Research & Projects
Image of a transition zone between stratified and mixed conditions in the North Sea 2014. -Image: HZG-
In large portions of the North Sea during summer, increased heat input leads to the development of a warm upper layer that is somewhat disconnected to the cooler bottom waters by a relatively thin transition zone called the thermocline.
This seasonal stratification is vital in determining the growth of phytoplankton, and therefore the food supply to the marine food web. In addition, the stratification also heavily influences the amount of particles suspended in the water (suspended sediments), as well as the formation of low-oxygen "dead zones". However, predicting the development and evolution of the stratification is not an easy task. It is critically dependent on understanding the amount of turbulence that is present in the North Sea. This turbulence is responsible for mixing away the stratification, and can be generated through a number of different processes.
Of particular interest is quantifying the turbulence that is generated as the tides push and pull water over the sea bed. Turbulence generated in this way contributes to an increase in sediment suspension as particles are dragged up from the sea bed, as well as contributes to the slow mixing of the stratification. The Small-scale Physics and Turbulence group is also examining the turbulent mixing processes that are acting directly within the thermocline.
The goal is to understand the processes that are dominant in setting the overall stratification in the North Sea, and to use this knowledge to model and predict possible changes. These changes could occur from, for example, the installation of offshore wind farms, or increasing temperatures due to climate change.
A so-called underwater glider is ready for a new measurement campaign. -Image: Raimo Kopetzky / HZG -
The Small-scale Physics and Turbulence group is currently using ocean gliders to measure turbulent processes in the North Sea and other regions.
Ocean gliders are underwater vehicles that can be operated remotely via satellite connection to collect long term and long range measurements. They can be deployed for periods of many weeks at a time since they are designed to operate with a very low power consumption. In line with this low power consumption, they use no propellers to move forwards. Instead, they sample the sea vertically by using a pump to adjust their buoyancy, and wings to propel them forward while diving and climbing, thus moving in a "saw-tooth" pattern through the ocean.
Though gliders have traditionally been used in deeper waters, our group can successfully operate them in the shallow and energetic North Sea coastal region. They also provide an ideal platform for turbulence measurements, and they are currently used to study ocean mixing and sediment suspension processes.