Royal Netherlands Institute for Sea Research
Phone number
+31 (0)222 36 9403
Ocean Systems (OCS)
Senior Scientist
Interview met Sjoerd Groeskamp op Vroege Vogels, zondag 2 Februari NPO Radio 1. Foto Rob Buiter

Dr. Sjoerd Groeskamp

Senior Scientist

Stirring the oceans

Physical oceanographer Sjoerd Groeskamp investigates the mixing of water masses in the  oceans to gain a better understanding of the Earth’s climate. ‘If you add sugar to your cup of coffee, then it will eventually mix in with the coffee, but that will take an awfully long time if you don’t stir it with a spoon. By that time, your coffee will be cold! In the oceans, a range of different water currents take on the teaspoon’s role and mix heat and salt, for example. These currents could be warm- and cold-water masses or the everyday tides. However, these could also be far more subtle currents, such as the rotating whirlpools or eddies several hundreds of kilometres in diameter or undulating layers of water with different densities deep below the surface.’

Indirect measurements

‘The mathematical models of the climate that we are now trying to construct still contain several annoying unknown factors. For example, in the atmosphere, we still do not understand the role of clouds well enough to capture them adequately in numbers. And in the oceans, those unknown factors are the various subtle currents. Climate models still currently calculate in units of about 100 x 100 kilometres, whereas the effects of waves in different layers of water take place at the scale of metres. Those scales cannot be simply combined in mathematical terms.’

‘In my work, I use existing measurements of temperature and salt concentration of the oceans to nevertheless try to incorporate information about the subtle mixing in the oceans into the climate models, albeit via a small detour. If I can see how heat and salt move across the oceans via the existing network of independent measurement stations, then I can hopefully derive what the effects of subtle currents were.’

Latte in the ocean

‘The immense coffee cup formed by our oceans also contains various layers with different densities, just like the brown and white layers of milk and coffee that you can see in a latte macchiato. In my research, I also try to discover how the transport of water occurs along and through those various layers with different densities. Ultimately, I hope that this research will contribute to a better understanding of the Earth’s climate and the way it is changing.’

Read more +


1. Understanding and quantifying ocean mixing from local to global scales.

2. Water mass transformation and transport of biogeochemical tracers.

3. Seal level rise: The Northern European Enclosure Dam (NEED).

4. Miscellaneous studies: Resonantly Generated Internal Waves

Keywords: Mixing, Inverse Methods, Internal Waves, Sea Level Rise, water masss transformation, tracers.

My extended webpages can be found under

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."
Why should we care about the Ocean?
  • 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.


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 3 meters in 2100 and possibly over 20 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.

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 in our atmosphere. 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.

Understanding and quantifying ocean mixing from local to global scales.

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.

​Water mass transformation and transport of biogeochemical tracers.

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.

Seal level rise: The Northern European Enclosure Dam (NEED).

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.

Society today is shaped by the climate of the past. Now that human activities rapidly change the climate, society will need to adapt. Among many other damaging effects, sea level rise (SLR) is also a consequence of climate change. Cities and agricultural land are now threatened by flooding and may become inhabitable or unusable.

The best solution to avoid problems is to mitigate (avoid) human-caused climate change. Considering the uncertainty of our ability to do so, I believe we need to simultaneously prepare for worst case scenario’s. I think about such solutions, while I also try to understand and quantify the effects on SLR due to various fundamental oceanographic processes such as mixing.

The figure above illustrates NEED: "The Northern European Enclosure Dam". It would possibly be a solution, if climate mitigation fails. However, this solution is so dramatic that it may feel unrealistic. And yet, it is merely an example of the scale of the problems we are facing. That is, the threat of SLR is so large, that we need to think in solution that are equally large. Let us hope we can limit climate change and SLR, such that solutions as NEED will not be required.

For more information on my research, my publication list and alternative research topics, please visit

Linked news

Friday 30 June 2023
Dossier: warm sea and ocean water
Global warming dominates global news. Here, rising atmospheric temperatures are usually the common thread. But of all the extra heat trapped by our greenhouse gas emissions, only 1 per cent remains in the air. The vast majority (89 per cent)…
Thursday 26 January 2023
Dossier: Dijkdoorbraken en natuurlijke kustverdediging
Op 1 februari 2023 herdenkt Nederland de Watersnoodramp van 70 jaar geleden. Hoewel ons laaggelegen land sindsdien goed beschermd is geweest, stellen de gevolgen van klimaatverandering, zoals zeespiegelstijging en weersextremen Nederland voor nieuwe…
Monday 28 February 2022
IPPC report on climate adaptation
Today the IPCC published their newest report on climate change and climate adaptation. It analyses the impacts of the climate crisis and how humanity can adapt, in addition to slashing emissions. The good news is that a liveable future remains within…
Thursday 18 November 2021
'Mapping of ocean mixing' project receives Open Competition Domain Science-M award
NIOZ oceanographer Sjoerd Groeskamp has received a NWO Open Competition Domain Science-M programme award for his 'Measuring the immeasurable: mapping of ocean mixing' project. The NWO Domain Board Science has approved sixteen grant applications today…
Friday 27 March 2020
Derde Natuurwijzer met NIOZ oceanograaf Sjoerd Groeskamp
In de Natuurwijzer van 29 maart op Radio Texel spreekt Sjoerd Groeskamp, geboren Texelaar en oceanograaf bij het NIOZ, met radiomaker en schrijver Mathijs Deen.
Friday 31 January 2020
Een dam dwars door de Noordzee: waarschuwscenario voor klimaatverandering
[English follows Dutch] Met een dam van 475 kilometer tussen het noorden van Schotland en het westen van Noorwegen, plus nog één van 160 kilometer tussen het westpuntje van Frankrijk en het Zuidwesten van Engeland, kunnen meer dan 25 miljoen…

Linked blogs

Thursday 21 April 2022
Mixation expedition, estimating ocean mixing
From April 21st researchers on the MIXATION expedition will deploy moorings to calibrate new methods that help us provide better estimates of ocean water mixing. Ocean water mixing is an important component in climate models.

NIOZ publications

Linked projects

UUNIOZ_The intermittency of large-scale ocean mixing
Sjoerd Groeskamp
Utrecht University
Project duration
1 Jan 2021 - 31 Dec 2025