PS113 - Weekly Report No. 4 | 28th May - 3rd June 2018
The Tropical North Atlantic: Cold in the Water, Dust in the Sky
[04. June 2018]
The whole week long we sailed in the North-east Trade Wind zone under skies with only a few clouds, but with strong head-winds for Polarstern. By crossing the northern tropic, the Tropic of Cancer, at 23° 26’ 05” North during the night of 31st May to 1st June we have left the Tropics and are again in the Subtropics. Although we have been moving through the Tropics and Subtropics, the water and air surrounding us with temperatures of around 20°C have been relatively cool, 10° colder than the daytime temperatures in much of Germany. That sounds like a topsy-turvy world, but it has a simple explanation.
On the west side of the continents, the east side of the oceans, the Trade Winds blowing offshore drive the warm surface water away from the coast. To replace this, colder water from below wells up to the surface. This colder water brings with it new nutrients, such as nitrate, into the sunlit upper ocean where phytoplankton can grow. For this reason, these coastal upwelling regions support some of the most important fisheries on earth.
In satellite images of the chlorophyll distribution, which thanks to the AWI-Phytooptics Group are also available here on board, the strong increase in the phytoplankton concentration near to the coast of Africa could clearly be seen. Isolated tongues of enhanced chlorophyll concentration stretched away from the coast to beyond our route over 200 miles out in the open ocean. We used this opportunity to test the instruments and sensors of the Triaxus/topAWI towed system for their suitability for detecting biogeochemical parameters. To this end, we towed Triaxus for 23 hours continuously through one of these tongues – with a very satisfactory result. We had a similar 32-hour long deployment crossing the equator. Then the emphasis was on testing the acoustic Doppler current profilers mounted on Triaxus for their ability to measure current shears. We chose the equator for this test because there, in 50 to 200 m depth, the Equatorial Undercurrent flows with a speed of 1 m/s towards the east, in contrast to the adjacent westward flowing North and South Equatorial Currents.
The upwelling region off Northwest Africa touched by our route also receives a trace nutrient from above: iron, which is contained in dust from the Sahara. In many ocean regions far from land there is a lack of iron, so that even with a goodly supply of other nutrients phytoplankton growth is limited. The images from weather satellites presented by the meteorologist on board showed gigantic clouds of dust blown by the Trade Winds out over the Atlantic and right across to America.
The Sahara dust was also measured here on board. During the transit voyag, the team from the Leibniz Institute for Tropospheric Research in Leipzig looks after a variety of remote sensing instruments, which as part of the OCEANET project, are regularly on board during the transit voyages and deliver continuous high-resolution observations. The Atlantic transits permit studies contrasting the anthropologically loaded Northern Hemisphere with the much cleaner Southern Hemisphere. One of the most important instruments is a Lidar (Light Detection and Ranging device), with whose laser beam the vertical aerosol and cloud distribution in the atmosphere can be physically characterised. Amongst other things, the measurements allow the aerosol type as well as the size and concentration to be determined as a function of the height. Other instruments register the solar radiation reaching the earth’s surface and other cloud parameters, such as the column total of water vapour and liquid water.
On reaching the Trade Wind zone north of the Inter-Tropical Convergence Zone things became more exciting for the atmospheric scientists. After 3 weeks with marine aerosols, the Sahara lay in exactly the direction from which the Trade Wind was blowing and layers of dust aloft were crossing our path on their way westwards. In the Lidar measurements it could be seen how the dust cloud above Polarstern increased in thickness and on the following days showed a height between 1.5 and 5 km. The haziness in the sky produced by the dust could be seen with the naked eye. On approaching the Canaries at the end of the week the thickness of the observed dust cloud gradually diminished.
The high values of aerosol optical depth combined with low water vapour content in the atmosphere were also measured by a colleague from the Max-Planck-Institute for Meteorology in Hamburg, using a MICROTOPS solar photometer. This instrument measures the attenuating effect of scattering and absorption by aerosols (such as dust and sea salt) and gases (water vapour) at five different wavelengths and so gives information on aerosol concentration and particle size. To do this the solar disk must be clearly visible and the instrument pointed towards it by hand. So long as the solar disk is not obscured by clouds, these measurements are repeated every 15-30 minutes, a labour intensive procedure. The data obtained are relayed, generally available, to NASA and serve to improve satellite observations and climate models.
In addition to the MICROTOPS measurements, a dual camera system continuously monitors cloud parameters. The visible spectrum camera is fitted with a fish-eye lens and records the cloud distribution in the sky. The infra-red camera records images which allow cloud characteristics such as height and cloud-base temperature to be derived.
Today we stopped briefly in Las Palmas on Gran Canaria, the staging point of our transit voyage to Bremerhaven. One colleague has unfortunately had to leave us here because of other professional obligations. However, in exchange four colleagues have joined us and are heartily welcomed on board.
With Best Wishes from Polarstern to all relatives, friends and colleagues at home,
Astrid Bracher, Martin Radenz, Jessica Vial and 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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