Oceanic heat and carbon sinks: The changing role of Global Ocean Mode Waters

Dr Dafydd Gwyn Evans, Bob Marsh, Dr Alice MarzocchiDr Peter Brown, Prof Helen Johnson (University of Oxford)

In the North Atlantic, Subtropical Mode Water (~18˚C water) is undergoing dramatic changes, declining in volume by up to 90% during the last decade (Stevens et al., 2020). Subtropical Mode Water and other mode waters in the Southern and Pacific Oceans account for a substantial fraction of the heat and carbon dioxide that is drawn from the atmosphere into the ocean, slowing climate change. Understanding the variability of mode water volume and the processes that control this variability are therefore vitally important if we want to better predict how the global climate system will change into the future.

The focus of this project will be on understanding the changing role of the ocean’s mode waters in the oceanic drawdown of heat and carbon dioxide. This project will use cutting edge techniques, develop novel methods of analysis and explore the use of machine learning within oceanographic data. Changes in mode water formation and volume will be analysed within observational data (Argo, hydrographic sections) state estimates (ECCO) and models (NEMO, CMIP6). The overarching goal will be to quantify the role of mode waters as a carbon and heat sink and determine how this sink is changing.


Key to understanding any oceanic change is separating diabatic (warming/cooling, freshening/salinification) and adiabatic (advection/heave) changes. In most analytical frameworks these components of change are very difficult to separate, and may lead to invalid conclusions if misinterpreted. This project will take advantage of recent advances in water mass frameworks that are used to separate diabatic and adiabatic changes in the ocean. The use of water mass frameworks has led to novel insight into key components of the Southern Ocean overturning circulation and successfully determined the drivers of recent interannual variability in the Atlantic Meridional Overturning Circulation. A water mass framework will be used in conjunction with machine learning-based tools to identify water mass classes and characterise water mass variability. This analysis will incorporate biogeochemical observations, in particular dissolved inorganic carbon, to quantify the heat and carbon budget of mode waters. Observational and model data will be used in a complementary manner, to gain a more comprehensive view of mode water volume and variability, and associated heat and carbon uptake. The analysis will provide a benchmark for the representation of mode waters in future climate models. The candidate will strive to develop open-source and accessible code and results for maximum academic impact.

NOC Southampton

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the National Oceanography Centre, Southampton. Specific training will include:

  • Programming (Python, Matlab etc.), development of open source software.  
  • Access to, and handling, a variety of observational and model data, including physical- and biogeochemical-based data.
  • Configuration and operation of ocean models with varying degrees of complexity
  • Potential to spend time working at the University of Oxford.
  • Participate in NOCS led hydrographic- and mooring-based cruises.
  • Attend national and international conferences.
Eligibility & Funding Details: 

Please see https://inspire-dtp.ac.uk/how-apply for details.

Background Reading: 

Evans, D.G., J. Toole, G. Forget, J.D. Zika, A.C. Naveira Garabato, A.J.G. Nurser, and L. Yu, 2017: Recent Wind-Driven Variability in Atlantic Water Mass Distribution and Meridional Overturning Circulation. Journal of Physical Oceanography, 47, 633–647, https://doi.org/10.1175/JPO-D-16-0089.1.

Maze G., H. Mercier, R. Fablet, P. Tandeo, M.L. Radcenco, P. Lenca, C. Feucher and C. Le Goff, 2017: Coherent heat patterns revealed by unsupervised classification of Argo temperature profiles in the North Atlantic Ocean, Progress in Oceanography, 151, 275-292, https://doi.org/10.1016/j.pocean.2016.12.008.

Stevens, S.W., R.J. Johnson, G. Maze and N.R. Bates, 2020: A recent decline in North Atlantic subtropical mode water formation. Nature Climate Change10, 335–341, https://doi.org/10.1038/s41558-020-0722-3.