ALBERT

Description: Optimum multiparameter (OMP) analysis is used to analyze mixing in the central water boundary of the tropical North Atlantic Ocean. Diapycnal mixing is found to be prevalent in the frontal region. OMP analysis shows that the mixing is unidirectional (South Atlantic Central Water is always mixed upward into North Atlantic Central Water) but cannot identify the process responsible for the observed diapycnal mixing. A histogram of stability ratios Rρ for all mixing lines shows Rρ values between unity and the value found in the parent water masses. It is suggested that this may indicate competition between isopycnal mixing and double diffusion. Double diffusive fluxes are likely to make a recognizable and significant contribution to diapycnal mixing between the Central Waters.

Description: The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.

Description: Auf 32 Seiten geben die beiden Wissenschaftsverbände einen verständlichen Überblick zum Meeresspiegelanstieg. 14 Wissenschaftlerinnen und Wissenschaftler unterschiedlicher Forschungseinrichtungen ordnen gemeinsam die Informationen ein, die immer wieder in der Öffentlichkeit diskutiert werden, und erklären die wichtigsten Zusammenhänge und zugrundeliegenden Prozesse in klaren Worten. Damit bietet die Broschüre Orientierung in Bezug auf plausible Zukunftsszenarien, und hilft, die Risiken besser einzuschätzen. Zusätzlich erläutern die Forschenden die Situation an den deutschen Küsten, denn Klimawandel und Meeresspiegelanstieg betreffen auch die Nord- und Ostsee.

Description: Since a pH sensor has become available that is principally suitable for use on demanding autonomous measurement platforms, the marine CO2 system can be observed independently and continuously by Biogeochemical Argo floats. This opens the potential to detect variability and long-term changes in interior ocean inorganic carbon storage and quantify the ocean sink for atmospheric CO2. In combination with a second parameter of the marine CO2 system, pH can be a useful tool to derive the surface ocean CO2 partial pressure (pCO2). The large spatiotemporal variability in the marine CO2 system requires sustained observations to decipher trends and study the impacts of short-term events (e.g., eddies, storms, phytoplankton blooms) but also puts a high emphasis on the quality control of float-based pH measurements. In consequence, a consistent and rigorous quality control procedure is being established to correct sensor offsets or drifts as the interpretation of changes depends on accurate data. By applying current standardized routines of the Argo data management to pH measurements from a pH / O2 float pilot array in the subpolar North Atlantic Ocean, we assess the uncertainties and lack of objective criteria associated with the standardized routines, notably the choice of the reference method for the pH correction (CANYON-B, LIR-pH, ESPER-NN, and ESPER-LIR) and the reference depth for this adjustment. For the studied float array, significant differences ranging between ca. 0.003 pH units and ca. 0.04 pH units are observed between the four reference methods which have been proposed to correct float pH data. Through comparison against discrete and underway pH data from other platforms, an assessment of the adjusted float pH data quality is presented. The results point out noticeable discrepancies near the surface of 〉 0.004 pH units. In the context of converting surface ocean pH measurements into pCO2 data for the purpose of deriving air–sea CO2 fluxes, we conclude that an accuracy requirement of 0.01 pH units (equivalent to a pCO2 accuracy of 10 µatm as a minimum requirement for potential future inclusion in the Surface Ocean CO2 Atlas, SOCAT, database) is not systematically achieved in the upper ocean. While the limited dataset and regional focus of our study do not allow for firm conclusions, the evidence presented still calls for the inclusion of an additional independent pH reference in the surface ocean in the quality control routines. We therefore propose a way forward to enhance the float pH quality control procedure. In our analysis, the current philosophy of pH data correction against climatological reference data at one single depth in the deep ocean appears insufficient to assure adequate data quality in the surface ocean. Ideally, an additional reference point should be taken at or near the surface where the resulting pCO2 data are of the highest importance to monitor the air–sea exchange of CO2 and would have the potential to very significantly augment the impact of the current observation network.

Description: The Eastern Boundary Current is an essential part of the water mass exchange between the subtropical and subpolar North Atlantic. Here, we study the offshore branch of the European Shelf Current (ESC) over the Goban Spur slope area off Ireland. Our analysis is based on current measurements obtained from a multiyear mooring effort, complemented by ship‐board observations along a hydrographic section, satellite‐derived estimates of absolute dynamic topography with geostrophic currents, and float trajectories. These data serve to quantify the offshore branch of the ESC on intraannual to interannual timescales. From the moored observations, we derive a mean poleward along‐slope volume flux of 3.7 ± 0.7 Sv for the period 2017–2019. Using a multilinear regression model and geostrophic surface velocities, we extend the time series to the period 1993–2019 and obtain a long‐term mean transport of 3.2 ± 0.4 Sv. Both time series show strong variability ranging from −7.5 to 15.7 Sv. The variability is linked to a dynamic eddy field, especially a stationary cyclonic circulation pattern near the mooring array, and meandering of current branches originating from the North Atlantic Current. We find no evidence of a consistent deep boundary current extending from the shelf break to the position of the offshore mooring (4,500 m depth), but confirm a persistent along‐slope flow at the shallower slope (1,500 m depth). Geostrophic surface velocities and float trajectories reveal that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions but rather indicate the intermittent character of the flow.

Description: Plain Language Summary: In this study, we describe the European Shelf Current (ESC) in the eastern North Atlantic. The study area is the deeper part of the Goban Spur area off Ireland. We use current measurements of instruments deployed in the water column from 2016 to 2019 combined with satellite observations of surface circulation and drifting float paths. We aim to quantify the contribution of the offshore branch to the ESC system. We calculate a poleward volume transport following the slope that is on average 3.7 ± 0.7 Sverdrup (Sv; 1 Sv = 1 million cubic meter per second) for the period 2017–2019. We use a mathematical relation between mooring data and satellite observations to extend the time series to the period 1993–2019 and obtain an average transport of 3.2 ± 0.4 Sv. There occur strong changes over time ranging from −5 to 15 Sv. These changes are caused by rotating vortices (eddies), especially a counter‐clockwise rotating circulation pattern near the moorings, and changing positions of currents branching from the North Atlantic Current. We find that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions of the North Atlantic. Instead, the flow is of rather unsteady character.

Description: Key Points: The analysis of multiplatform observational efforts is used to study the circulation at the North Atlantic Eastern Boundary. The volume transport of the offshore branch of the European Shelf Current is on average 3.2 ± 0.4 Sv for the period 1993–2019. Transport variability is in the range of −7.5–15.7 Sv and induced by eddies and meandering North Atlantic Current branches.

Description: Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347

Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659