PS113 - Weekly Report No. 3 | 21st - 27th May 2018
Cruising through the Blue
[28. May 2018]
After entering the Tropics in the Southeast Trade Wind region and sailing under cloudless skies, the second half of the week was largely cloudy with frequent rainfall. We had arrived in the Inter-Tropical Convergence Zone, in which the air masses transported by the Southeast Trade Winds and their northern hemispheric counterpart, the Northeast Trade Winds, rise up and produce thick clouds reaching high into the atmosphere.
At this season in the northern summer the Inter-Tropical Convergence Zone has moved a few degrees of latitude north of the equator. We crossed the equator itself on the morning of the 25th May. Last night we reached the NE Trade Wind region and are pleased to enjoy radiant sunshine again. So much to the atmospheric climate regions we experienced so far.
The ocean we have been passing through is really a desert. The visible sign of oceanic deserts is deep blue seawater, which we have been sailing through since 40° south - for the best part of the last 3000 nautical miles. The water has this deep blue colour when it is very pure. And it is so pure here in the open tropical and subtropical ocean, because of the lack of nutrient input from land, or from the upwelling of deep water that there is hardly any growth of phytoplankton, those microscopic algae which float in the water. In the centre of the oceanic subtropical gyre between 20° and 10° south we could see from the fluorometers mounted both on the CTD and the Triaxus towed ocean profiler of the AWI that only at a depth of 140 m, in which there were nutrients but hardly any sunlight for photosynthesis, a little chlorophyll was present. If there is little primary productivity by phytoplankton then the whole basis of the marine food web is missing. It is therefore hardly surprising that for days on end we have seen no sea birds, nor whales for even longer.
The optical properties of the waters, the changes of which in terms of intensity and colour with depth, as well as the influence on these of the phytoplankton concentration and species composition, are the focus of the phyto-optics group. Precise knowledge of how the light changes under water and with which intensity and colour it is scattered back into the atmosphere are also necessary to improve the remote sensing of phytoplankton by satellite. The phyto-optics group develops robust algorithms which allow the production of continuous global maps with greater temporal (daily) and spatial (300 m) resolution of the quantity and composition of phytoplankton and its decay products based on satellite measurements.
Every day around noon a package of optical sensors is lowered beside the ship to a maximum depth of 150 m to record profiles of optical parameters. These include a hyperspectral radiometer called RAMSES, for measuring the intensity and colour of the sunlight within the sea coming from above. With a spectrophotometer type ACS absorption and scattering are also measured. To calibrate these optical measurements, water samples, collected at six different depths within the sunlit upper ocean layer using the water bottles on the CTD/Rosette sampler, are analysed for their pigment composition and the optical properties of the various components of the water. The analysis so far indicate that near the Argentinian coast the phytoplankton concentration was high and the most populous form were diatoms growing in the upper 20 m of the water. In contrast in the warm (>28°C) and saline subtropical waters around 15°S there are only a tenth as many algae. These instead, predominantly cyanobacteria, grow best at 140 m.
A RAMSES and an ACS are also installed in the Triaxus towed vehicle. An advantage of making measurements from this towed system is that they are beyond the influence of the ship and can be made with a significantly higher data rate. A disadvantage could be that the data quality might be lower because Triaxus is towed through the water much more quickly, mostly with 4 m/s horizontally and 1 m/s vertically, than the speed at which the optical package is lowered and raised through the water from the ship. The effect of the fast measurement on the data quality must be determined through comparison of the two techniques.
A further ACS photometer is used to continuously measure the scattering and absorption properties of the surface sea water taken in at keel-depth and distributed to the laboratories via a seawater circulation system. These data are also regularly calibrated with direct measurements of water samples. From these continuous optical measurements much can be learned about the quantity and composition of the phytoplankton. With this dataset many thousands of pixels of satellite data can be validated later on.
On behalf of all the cruise participants I send greetings from on board Polarstern.
Volker Strass.
Contact
Scientific Coordination
Rainer Knust
+49(471)4831-1709
Rainer Knust
Assistant
Sanne Bochert
+49(471)4831-1859
Sanne Bochert
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