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Granite Beach, South Coast Track, Tasmania, 2014. Photo by Alinta Robinson-Herbert.

Why Care

​From the weather we experience to the climate we live in.

From the oxygen we breathe to the food we eat.

From the resources mined and extracted from the ocean bottom, to the first signs of life on this planet.

The ocean is key to (our) life on earth.


The ocean can both be the source and the solution for the many problem we face.


Although the Amazon is often considered to be the lungs of the planet, as it produces about 20% of the oxygen we breath, it is actually ocean phytoplankton (single celled algae) that produce about 50% of the oxygen on Earth. This picture shows phytoplankton near Finland. Phytoplankton growth depends strongly on ocean circulation and mixing processes. This is a NASA Earth Observatory images by Joshua Stevens and Lauren Dauphin, using Landsat data from the U.S. Geological Survey and MODIS data from LANCE/EOSDIS Rapid Response on July 18, 2018.

The ocean has absorbed about 30% of the CO2 emitted by humans and 90% of the extra warmth (heat) added to the atmosphere by human activity. Imagine how warm it could have been now without this effect! Small changes in these numbers can have enormous impacts on our weather and climate. So how much heat and CO2 will the ocean keep absorbing in the future? 


Another example is sea level rise predictions that vary from 1 to 2 meters in 2100 and possibly over 15 meters in the following centuries. Which of these predictions are most accurate? How fast will sea level rise occur? And how do we cope with such changes?

These are only a few of many reasons why we should care about the ocean.

Research Interest

My Research Interests

No one-person can answer these important questions discussed above in perfect detail, but we can all contribute our piece of the puzzle. I study ocean processes that influence the answers to these questions, and I deliver these pieces of the puzzle to the broader science community. As a community we then try to solve the puzzle and answer these questions as a whole.

"My work is to contribute to our understanding of the role of the ocean in climate and ecosystems."

Oceanography is the study of the biological, chemical and physical processes in the ocean. I’m a physical oceanographer and study the physical processes that occur in the ocean. My research can be roughly categorised into the following topics:

  1. Understanding and quantifying ocean mixing.

  2. Water mass transformation and transport of biogeochemical tracer.

  3. The Northern European Enclosure Dam (NEED).

  4. Miscellaneous topics such as Resonantly Generated Internal Waves or Drifting dynamics of blue bottles, amongst others.


Understanding and quantifying ocean mixing.


A cartoon by Malou Zuidema Illustrations, showing a combination of external ingredients and ocean mixing, will lead to the creation of different types of water (fresher Vs Saltier, Warmer Vs Cooler, etc).

Mixing is a fundamental processes occurring in the ocean. With fundamental, I refer to both the small scales at which mixing occurs, as the fact that mixing feeds into many other important ocean features. Examples are transport of heat and carbon, ocean-atmosphere interaction and food availability for marine animals. Clearly, understanding mixing is important for understanding many other things that happen in the ocean.

Not only should we understand ocean mixing processes, we also need to quantify them. Evidently, mixing is not the same everywhere (think of the difference between calm waters and the surf zone). However, the spatial and temporal distribution of mixing strengths are not well known. This is because mixing is difficult to measure due to do technological, logistical and financial limitation.


Therefore, I develop techniques that estimate mixing indirectly from observations that are available, such as temperature and salinity. These estimates help understand how to implement mixing in numerical simulations of our future climate and sea level.

Related Publications:

2020 - The Oceanic deformation radius over bathymetry

2020 - Full-Depth Global Estimates of Ocean Mesoscale Eddy Mixing from Observations and theory.

2019 - VENM An algorithm to accurately calculate neutral slopes and gradients.

2017 - Comment on Tailleux, "R. Neutrality versus Materiality: A Thermodynamic Theory of Neutral Surfaces".

2017 - Mixing Inferred from an ocean hydrography and surface fluxes.

2014 - A thermohaline inverse method for estimating diathermohaline circulation and mixing.

2014 - On geometrical aspects of interior ocean mixing.


Water mass transformation and transport of biogeochemical tracer.


A cartoon by Malou Zuidema Illustrations, illustrating the processes that influence water mass transformation. This figure is part of the paper Groeskamp et al (2019) - The Water Mass Transformation Framework for Ocean Physics and Biogeochemistry.

Water mass transformation described how properties of a bit of water in the ocean (a water mass) can change (transform). These properties can be heat, density, carbon, oxygen, amongst many others. Studying water mass transformation improves improve our understanding of how these properties are absorbed, transported, stored in the ocean.

In an abstract way, water mass transformation is a form of ocean circulation, but instead of using “location” as a property, we use a tracer as property. This trick provides completely new description and insights into the movement of these properties through the ocean.

Related Publications:

2021 - The Origin and Fate of Subantarctic Mode Water in the Southern Ocean.

2019 - Climate recorded in seawater: A workshop on water-mass transformation analysis for ocean and climate studies.

2019 - The Water Mass Transformation Framework for Ocean Physics and Biogeochemistry.

2018 - The effect of air-sea flux products, Short Wave Radiation Depth Penetration and Albedo on the Upper Ocean Overturning Circulation.

2016 - Anthropogenic Carbon in the Ocean, Surface to Interior Connections.

2016 - Water Mass Transformation by Cabbeling and Thermobaricity​
2014 - The representation of ocean circulation and variability in thermodynamic coordinates.


The Northern European Enclosure Dam


A sketch of the Northern European Enclosure Dam, conceptualised to protect many countries and cities against Sea Level Rise. However, I emphasise that this is the very last thing we actually want to do, due to its immense consequences for nature, the maritime industry, the weather and many other things. The best approach is to avoid SLR as much as possible, as a whole, by reducing climate change to a minimum.

For More information on NEED, please go here.

For a Keynote presentation about NEED, please go here.



Resonantly Generated Internal Waves


A Photo taken by an Astronaut on board of the International Space Station, showing the effects of ocean waves underneath the surface, on the surface of the ocean. Such "Internal waves" are important for transport of tracers and sediment. Breaking internal waves also lead to mixing, a little bit like breaking waves at the beach. More details can be found in Groeskamp et al. (2011, pdf)

This study presents one of the first to present observational evidence of the existence of internal waves that are generated by resonance.


The Marsdiep is a channel between the Wadden Sea and the North Sea in the Netherlands. Tides cause strong currents of water flowing in and out of the Wadden Sea, through the Marsdiep channel. When the current is strong, the water is well mixed from top to bottom. This means, the water is almost the same density, temperature and salinity, from top to bottom. However, when the currents are not strong (slack-tide), there is no mixing and vertical ‘layering’ of density occurs. In particular, we find light waters above dense waters. Due to special conditions (resonance conditions) in the Wadden Sea, disturbances between these lighter and denser layers, can grow into waves between the layers. These waves are called Internal Waves as they are below the sea surface.


Why is this important?

There are different kind of Internal Waves in the global ocean, and also different causes that form these waves. They vary in height from centimeters to kilometers, and like waves at the surface, also internal waves can break. Apparently, Dolphins surf on internal waves, while submarines can get in trouble if they collide with one. Internal Waves can travel long distances before they break. However, when they break, they can cause mixing between water masses that would otherwise not mix. This mixing leads to exchange of properties (e.g. plankton, nutrients, carbon, heat, salt) between the different water layers. This is one of few mechanisms in the ocean by which such an exchange can take place. Hence understanding internal waves is important for understanding for climate science, fisheries and maritime industry. The Internal Waves observed in the Marsdiep, as small as they might be, will improve our understanding of how internal waves are generated, move and break.

Related Publications:

2011 - Observations of estuarine circulation and solitary internal waves in a highly energetic tidal channel.

2012 - Ship-borne contour integration for flux determination [This publication is not about the internal waves, but on mussel seed installations nearby]

Blue Bottle "Jellyfish"


Photograph of a Bluebottle taken at Coogee Beach, Sydney. Courtesy of Lisa Clarke. Here we are measuring the dimensions of a blue bottle that we captured. These Blue Bottles are nasty buggers when they sting you.

When I lived in Sydney (Australia) and regularly swam at Coogee Beach, I always wondered why Blue Bottles would suddenly swarm the beaches. It had to do with the wind direction, but perhaps more was at stake. To satisfy this curiosity we started a project in which we treat the Blue Bottles as little sailboats so we can derive some equations that can describe their drifting dynamics. We can now use this to understand their drifting direction depending on the wind, waves and other factors. So far, the work has been mathematical. Comparison with observations and experiments to improve the model, are yet to be conducted.

Related Publications:

2021 - Drifting Dynamics of the Bluebottle.

Research Group: 

Since November 2019 I started a Tenure Track at the NIOZ.

In this position I'm privileged to be able to supervise various PhD students.

Currently the PhD students I'm supervising are:

2018 - ongoing --- Zhi Li (together with Matthew England)

2021 - ongoing --- Niek Kusters

2021 - ongoing --- Miriam Sterl (together with Michiel Baatsen) 

Master Students currently working are:

Simen Bootsma --- (Together with Matthew Humprheys)

Athina Karaoli    --- (with Johan van der Molen and Leo Maas)

Pim Koch           --- (With Bas Jonkman en Coen Kuipers)

Peter Farrel        --- (with Casimir de Lavergne)

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