Description: The formation of secondary ice in clouds, i.e. ice particles that are created at temperatures above the limit for homogeneous freezing without the direct involvement of a heterogeneous ice nucleus is one of the longest standing puzzles in cloud physics. Here we present comprehensive laboratory investigations on the formation of small ice particles upon the freezing of drizzle-sized cloud droplets levitated in an electrodynamic balance. Four different categories of secondary ice formation (bubble bursting, jetting, cracking, breakup) could be detected and their respective frequencies of occurrence as a function of temperature and droplet size are given. We find that bubble bursting occurs more often than droplet splitting. While we do not observe the shattering of droplets into many large fragments, we find that the average number of small secondary ice particles released during freezing is strongly droplet-size dependent and may well exceed unity for droplets larger than 300 μm in diameter. This leaves droplet fragmentation an important secondary ice process effective at temperatures around -10 °C in clouds where large drizzle droplets are present.

Description: Meteor Cruise M121 was dedicated to the investigation of the distribution of dissolved and particulate trace metals and their isotopic compositions (TEIs) in the full water column of the Angola Basin and the northernmost Cape Basin. A key aim was to determine the driving factors for the observed distributions, which includes the main external inputs, as well as internal cycling and ocean circulation. The research program of the cruise is official part of the international GEOTRACES program (www.geotraces.org) and cruise M121 corresponds to GEOTRACES cruise GA11. Subject of the cruise was the trace metal clean and contamination-free sampling of waters and particulates for subsequent analyses of the TEIs in the home laboratories of the national and international participants. Besides a standard rosette for the less contaminant prone metals, trace metal clean sampling was realized by using for the first time a new dedicated, coated trace metal clean rosette equipped with Teflon-coated GO-FLO bottles operated via a plastic coated cable from a mobile winch of GEOMAR Kiel. The particulate samples were collected under trace metal clean conditions using established in-situ pump systems operated from Meteor’s Aramid line. The cruise track led from Walvis Bay northwards along the West African margin until 3°S, then turned west until the Zero Meridian, which was followed southwards until 30°S. Then the cruise track turned east again until the Namibian margin was reached and then completed the near shore track northwards until Walvis Bay. The track crossed areas of major external inputs including dust from the Namib Desert and exchange with the west African continental margin and with the oxygen depleted shelf sediments of the Benguela upwelling, as well as with the plume of the Congo outflow, that was followed from its mouth northwards. Our investigations of internal cycling included the extremely high productivity associated with the Benguela Upwelling and the elevated productivity of the Congo plume contrasting with the extremely oligotrophic waters of the southeastern Atlantic Gyre. The links between TEI biogeochemistry and the nitrogen cycle forms an important aspect of our study. The major water masses contributing the Atlantic Meridional Overturning Circulation were sampled in order to investigate if particular TEI signatures are suitable as water mass tracers, in particular near the ocean margin and in the restricted deep Angola Basin. A total of 51 full water column stations were sampled for the different dissolved TEIs, which were in most cases accompanied by sampling for particulates and radium isotopes using the in-situ pumps. In addition, surface waters were continuously sampled under trace metal clean conditions using a towed fish and aerosol and rain samples were continuously collected.

Description: M159 (29.10. – 20.11.2019) 2. Wochenbericht vom 10. November 2019

Description: M159 (29.10. – 20.11.2019) 3. Wochenbericht vom 17. November 2019

Description: The cruise M133 SACROSS (South Atlantic Crossing) was a multidisciplinary ocean survey of the South Atlantic gyre roughly along 34.5°S. This transect is covered by the international SAMOC moored array and also the path of the internationally agreed AX18 XBT line. Most of the measurements were based on using underway methods including near-surface water sampling for the determination of SST, and SSS as well as shipboard ADCP current observations. Moreover, an underway CTD allowed to sample the upper 300-400 m every hour. Chemical analysis of surface waters as well as atmospheric parameter were of scientific interest to both compare different regions with each other but also to document long term trends. At the western and eastern boundary current regime full water column water mass properties were measured. Upper ocean 10-700m plankton assemblages allow improving the calibration of sediment proxies. Water samples for later lab-based biodiversity analysis were taken. A number of smaller student projects were carried out as part of a global ocean learning and capacity building effort. Finally, continuous swath bathymetry mapping was made, and a number of floats and drifters were launched in support of the global ocean observing system arrays. The cruise was very successful, all objectives were reached, and the measurements were carried out as planned.

Description: The subpolar North Atlantic (SPNA) circulation is comprised of a complex interplay between the wind- driven gyre circulation and the buoyancy driven meridional overturning circulation (MOC). As the Atlantic MOC (AMOC) plays an essential role in our climate system due to the associated meridional transport of heat, mass and freshwater it is of fundamental importance to understand its forcing mechanisms, variability and impacts on various different time scales. Due to its role in the formation of North Atlantic Deep Water (NADW), the SPNA is of crucial importance to the understanding of the AMOC. This thesis presents se- lected aspects of the SPNA circulation dynamics, based on various observational data sets in combination with two high-resolution ocean general circulation models (OGCMs; VIKING20, VIKING20X). In order to understand observations in correspondence with OGCM output, the model fidelity in comparison to observed quantities has to be secured. These quantities should be available for sufficiently long time scales and should be determined similarly in the OGCM and the observations. Using observational data in the vicinity of 53◦N in the Labrador Sea and the ocean model VIKING20, the following comparable robust integral quantities were defined: the magnitude and spatial and temporal variability of integral circulation elements on the regional scale (NADW transport at 53◦N; 33 Sv model, 31 Sv observations), the horizontal and vertical extend of the March Mixed Layer Depth in the Labrador Sea and the gyre scale baroclinicity. The models’ boundary current system is more barotropic and indicates stronger monthly to interannual transport variability compared to the observations. Furthermore, during periods of enhanced deep convec- tion an increased correlation between different components of NADW is found in the model, which is found to be the result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale. Apart from the challenging to measure AMOC strength, these above mentioned regional and basin scale quantities were found to be suitable for model verification. The export routes of deep water from the Labrador Sea with a specific focus on the connection to the Irminger Sea are further investigated using different Lagrangian particle tracking experiments based on both the Argo observations as well as the VIKING20X model output. The connection between the Labrador- and Irminger Sea on the Labrador Sea water (LSW) depth is evaluated with pure advective trajectory integra- tion as well with a simple additional diffusion parametrization. Advective experiments with the temporarily varying model output and seeding in the central Labrador Sea and the advective-diffusive experiments with the Argo based Eulerian velocity fields resulted in ∼ 20% of the total particles connecting the two regions v within 1.5 - 2.5 years, with shorter transit times in the model (∼ 1.5 years). The DWBC export of LSW towards the south was found to be strongly decreased with the applied diffusion parameterization. A rela- tively simple method reproduced a similar connection of the two regions as derived from various observations. The relation between the SPNA AMOC and the horizontal circulation of the subpolar gyre was then subject to a model only study with VIKING20. Current transports and the AMOC strength are evaluated along the “Overturning in the Subpolar North Atlantic Program” (OSNAP) array and a section spanning the Greenland-Scotland Ridge (GSR), where for both recently observational estimates became available. The AMOC strengths (GSR 6.6 ± 0.9 Sv, OSNAP eastern leg 17.2 ± 3.0 Sv) are comparable with observational values. However, due to the limited time series of observations, the variability of the AMOC can only be determined with the model on longer than interannual time scales. In VIKING20 all SPNA currents, the AMOC and the subpolar gyre index exhibit strong interannual to decadal variability. Using a simple box model, water mass transformation in the SPNA showed similar formation rates in the Labrador Sea (7.7 ± 3.0 Sv) compared to the Irminger Sea (1.6 ± 2.8 Sv) and Iceland basin (5.1 ± 1.2 Sv) combined. During periods of deep convection (1990’s) the transport of the boundary currents and the North Atlantic current import through the Iceland basin is particularly enhanced suggesting a similar forcing mechanism.

Description: Long‐term observations from a 17 year long mooring array at the exit of the Labrador Sea at 53°N are compared to the output of a high‐resolution model (VIKING20). Both are analyzed to define robust integral properties on basin and regional scale, which can be determined and evaluated equally well. While both, the observations and the model, show a narrow DWBC cyclonically engulfing the Labrador Sea, the model's boundary current system is more barotropic than in the observations and spectral analysis indicates stronger monthly to interannual transport variability. Compared to the model, the observations show a stronger density gradient, hence a stronger baroclinicity, from center to boundary. Despite this, the observed temporal evolution of the temperature in the central Labrador Sea is reproduced. The model results yield a mean export of North Atlantic Deep Water (NADW) (33.0 +/‐ 5.7 Sv), which is comparable to the observed transport (31.2 +/‐ 5.5 Sv) at 53°N. The results also include a comparable spatial pattern and March mixed layer depth in the central Labrador Sea (maximum depth ∼ 2000 m). During periods containing enhanced deep convection (1990's) our analyses show increased correlation between LSW and LNADW model transport at 53°N. Our results indicate that the transport variability in LSW and LNADW at 53°N is a result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale.