Picture of participants (above): First row, left to right: Zanna Chase, Abhishek Savita, Gurvan Madec, Stephen Griffies, Veronica Tamsitt, Ivy Frenger, Sjoerd Groeskamp, Ivana Cerovecki, Francois Primeau, Daniele Iudicone, Matthew England, Victoria Coles. Second row, left to right: George Nurser, Andreas Klocker, Yuehua Li, Bob Marsh, Trevor McDougall, Richard Matear, Damien Irving, John Church, Jonathan Gregory, Emily Newsom, Elaine McDonagh. Third row, left to right: Aaron Lang Yangdong, Paul Spence, Graeme MacGilchrist, Chris Bladwell, Aitor Aldama-Campino, Violaine Pellichero, Geoff Stanley, Catia Domingues, Kristoffer Döös, Casimir de Lavergne. Fourth row, left to right: Jan Zika, Chris Chapman, Bernadette Sloyan, Joakim Kjellsson. (Participants not in photo: Ryan Holmes, Mark Holzer). Photo by: Veronique Lago.

by Ryan Holmes, Sjoerd Groeskamp and Casimir de Lavergne


In early February 2019 an international cohort of around 40 oceanographers, marine biogeochemists and climate modellers gathered at UNSW to discuss the use of the Water Mass Transformation (WMT) framework for studies of ocean physics, biogeochemistry and climate. The workshop was an initiative of several CLEX early career researchers and gained interest from a diverse international community. Funding from the ARC Centre of Excellence for Climate Extremes and the UNSW School of Mathematics and Statistics facilitated attendance by several high-profile international scientists. The workshop was a resounding success, leading to a better understanding of the potential of WMT to aid ocean physics and biogeochemistry studies, and to extensive ongoing activities.

What is WMT?

In contrast to the atmosphere, the ocean is heated from above by solar radiation. Consequently, the ocean tends to be stably stratified with warm light surface waters overlying colder, denser deep waters. This stable stratification restricts direct vertical exchange in the ocean, meaning that interior waters are sheltered from change. Water masses acquire their core properties (such as their heat content and chemical composition) in a thin surface layer and, once they move into the interior, can maintain these properties for periods ranging from years to millennia. Water masses, therefore, serve as a long-lived record of the climate and help to sequester properties such as carbon and heat away from the surface. The WMT framework describes the ocean in terms of these water masses as a series of moving layers defined by particular tracer properties. The framework complements more traditional Eulerian and Lagrangian approaches that frame the ocean circulation in terms of fixed boxes (Eulerian) or moving water parcels (Lagrangian). Understanding ocean circulation using the WMT framework reduces to understanding how water masses are created at the surface by air-sea fluxes and how they are ultimately destroyed (or transformed into other water masses) through interior mixing or re-emergence into the surface layer (many of these processes are illustrated in Figure 1). The main advantages of the WMT framework over more traditional approaches are that 1) “unimportant” adiabatic displacements that do not alter the seawaters’ properties are automatically filtered out and 2) bulk properties of the ocean circulation that are often difficult to measure, such as net mass or heat fluxes, can be estimated from more measurable quantities such as temperature and salinity distributions.

Figure 1: Illustration of the zoo of processes that modify the properties of a water mass. Reproduced from Groeskamp et al. (2018).

Big potential

The WMT 2019 workshop was aimed specifically at bringing together a wide range of expertise to explore ways in which the WMT framework could be expanded for use in interdisciplinary studies. Instead of the traditional focus on research talks, the workshop was centred around a series of afternoon discussion sessions aimed at tackling prominent questions and identifying ways forward. Core subjects discussed included:

  1. The potential for leveraging new observational data sets, such as the worldwide hydrographic measurements available through the recent Argo float program, along with WMT to understand climate change and variability, and how it impacts on the ocean.
  2. The application of WMT techniques, traditionally used in the physical oceanography community, to biogeochemical tracers (such as carbon and nitrate). This has potential not only for improving our understanding of carbon and nutrient cycles, but the biogeochemical budgets can also be used to provide additional constraints on ocean physics.
  3. The use of WMT as a tool for ocean and climate model inter-comparison and evaluation.

The group identified a range of challenges to the adoption of these techniques and discussed potential avenues to overcome these challenges.

Where to next?

Several tasks are underway to ensure that the workshop delivers on its promise to stimulate and maintain progress beyond the workshop itself. These include the development of a forum and website for hosting code, a session proposal at the international Ocean Sciences conference to be held in San Diego on February 2020 and the organisation of a follow-up WMT meeting, to be held on April 2021 in Italy. With a rapidly expanding community of energetic and interdisciplinary scientists and many untapped potential applications, the WMT framework promises to become a forefront technique in oceanography and climate science.

For more information:

  • Groeskamp et al. (2019) Climate recorded in seawater: A workshop on water-mass transformation analysis for ocean and climate studies, Bulletin of the American Meteorological Society, in press, http://dx.doi.org/10.1175/BAMS-D-19-0153.1