Description: The MSM89 expedition of Maria S Merian was a contribution to the international research initiative EUREC4A (www.eurec4a.eu). The cruise was carried out in concert with the M161 campaign on RV METEOR (Germany) and the EUREC4A-OA campaign on NO L’ATALANTE (France). Airplane and drone operations as well as well as continuous observations from the ground-based site on the Island of Barbados (BCO) were considered during the MSM89 campaign. Moreover, the cruise was coordinated with ships and Saildrone© operations in the context of the US American ATOMIC project. The overall goal of the EUREC4A field campaign was to collect observational data that will enable research on dynamic and thermodynamic processes in the atmosphere and ocean that will bring the understanding of the role of clouds in the climate system to a new level. MSM89 had its focus on the ocean/atmosphere coupling across ocean mesoscale vortices. For this purpose, both ocean and atmosphere profile measurements were carried out to observe the temporal evolution and spatial heterogeneity of the atmospheric and oceanic boundary layer. Autonomous observing platforms (underwater glider) and a ship towed platform (Cloudkite) augmented the ship-based observations. Incubation experiments were performed to determine Nitrogen fixation rates, the gas exchange for carbon dioxide and oxygen uptake.

Description: The scientific program of the MARIA S. MERIAN MSM74 expedition was dedicated to studies on the intensity of water mass transformation and the southward transport of water masses in the boundary current systems off Labrador and at the southern tip of Greenland. During the expedition we recovered 17/deployed 8 deep sea moorings. Measurements of the vertical structure of temperature, salinity, density, oxygen, optical properties and the flow along selected sections have been surveyed during the MSM74 expedition. Close to the surface, permanent registrations are carried out with the thermosalinograph (temperature, salinity) and meteorological data are continuously collected. Flow measurements up to 1000m depth are performed with the ships installed ADCPs. The expedition is a contribution to national (RACE) and international projects (OSNAP, AtlantOS).

Description: Mesoscale eddies play an important role in lateral property fluxes. Observational studies often use sea level anomaly maps from satellite altimetry to estimate eddy statistics (incl. eddy kinetic energy). Recent findings suggest that altimetry derived eddy characteristics may suffer from the low spatial resolution of past and current satellite-tracks in high-latitude oceans associated with small Rossby radii. This study presents results of an eddy reconstruction that provides an alternative perspective on the eddy field based on a nonlinear, damping Gauss-Newton optimisation algorithm using ship-based current profiler observations from two research expeditions in the Labrador Sea in 2014 and 2018. The derived eddy characteristics are compared to the high-resolution submesoscale permitting NATL60 model (1/60°). Overall, a total of 14 eddies with radii ranging from 7 to 35km and azimuthal velocities ranging from 20 to 60 cm s−1 were detected, of which 3 eddies were identified as Irminger rings. Irminger rings are characterised by a warm, salty lens either surface trapped or overlaid by a cold, fresh cap of water. The modelled eddies are on average 25% smaller in radius and 20% higher in azimuthal velocity accompanied by increased nonlinearity. The inner ring velocity structure for observations and NATL60 suggests solid-body rotation for 1/2 to 2/3 of the radius. The eddy boundary and outer ring velocity structure follows a more Gaussian-like shape, best described by a exp(−r2) dependency. Furthermore, the maximum azimuthal velocity may deviate from solid-body rotation by more than 30% for the observations and 50% for the model. In order to verify the skill of eddy reconstruction, the NATL60 served as a reference data set for the skill assessment of the Gauss-Newton method. It is shown that the eddy characteristics are affected by the location of the ship track through the velocity field. In most cases, however, the derived properties are not expected to deviate by more than 10%. Spectral analysis of the horizontal velocity implies that the mesoscale regime is well represented in NATL60 compared with the observations. The representation of the submesoscale flow decreases in the model with increasing depth. In particular, observations and model spectra diverge in the pycnocline by roughly one order of magnitude at scales smaller than 50km. Besides, the transition of power spectra slopes from k−2 to k−3 in ship-based measurements and model suggests a weak seasonal signal.

Description: Mesoscale eddies modulate the ocean's physical, chemical, and biological properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophy and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To fill this gap, we investigated the horizontal and vertical variability in auto- and heterotrophic microbial activity (biomass production and respiration) within a CE that formed off Mauritania and along the ∼ 900 km zonal corridor between Mauritania and the Cape Verde islands in the Eastern Tropical North Atlantic (ETNA). Our results show how the physical disturbances caused by the CE affected the biomass distribution of phyto- and bacterioplankton and their metabolic activities. The injection of nutrients into the sunlit surface resulted in enhanced autotrophic pico- and nanoplankton abundance and generally increased autotrophic activity as indicated by chlorophyll a (Chl a) concentration, primary production (PP), and extracellular release rates. However, the detailed eddy survey also revealed an uneven distribution of these variables with, for example, the highest Chl a concentrations and PP rates occurring near and just beyond the CE's periphery. The heterotrophic bacterial activity was similarly variable. Optode-based community respiration (CR), bacterial respiration (BR) estimates, and bacterial biomass production (BP) largely followed the trends of PP and Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial abundance and activities occurred within the CE that was closely related to variability in autotrophic production. Consistent with this, we found a significant positive correlation between concentrations of semi-labile dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable amino acids and dissolved combined carbohydrates) and BR estimates. Extracellular release of carbon as indicated by primary production of dissolved organic carbon (PPDOC) was variable with depth and laterally and not always sufficient to compensate the bacterial carbon demand (BCD: BR + BP), with PPDOC accounting for between 28 % and 110 % of the BCD. Bacterial growth efficiency (BGE: BP / BCD) ranged between 1.7 % and 18.2 %. We estimated the metabolic state to establish whether the CE was a source or a sink of organic carbon. We showed that the CE carried a strong autotrophic signal in the core (PP  CR 〉 1). Our results suggest that submesoscale (0–10 km) processes lead to highly variable metabolic activities in both photoautotrophic and heterotrophic microorganisms. Overall, we revealed that the CEs not only trap and transport coastal nutrients and organic carbon to the open ocean but also stimulate phytoplankton growth, generating freshly produced organic matter during westward propagation. This drives heterotrophic processes and may contribute to the previously observed net heterotrophy in open Atlantic surface waters.