Climate-Relevant Exchange Processes between Atmosphere and Ocean

The Heidelberg Aelotron | © Bernd Jähne
The Heidelberg Aelotron | © Bernd Jähne

Environmental physicist Bernd Jähne from Heidelberg University is pursuing a new approach to exploring the processes that ensue with the exchange of climatically relevant gases and volatiles between the atmosphere and the ocean. To this end, the scientist will use two imaging measurement procedures for experiments in the Heidelberg Aelotron, a wind-wave tank. The German Research Foundation is financing the project within the Reinhart Koselleck Programme. Funds amounting to approximately 1.2 million euros will be available over a period of five years. The research studies at the Institute of Environmental Physics will be conducted in collaboration with the Heidelberg Collaboratory for Image Processing, which is based at the Interdisciplinary Center for Scientific Computing (IWR).

In the words of Prof. Jähne, the mechanisms of small-scale exchange processes on the ocean surface have not been sufficiently well understood to date, despite their significance for global energy and material cycles. With respect to climate change, it is particularly important to find out how CO2 is transferred from the atmosphere into the sea. "Field measurements have so far not contributed much to the understanding of the underlying physical mechanisms. Above all, there is a lack of reliable results for low wind speeds. Laboratory measurements in linear wind-wave tanks have the disadvantage that the shape of the waves generated by the wind is often different on the water surface from the waves on the open ocean," underlines Bernd Jähne, senior professor at the IWR and a research group leader at the Institute of Environmental Physics.

The scientists now want to simulate the oceanic conditions at low and medium wind speeds in a considerably more realistic way by using the Heidelberg Aelotron, an annular wind-wave tank ten meters wide, located at the Institute of Environmental Physics. That involves using two advanced imaging techniques. The heat exchange rate is to be measured by active thermography and a new opto-chemical technique will visualise the mass border layer on the water surface, which is less than a millimeter thick, in order to determine the local gas exchange rate. "By using these fast techniques we can, for the first time, examine the processes controlling gas and heat exchange in the presence of growing and decaying wave fields," underlines Prof. Jähne. At the same time, at low wind speeds, the aim is to investigate the influence of surface-active materials on gas transfer - this influence is known but has so far been less quantified. Surface-active materials are produced as waste products in the metabolism of marine organisms and they also attenuate wind-generated waves. "Our measurements will enable a physically based description of the mechanisms of gas exchange under oceanic conditions," says the Heidelberg environmental physicist.

The aim of the second phase of the project "Quantifying the Mechanisms of Gas Exchange between Ocean and Atmosphere - Bridging Laboratory and Field by Imaging Measurements" is to test a technique to measure gas and heat exchange in less than a minute with metre-scale resolutions. The research instrument is a simple, reasonably priced thermal imaging camera to determine the transfer rate of the exchange, as well as the mechanisms involved, from the small-scale temperature patterns on the water surface caused by natural heat transfer over the ocean surface. "That way, we can verify at the same time whether our laboratory measurements have included all mechanisms relevant to the ocean," says Prof. Jähne. These measurements are being carried out in cooperation with the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Institute for Chemistry and Biology of the Marine Environment at the University of Oldenburg.

The Reinhart Koselleck Programme of the German Research Foundation offers scientists the opportunity to conduct highly innovative or, in a positive sense, risky projects.