couple of links mentioned in the podcast:

Implementation of GEOMETRIC in an Ocean Circulation Model

Julian Mak‘s paper on the Implementation of a Geometrically Informed and Energetically Constrained Mesoscale Eddy Parameterization in an Ocean Circulation Model has been published in the Journal of Physical Oceanography and is freely available.

This is the first paper in which we implement and test GEOMETRIC in an ocean general circulation model, albeit in idealised basin geometries. We find that integrations at coarse resolution, in which eddies are parameterised by GEOMETRIC, display broad agreement in the sensitivity to surface wind stress of the circumpolar transport, meridional overturning, and depth-integrated eddy energy pattern as analogous reference calculations at eddy-permitting resolutions. Differences can be ascribed to the overly simple parameterised eddy energy budget employed in this calculations, which is likely to be a key area of focus for the eddy parameterisation problem over the next decade.

Variation of circumpolar transport with wind stress in a reference calculation with explicit eddies (green), with a constant eddy diffusivity (blue) and with GEOMETRIC (red, purple). See Mak et al. (2018) for full details.