Eddy Study to Understand Physical-Chemical-Biological Coupling and the Biological Carbon Pump as a Function of Eddy Type off West Africa, Cruise No. M160, 23.11.2019 - 20.12.2019, Mindelo (Cabo Verde) - Mindelo (Cabo Verde)

Cruise M160 is part of concerted MOSES/REEBUS Eddy Study featuring three major research expeditions (M156, M160, MSM104). It aims to develop both a qualitative and quantitative understanding of the role of physical-chemical-biological coupling in eddies for the biological pump. The study is part of the MOSES “Ocean Eddies” event chain, which follows three major hypotheses to be addressed by the MOSES/REEBUS field campaigns: (1) Mesoscale and sub-mesoscale eddies play an important role in transferring energy along the energy cascade from the large-scale circulation to dissipation at the molecular level. (2) Mesoscale and sub-mesoscale eddies are important drivers in determining onset, magnitude and characteristics of biological productivity in the ocean and contribute significantly to global primary production and particle export and transfer to the deep ocean. (3) Mesoscale and sub-mesoscale eddies are important for shaping extreme biogeochemical environments (e.g., pH, oxygen) in the oceans, thus acting as a source/sink function for greenhouse gases. In contrast to the other two legs, MOSES Eddy Study II during M160 did not include any benthic work but focused entirely on the pelagic dynamics within eddies. It accomplished a multi-disciplinary, multi-parameter and multi-platform study of two discrete cyclonic eddies in an unprecedented complexity. The pre-cruise search for discrete eddies suitable for detailed study during M160 had already started a few months prior to the cruise. Remote sensing data products (sea surface height, sea surface temperature, ocean color/chlorophyll a) were used in combination with eddy detection algorithms and numerical modelling to identify and track eddies in the entire eddy field off West Africa. In addition, 2 gliders and 1 waveglider had been set out from Mindelo/Cabo Verde for pre-cruise mapping of the potential working area north of the Cabo Verdean archipelago. At the start of M160, a few suitable eddies – mostly of cyclonic type – had been identified, some of which were outside the safe operation range of the motorglider plane. As technical problems delayed the flight operations, the first eddy (center at 14.5°N/25°W) for detailed study was chosen to the southwest of the island of Fogo. It was decided to carry out a first hydrographic survey there followed by the deployment of a suite of instruments (gliders, waveglider, floats, drifter short-term mooring). Such instrumented, we left this first eddy and transited – via a strong anticyclonic feature southwest of the island of Santiago – to the region northeast of the island of Sal, i.e. in the working range of the glider plane. During the transit, a full suite of underway measurements as well as CTD/RO section along 22°W (16°-18.5°N) were carried in search for sub-surface expressions of anticyclonic eddy features. In the northeast, we had identified the second strong cyclonic eddy (center at 18°N/22.5°W) which was chosen for detailed study starting with a complete hydrographic survey (ADCP, CTD/RO, other routine station work). After completion of the mesoscale work program, we identified a strong frontal region at the southwestern rim of the cyclonic eddy, which was chosen for the first sub-mesoscale study with aerial observation component. There, the first dye release experiment was carried out which consisted of the dye release itself followed by an intense multi-platforms study of the vertical and horizontal spreading of the initial dye streak. This work was METEOR-Berichte, Cruise M160, Mindelo – Mindelo, 23.11.2019 4 – 20.12.2019 supported and partly guided by aerial observation of the research motorglider Stemme, which was still somewhat compromised by technical issues and meteorological conditions (high cloud cover, Saharan dust event). Nevertheless, this first dye release experiment was successful and showed rapid movement of the dynamic meandering front. After completion of work on this second eddy and execution of a focused sampling program at the Cape Verde Ocean Observation, RV METEOR returned to the first eddy for continuation of the work started there in the beginning of the cruise. This was accompanied by a relocation of the airbase of Stemme from the international airport of Sal to the domestic airport of Fogo. The further execution of the eddy study at this first eddy, which again included a complete hydrographic survey followed by a mesoscale eddy study with dye release, was therefore possible with aerial observations providing important guidance for work on RV METEOR. Overall, M160 accomplished an extremely intense and complex work program with 212 instrument deployments during station work, 137 h of observation with towed instruments and a wide range of underway measurements throughout the cruise. Up to about 30 individually tracked platforms (Seadrones, glider, wavegliders, drifters, floats) were in the water at the same time providing unprecedented and orchestrated observation capabilities in an eddy. All planned work components were achieved and all working groups acquired the expected numbers of instrument deployments and sampling opportunities

Untersuchungen zur Aerolgie und zum Wärmehaushalt der Atmosphäre über den westlichen Arabischen Meer während der Nord-Ost-Monsun-Periode

The atmospheric space from 11° N to 5° S along the Somali-coast of East-Africa over the western part of the Arabian Sea has been investigated making use of the aerological measurements, gained during the International Indian Ocean Expedition (IIOE) 1964/65. On the basis of the analyses obtained, an attempt is made to compute a heat budget for this space. In particular the following components have been taken into account: the net-radiation flux in the atmosphere (GN), the turbulent flux of sensible heat (Qs) and of latent heat (QL) from ocean to the atmosphere above, the potential (P), the sensible (I) and the latent energy (E). The vertical velocity component (ω) has been carefully determined making use of the adiabatic method but considering nonadiabatic effects of various kind lateron. From the vertical divergence of the net-radiation flux the cooling by long wave radiation was computed. Maximum effect of this process was obtained in the monsoon inversion (-5.4°C/day). Above the tropical tropopause the atmosphere shows maximum warming of 1.8°C/day. The intensity of the turbulent flux of sensible and latent heat is mainly determined by the windfield and by the local differences in the watertemperature. Maximum values are for Qs = 12·10-4 cal cm2 sec-1 and for QL = 140·10-4 cal cm2 sec-1 (Bowen-ratio R = Qs/QL = 8.5%, mostly not more than 4-5%). The computation of the vertical velocity rests besides the thermodynamic method (adiabatic) on a consideration of various non-adiabatic effects such as radiation, flux of sensible and latent heat. The following distribution has been obtained: Below the monsoon inversion a rising motion (maximum -30·10-4 mb/sec ≈ +2.8 cm/sec) and above a sinking motion (maximum +20·10-4 mb/sec ≈ -3.4 cm/sec) upwards to the tropical tropopause (near 90 mb). Above the tropopause weak rising motion (-2·10-4 mb/sec ≈ +1.3 cm/sec) was obtained. An attempt to compute various terms (in total 7) of the heat budget equation shows, that horizontal advection of various energy forms are only of importance in the monsoon layer (below the inversion) and above the tropical tropopause. In all height-intervalls considered the vertical advektion of the energy forms is dominant. The results of the budget-computation are presented in graphical form

Surface radiation at sea validation of satellite-derived data with shipboard measurements

Quality-controlled and validated radiation products are the basis for their ability to serve the climate and solar energy community. Satellite-derived radiation fluxes are well preferred for this task as they cover the whole research area in time and space. In order to monitor the accuracy of these data, validation with well maintained and calibrated ground based measurements is necessary. Over sea, however, long-term accurate reference data sets from calibrated instruments recording radiation are scarce. Therefore data from research vessels operating at sea are used to perform a reasonable validation. A prerequisite is that the instruments on board are maintained as well as land borne stations. This paper focuses on the comparison of radiation data recorded on board of the German Research Vessel "Meteor" during her 13 months cruise across the Mediterranean and the Black Sea with CM-SAF products using NOAA- and MSG-data (August 2006-August 2007): surface incoming short-wave radiation (SIS) and surface downward long-wave radiation (SDL). Measuring radiation fluxes at sea causes inevitable errors, e.g.shadowing of fields of view of the radiometers by parts of the ship. These ship-inherent difficulties are discussed at first. A comparison of pairs of ship-recorded and satellite-derived mean fluxes for the complete measuring period delivers a good agreement: the mean bias deviation (MBD) for SIS daily means is −7.6 W/m2 with a median bias of −4 W/m2 and consistently the MBD for monthly means is −7.3 W/m2, for SDL daily means the MBD is 8.1 and 6 W/m2 median bias respectively. The MBD for monthly means is 8.2 W/m2. The variances of the daily means (ship and satellite) have the same annual courses for both fluxes. No significant dependence of the bias on the total cloud cover recorded according to WMO (1969) has been found. The results of the comparison between ship-based observations and satellite retrieved surface radiation reveal the good accuracy of the satellite-based CM-SAF products over sea