ALBERT

Description: In the tropical North Atlantic, mean winds introduce relatively little energy into the internal wave field, but hurricanes act as very energetic sources for near-inertial waves. In addition to the eventlike passage of such tropical cyclones, changes in the wind speed north of the trade wind system induce a seasonal cycle in surface swell, with potential implications for the high-frequency part of the internal wave field. Using a 5-yr mooring time series in the interior of the tropical North Atlantic at 16°N, the temporal variability of internal wave energy south of the main hurricane track in different frequency bands is studied, and the magnitude of its variability, along with possible energy transfer mechanisms, is analyzed. The results show that changes in near-inertial energy are dominated by the passage of internal waves generated by hurricanes centered several hundred kilometers north of the mooring. The major role of hurricanes in the generation of near-inertial waves is also seen in an extended slab model that takes the horizontal divergence of the near-inertial current field at the mixed layer base into account. A seasonal cycle is observed in the energy at the high-frequency end (frequencies above 6 cpd) of the internal wave spectrum. It is not in phase with the near-inertial energy variability but covaries with changes in the local surface waves. These high-frequency internal waves are most energetic at times when large-amplitude surface swell with long periods and correspondingly long wavelengths is observed.

Description: The RV Poseidon cruise 516 (POS516) is part of the observational program of the TRR 181 'Energy Transfers in Atmosphere and Ocean', and focussed on the energy transfer by low-mode internal waves. The goals of the cruise were to deploy a mooring to record the temporal variability of the internal wave field and associated energy fluxes, and to use time series CTD/LADCP stations to assess locally the temporal variability of mixing, dissipation, and internal wave fluxes. The region south of the Azores in the east Atlantic is ideally suited for this kind of process study, because it is an area of a strong internal tide signal radiating away from the islands. The cruise track is located along a convergence of tidal beams south of the archipelago, crossing a chain of sea mounts as well as the critical latitude for parametric subharmonic instability (PSI). During the cruise, we collected CTD/LADCP time series between 39h and 52h length on 7 stations in up to 5000 m water depth along the tidal beam between 27°30’N and 37°N latitude, a total of 92 casts. The mooring equipped with of 7 current meter/temperature logger pairs and an acoustic Doppler current profiler was deployed along the track at 30°29.04’N, 30°11.7’W in a water depth of 4500 m (to be retrieved in 2018). All anticipated goals of the cruise were accomplished.

Description: TRR181 Cruise Poseidon 533/2 25. March - 1. April 2019 1. Weekly report!

Description: The Poseidon cruise POS533/2 was part of the observational program of the TRR181, an interdisciplinary research program of the Deutsche Forschungsgemeinschaft (DFG). The overarching aim of the TRR181 ‘Energy transfers in atmosphere and ocean’ is the energetically consistent representation of the oceanic and atmospheric energy cycles in climate models. After POS516 (2017) and POS523 (2018) the cruise POS533/2 was the third expedition to the area south of the Azores within the TRR181. Tidal forcing excites so-called internal tides at steep topography such as seamounts and the continental shelf. These internal tides have the form of underwater waves that may travel hundreds of kilometers across ocean basins. Along their paths, the waves lose their energy by a range of processes and interactions, but the how and where is mostly unknown. South of the Azores seamounts generate very energetic internal tides which radiate energy towards the Cap Verde Islands. During POS516 and POS523 we conducted a spatial survey to better understand the fluxes and dissipation of the internal tides. A mooring measuring time series of temperature and current velocities was redeployed at 30°29’N, 30°12’W during POS523 in May 2018. Its data will be used to study the temporal variability in the energy of the internal tides. With the successful retrieval of this mooring, the goal of POS533/2 was fully achieved.

Description: Highlights • Hydrothermal survey in the 13°-33°S region of the Mid-Atlantic Ridge based on hydrographic casts, noble gas observations and AUV dives. • Discovery of hydrothermal plumes above ten ridge segments pointing to 14 unknown active vent sites. • Rio de Janeiro Transform (22°S) likely represents a barrier separating different vent endemic faunal communities to the north and south. Abstract The oceanic crust is initially cooled and deep-sea chemosynthetic ecosystems are largely fed by hydrothermal circulation and venting on the seafloor. Much of this venting takes place at mid-ocean ridges and in order to make realistic models of the crust's thermal budget and to understand chemosynthetic biogeography it is important to have a detailed inventory of vent sites. Until recently, a major gap in this inventory was the Mid-Atlantic Ridge south of 13°S, a key region for vent fauna biogeography as it is the corridor linking the Atlantic to the Indian and Pacific Oceans. In spring 2013 we systematically surveyed the axial region between 13°S and 33°S for hydrothermal signals in the water column, using turbidity, oxidation-reduction-potential (ORP) and noble gases as indicators. Standard conductivity-temperature-depth (CTD) rosette water-sampler deployments were complimented by a novel autonomous underwater vehicle (AUV) deployment strategy, in which the AUV made single-pass, segment-scale (up to 100 km long) dives close to the seafloor to detect small vents. The ca. 2100 km-long survey covered 16 ridge segments and we identified previously unknown hydrothermal plumes above ten segments that point to 14 new hydrothermal vent fields. The majority of plumes are located at high-relief segment centers, where magmatism is robust. A wide gap in the distribution of vents in the 19°S-23°S region coincides with the Rio de Janeiro Transform, the maximum southward progression of North Atlantic Deep Waters and the maximum northwards extent of 3He-enriched waters with Pacific origins. Crossflowing currents in the transform and the large gap between adjacent vents may prevent a meridional connection between the vent fauna communities in the North Atlantic and along the Antarctic Ridges. This makes the region a prime target for future biogeographical studies.

Description: The RV Poseidon cruise 523 (POS523) is the second cruise to the work area as part of the observational program of the TRR 181 'Energy Transfers in Atmosphere and Ocean', and focussed on the energy transfer by low-mode internal waves. The goals of the cruise were to recover and redeploy a mooring to record the temporal variability of the internal wave field and associated energy fluxes, and to use time series CTD/LADCP/microstructure stations to assess locally the temporal variability of mixing, dissipation, and internal wave fluxes. The region south of the Azores in the east Atlantic is ideally suited for this kind of process study, because it is an area of a strong internal tide signal radiating away from the islands. The cruise track is located south of a chain of seamounts in a tidal beam formed by constructive interference of internal tides, and crosses the critical latitude for parametric subharmonic instability (PSI). During the cruise, we collected time series of CTD/LADCP and microstructure between 36 h and 52 h length on 5 stations in up to 4600 m water depth along the tidal beam between 29°20’N and 32°N latitude. In total 64 CTD/LADCP casts and 18 microstructure data sets were measured. The mooring equipped with current meter/temperature logger pairs and acoustic Doppler current profiler was successfully recovered and later redeployed along the track at 30°29’N, 30°12’W in a water depth of 4500 m (to be retrieved in 2019). All anticipated goals of the cruise were accomplished.