Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP-Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014-2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.

A sea change in our view of overturning in the subpolar North Atlantic

To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

Subpolar North Atlantic Overturning and Gyre-Scale Circulation in the Summers of 2014 and 2016

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system through its transport of heat and freshwater. The subpolar North Atlantic (SPNA) is a region where the AMOC is actively developed and shaped though mixing and water mass transformation and where large amounts of heat are released to the atmosphere. Two hydrographic transbasin sections in the summers of 2014 and 2016 provide highly spatially resolved views of the SPNA velocity and property fields on a line from Canada to Greenland to Scotland. Estimates of the AMOC, isopycnal (gyre-scale) transport, and heat and freshwater transport are derived from the observations. The overturning circulation, the maximum in northward transport integrated from the surface to seafloor and computed in density space, has a high range, with 20.6 ± 4.7 Sv in June-July 2014 and 10.6 ± 4.3 Sv in May-August 2016. In contrast, the isopycnal (gyre-scale) circulation was lowest in summer 2014: 41.3 ± 8.2 Sv compared to 58.6 ± 7.4 Sv in 2016. The heat transport (0.39 ± 0.08 PW in summer 2014, positive is northward) was highest for the section with the highest AMOC, and the freshwater transport was largest in summer 2016 when the isopycnal circulation was high (-0.25 ± 0.08 Sv). Up to 65% of the heat and freshwater transport was carried by the isopycnal circulation, with isopycnal property transport highest in the western Labrador Sea and the eastern basins (Iceland Basin to Scotland).

Low oxygen eddies in the eastern tropical North Atlantic: Implications for N2O cycling

Nitrous oxide (N₂O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N₂O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N₂O concentrations much higher (up to 115 nmol L⁻¹) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N₂O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N₂O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N₂O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N₂O at the regional scale, and point out the possible need for a reevaluation of how we view N₂O cycling in the ETNA.

Endoscopic ultrasound-guided needle confocal laser endomicroscopy in pancreatic masses

INTRODUCTION

Endoscopic ultrasound (EUS) is an established tool in diagnosing pancreatic masses and enables guided fine-needle aspiration (FNA). Confocal laser endomicroscopy (CLE) has allowed in vivo microscopic analysis during on-going endoscopy. Recently, CLE has gone beyond luminal indications with the development of a new microprobe (nCLE). The aim of this case series was to study the feasibility of EUS-guided nCLE and to correlate the findings with microscopy.

METHODS

A total of 25 patients with pancreatic masses were included. During the procedure, an nCLE fiber preloaded into a 19 gauge FNA needle was advanced into the lesion under EUS guidance. Fluorescein was administered intravenously and imaging performed. Afterwards EUS-FNA was performed in the same location. Safety and feasibility were evaluated and CLE structures were registered and correlated to the standard hematoxylin and eosin cytopathology specimens. Moreover, additional topical acriflavine-enhanced ex vivo examinations on fresh pancreatic specimens were conducted.

RESULTS

EUS-guided nCLE procedures were accomplished in all patients. No adverse advents were registered. Furthermore, it was feasible to do nCLE inside pathological lesions and relatively easy to visualize organ specific tissue. Despite selecting predefined structures the diagnostic value was limited mainly due to the missing ability to elucidate the cell nuclei, In the ex vivo examinations, where acriflavine was administered topically on excised pancreatic tissue, the nuclei were clearly visualized, thus increasing the diagnostic value.

CONCLUSION

EUS-guided nCLE procedures on focal pancreatic masses are feasible and safe, but the diagnostic value seems limited. Thus, further studies using different contrast agents are required to optimize the diagnostic accuracy.