PS107, 2nd weekly report | 31 July - 6 August 2017
How to find an eddy
On HAUSGARTEN cruises we typically study and sample the oceanographic conditions and the biological communities on a relatively large scale. Consecutive stations are often separated by 20-40 km or more. However, many important processes and physical-biological interactions take place on much smaller spatial scales. In order to show how new measurement and sampling techniques capable of achieving a high horizontal resolution in combination with interdisciplinary cooperation can study such small scale processes, we decided to launch a complementary campaign and set out to find a front and survey it.
On July 26th we received a satellite radar image that depicted a surprisingly straight line on the ocean surface. What appeared to be a very narrow (500 m wide) band of ice extended for 50 km from northeast to southwest. Since this was just 3 hours from our working area at the time, it seemed like a good opportunity to survey a front. Thus, planned to cross the feature at a right angle.
Our survey began with the deployment of the underway CTD. This is a small device of just 4 kg that is attached to the ship through a---what looks to be waaay too thin---Kevlar line of just 3 mm thickness. The device is dropped from the stern on a winch with the Kevlar line set to free spooling mode. On its way down, the device records temperature, salinity, and pressure, from which oceanographers love to identify different bodies of water. We let it drop for 90 seconds during which it went down to about 220 m before it was pulled back with the winch. This was repeated every 5 minutes for 3 hours. At the halfway mark of 1.5 hours, we had to take a break for 10 minutes, because the ship went through a thick band of ice. This was a first hopeful sign that the front was still where the satellite image 3 days prior had suggested it to be based on the long thin stretch of ice. We, therefore, collected our first section across the front, from 15 km on one side of the feature to 15 km on the other side of it.
Then, it was time to pull the UCTD back onto deck and look at the data it had recorded. After some quick calculations, the ocean had revealed its structure along this line: Underneath the long line of ice was a dome of much warmer water that had possibly been raised from greater depth. Such upward motion of water may bring nutrient rich water into depths where there is still enough light for phytoplankton to grow and can also drag copepods and other zooplankton up. This is why the oceanographic pattern is also of great biological interest. In respect to this we carried out a number of different biological surveys in the area of the front. Along the transects we used the underway filtration device AUTOFIM to collect water samples every ~15 min for cutting edge molecular genetic studies of composition and distribution of phytoplankton and other marine microbes across the front. AUTOFIM is installed deep down in the bug of the ship 10 m below sea surface. All steps related to sampling and filtrations happen automatically and computer-controlled. Due to the full automation of the sampling process the time needed to take an individual sample is reduced by at least a factor of three and allows sampling with higher spatial and temporal resolution, than manual filtration. This approach, combining AUTOFIM and molecular genetic analyses allows us to complement the front study with meaningful high-resolution information on biological communities. In addition to the automated underway sampling we selected a few close locations within the center of the feature, at its rim and outside of it as target stations for complementary biological water column sampling.
As the first station with the CTD water sampler, marine snow catcher, LOKI, and multinet (that is we collected water samples there and plankton hauls) took place, we recharged the battery of the UCTD and decided on the next section locations. The sections were laid parallel to the first one in order to be able to understand what was going on in those directions. Did the front just continue in the same fashion along those 50 km seen in the satellite image or did it change over some distance and what would that distance be? Onwards to the next section, where it, again, was: “drop the UCTD”, “10 more seconds of free fall”, “stop and rewind”, gaze out into the sunny sea with ice floes going by that the mate on the bridge had to dodge such that the Kevlar line would not get caught, oh, and then “ready again? Ok, drop the UCTD!” As we were out there working, that is standing at the stern of Polarstern, there were jokes about getting sunburnt at 79°N. A puffin also came to visit us where he (or was it a she?) flew around the ship multiple times and led to joy. After 3 hours, we went inside again to look at the newly recorded data while the next station took place. Ok, data, please tell us where to go next with our stations and sections! After three iterations of this, it became clear that the front indeed was no longer a straight line of 50 km length. Instead, we had been rather lucky in the location of our first section as further to the northeast and southwest, the anomalies along the sections decreased and slowly a rather circularly symmetric shape with a radius of 5 km emerged. It appears as if we actually had mapped an eddy with the stations located inside the eddy, at its rim, and outside of it. Additional instruments that ran on the ship while we were doing our sections, such as the ship hull mounted ADCP and the thermosalinograph, supported this view. This means that the front probably broke apart into eddies, one of which we surveyed, between the time of the satellite radar image and our survey.
Already those very preliminary results inspire much confidence for further analysis and combination of our data, to finally get the big picture for the oceanographic and biological processes occurring at our investigated feature.
Wilken-Jon von Appen, Holger Auel, Nicole Hildebrand, Katja Metfies und Ingo Schewe