Gulf Stream: Does it weaken or not?

Measuring campaign on oceanic deep circulation started in the North Atlantic

Research vessel Maria S. Merian in action © Institute for Environmental Physics, University of Bremen
Read out

Oceanographers keep asking themselves what changes the Gulf Stream and Oceanic Deep Circulation in the North Atlantic are undergoing. With the newest member of the German research vessel fleet, Maria S. Merian, scientists from the Institute of Environmental Physics (IUP) are now on the road to collect another piece of the puzzle.

{1l}

"We now have data from ten years that show that since 1997, the depth of water education has decreased by 70 percent, " said Professor Monika Rhein from the IUP shortly before her departure for Las Palmas, the starting point of the five-week trip. Between Greenland and the coast of Labrador in northern Canada, water sinks into the depths as it is cold. Just like cool air, cool water is heavier than warm. In the depth, it then flows to the south and thus begins the long journey which brings it in the North Pacific back to the surface and eventually back to the North Atlantic. Along with nutrients and organisms, it also carries a lot of heat on this journey. This determines our climate especially in northern Europe.

Climate models put to the test

Climate models show that the transport of warm water through the Gulf Stream is closely related to this global circulation and thus to the formation of cold deep water between Greenland and Canada. Since the same climate models are also to be used to predict future climate changes, there is understandably a great deal of interest in checking that they are accurate.

With sophisticated technology, the team around Monika Rhein now want to collect a whole lot of new data on the changes in the transport of the Gulf Stream. For example, they use four permanently anchored long-term sensors along the mid-Atlantic ridge between 46 degrees and 53 degrees north. These actually do nothing other than a ship's echo sounder: they measure the time it takes for a sound wave to measure the water depth, but in the opposite direction to the surface. "If we have the running time, all we have to do is find out which salt and temperature profile is right for it, " says Monika Rhein. display

But only good is: For this, the scientists need more data from other devices, either collected from the ship or by so-called drifters. These are tubular devices that move up and down the water on their own while they drift with the currents. They come to the surface regularly and send their data by satellite to the researchers' lab. Their advantage is that they can collect more data much cheaper than is possible from the ship. Both methods can be used to measure how the salinity, pressure and temperature of seawater changes from the surface to the seabed. Professionals call this type of data series a profile.

Determination of salinity, temperature and pressure

Since each such profile has a certain transit time, which is determined by salinity, temperature and pressure, the long-term sensors can collect a large amount of data with little effort. They stay on the seabed for over five years. But that's not the time researchers have to wait for data. The sensors reveal their treasures to an acoustic signal when the research vessel is near. There are also data from satellite surveys. WennOnly when we collect all the data, can we estimate how much the transports in the Gulf Stream are changing. The reason why Rhein demands the necessary combination of different measuring instruments.

Research vessel Maria S. Merian in action Institute for Environmental Physics, University of Bremen

The formation of deep water in the Labrador Sea is investigated by the team of Monika Rhein with the help of trace material inventories. These trace substances are introduced into the sea surface via contact with the atmosphere and are taken into the depths when the cold water sinks. Changes in these inventories from year to year are the best way to determine the changes in the sunken volume of the deep water. From the measurements, the group found that since 1997 the amount of deep water formed has decreased by 70%.

Interpretation still difficult

This data still does not tell us whether the circulation has really slowed down in the long run, or whether it is just a natural variation in intensity. We also do not know for sure whether the waning of the formation of deep water will be accompanied by a lesser transport of Gulf Stream, "explains Monika Rhein. Unfortunately, our data series are shorter than the cycles of natural fluctuations in deep water formation. However, if we find that more deep water has been re-formed since 2005, it would be argued that in recent years we have seen only a natural variation and not the influence of man., explains Monika Rhein. However, if the value were to remain on the same low order of magnitude, that would be another piece of the puzzle in our understanding of the ocean circulation in the North Atlantic

(Kirsten Achenbach, MARUM_Forschungszentrum Ozeanr nder, 19.04.2007 - AHE)