Millions of people live on the North Sea coasts and use these areas for fishing, cargo transport, energy production, recreation and many other purposes. People were and are, therefore, dependent on the “moods” of the sea.

Protection from storms and floods has always been a vital issue on the coasts.
To better prepare for possible dangers, it is important to study and understand the processes in the sea and to investigate if, and how, they change over the course of time. It isn’t enough, however, to only study the sea surface: what happens in the depths of the North Sea is of crucial importance in understanding the system in its entirety.

We still know very little about ocean processes because underwater studies are time-consuming, expensive and sometimes even dangerous. Scientists at the Institute of Coastal Research at the Helmholtz-Zentrum Geesthacht utilize different measurement methods to also uncover the secrets beneath the surface of the North Sea. In order to capture the ocean in three-dimensions, the researchers deploy ship-based and independent (autonomous) systems.

“Gliders,” autonomous underwater vehicles, can travel through the ocean on their own for weeks or months. With small changes to their buoyancy, and thanks to their “wings,” they can change depth, allowing them to continuously measure water properties as they steer a zig-zag course between the water surface and the sea floor. After thirty to forty such dives, the glider returns to the surface after approximately four hours, relays the results via satellite to the scientists, and receives new commands for further operation.

Another method for researching the depths of the North Sea is by using instrument carriers that are pulled behind the ships during research cruises; the carriers oscillate between the surface and sea floor, recording data for temperature, salinity, suspended matter, oxygen content and chlorophyll.

The scientists at the Institute of Coastal Research can use the collected data to generate maps of these quantities, which can, thereby, help us understand the processes in the North Sea. The figure provides an example of the impact a storm can have on the mixing in the North Sea: the water first shows a clear stratification with warmer water in the upper ten meters. The warm water at the surface is then mixed with cold deep water due to the storm, while sediment from the sea floor is simultaneously brought to upper layers. The cold water draws nutrients to the surface, leading to an algae bloom, which can be seen in increased chlorophyll concentrations.