Heather Hunt Furey

This report details the field work completed out of Toamasina, Madagascar as part of the East Madagascar Current Experiment in November and December 2024. The field work consisted of 76 subsurface (RAFOS) float deployments and 30 surface drifter deployments, deployed in sets across the East Madagascar Current using a tugboat chartered out of Toamasina, Madagascar. Two people, Woods Hole Oceanographic Institution employee Sebastien Bigorre, a native French speaker and member of the Physical Oceanography technical staff with extensive field work experience, and contracted Captain Steinar Ström, a person with extensive expertise captaining and working on chartered vessels to complete field missions, worked together to complete the instrument deployments during the experiment. This report details the background of the project, the deployment details, and includes the set of daily reports completed by Captain Ström.

The Deep Madagascar Basin (DMB) Experiment aims to identify deep and abyssal water pathways in the Southwest Indian Ocean. Abyssal waters store and redistribute heat, freshwater, and carbon, which, in turn, help to regulate Earth’s climate. The observational component of this program focused on both the Eulerian and Lagrangian framework: a field program that included synoptic hydrographic property and velocity surveys within the Madagascar Basin and along the Southwest Indian Ridge, deep SOLO and RAFOS float deployments, and mooring deployments to support the RAFOS float tracking. The goal of the Lagrangian observations was to determine pathways of younger abyssal waters of southern origin, which enter the basin through deep fracture zones in the Southwest Indian Ridge and spread northward. The resulting observations provide a new view on abyssal circulation pathways in the DMB and offer insight into the structure of the Indian Ocean Meridional Overturning Circulation. This report details the RAFOS float and sound source mooring components of the program. Twenty-two floats were deployed during the DMB cruise (R/V Roger Revelle RR2303) from April 9 to May 13, 2023, at two cross-slope sections on the western side of the Madagascar Basin and were ballasted to drift at 3000 dbar for a nominal duration of 730 days, recording temperature, pressure, and position data every 12 hours.

Denmark Strait Overflow Water (DSOW) is formed in the Nordic Seas and is a key water mass transported by the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). DSOW facilitates the transmission of climate signals into the deep ocean, both directly from the Nordic Seas and indirectly from the Irminger Sea via entrainment. Here, we use new hydrographic and oxygen observations from BGC-Argo profiling floats and moored instruments along with a simple streamtube model to investigate along-stream changes and seasonality in DSOW properties. We suggest that the along-stream changes in DSOW oxygen are consistent with mixing with entrained ambient waters. Novel year-round moored oxygen measurements offshore of Cape Farewell, Greenland reveal seasonality in DSOW oxygen properties that is similar to the seasonality in DSOW salinity. Model results suggest that this observed seasonality reflects seasonality in the source waters, rather than seasonality in the properties of the entrained waters or the strength of entrainment. We suggest that future changes in dense water properties in the Nordic Seas could substantially influence overflow properties. This finding may have consequences for future projections of AMOC strength and the ventilation of the deep North Atlantic.

The Indian Ocean plays a crucial role in the abyssal overturning circulation, transforming Antarctic Bottom Water (AABW) into lighter water. In the western Indian Ocean, the only path for AABW to reach the subtropics as part of the global overturning is through the fracture zones of the Southwest Indian Ridge. This mid‐ocean ridge has seven known fracture zones in this area, but until recently, only Atlantis II has been investigated, given the historical shortage of in situ observations in the region. To address this gap, we collected as part of the Deep Madagascar Basin (DMB) Experiment (DMB) the first comprehensive surface‐to‐bottom measurements of velocities, water properties, and tracers at four fracture zones deemed to transport AABW northward: Gallieni, Atlantis II, Novara, and Melville. Based on these new observations, we estimate the relative contributions of each fracture zone to the overall northward transport in the abyss and assess the strength of diapycnal turbulent mixing. Our observations reveal two strong northward deep jets transporting AABW, one at the Novara (46.0 cm/s at 4,370 dbar) and another at the Melville (23.8 cm/s at 4,360 dbar) fracture zones, and provide evidence of the persistence of the previously observed Atlantis II jet. Contrary to earlier assumptions, we find that the Melville fracture zone, rather than the Atlantis II, is the primary contributor to the northward transport of deep and bottom water, at least during the DMB period.

The DMB-2023 cruise, aboard the UNOLS vessel R/V Roger Revelle, took place from April 9 to May 13, 2023, as a key component of the “Deep Madagascar Basin (DMB) Experiment – A Quest to Find the Abyssal Water Pathways in the Southwest Indian Ocean”, a project funded by the National Science Foundation. The DMB Experiment aims to understand the still largely unknown abyssal circulation of the Madagascar Basin in the Southwest Indian Ocean, how abyssal temperature varies within the basin interior, and how the complex seafloor topography shapes the abyssal circulation patterns there. The primary objective is to determine the pathway(s) by which younger abyssal water of southern origin, entering the basin through deep fracture zones on the Southwest Indian Ridge, spreads northward across the basin. This knowledge is essential for characterizing the Indian Ocean Meridional Overturning Circulation and its variability. The DMB Experiment includes a fieldwork component, which this report details, along with a state-of-the art modeling component, both of which are used to investigate the fundamental dynamics and development of the deep flow field.

Formed by deep convection in the Nordic Seas, cold, oxygen-rich Denmark Strait Overflow Water (DSOW) is the densest water mass in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). As DSOW spreads southward from the Denmark Strait into the Irminger Sea, the overflow water entrains ambient water masses. This entrainment is critical for the transfer of climate signals and biogeochemical tracers from the surface to the deep ocean. Although the impact of entrainment on DSOW properties in the immediate vicinity of the Denmark Strait (< 100 km downstream of the sill) has been studied extensively, the entrainment dynamics contributing to DSOW transformation further downstream in the Irminger Sea and its influence on variability in DSOW properties remain unclear. Here, we use observations from BGC-Argo floats and moorings along with an idealized numerical model of entrainment mixing to investigate the dynamics contributing to the observed DSOW transformation in the western Irminger Sea. We find that DSOW experiences continual warming and deoxygenation as it spreads along the Deep Western Boundary Current in the Irminger Sea. Furthermore, novel dissolved oxygen measurements from moored instruments offshore of Cape Farewell, Greenland reveal seasonality in the oxygen content of DSOW for the first time. Numerical model simulations of supercritical entrainment dynamics predict DSOW properties offshore Cape Farewell that are cooler and more oxygenated than in the observations. This disagreement suggests that sub-critical entrainment influences DSOW transformation in the western Irminger Sea, which we test by accounting for this process in the model. Model sensitivity experiments further suggest that the observed seasonal variation in the oxygen content of DSOW likely originates from the source overflow water upstream of the Denmark Strait. Overall, this work is the first to explore the entrainment dynamics of DSOW in the western Irminger Sea and its influence on the oxygen content of the overflow water, with implications for the response of the AMOC structure and dissolved gas uptake to changes in the climate system.

An important component of the deep limb of the Atlantic Meridional Overturning Circulation – the Deep Western Boundary Current (DWBC) – forms in the North Atlantic subpolar gyre from the combination of cold, dense water from the Nordic Seas and the ambient water within the gyre. Much of this water merges south of the Denmark Strait along the eastern flank of Greenland and flows southwestward toward the tip of Cape Farewell where the Overturning in the Subpolar North Atlantic Program (OSNAP) has continually monitored the DWBC since 2014 using moorings consisting of current meters, acoustic doppler current profilers, and temperature-salinity recorders. Previous estimates of the DWBC (σθ > 27.8 kg m-3) at this location found 9-13 Sv of transport, including 10.8 Sv from the first two years of OSNAP data. This presentation extends the OSNAP estimates of the DWBC through 2022 and finds a 22% decrease in transport over the eight-year record from a thinning of the traditionally defined DWBC layer (σθ > 27.8 kg m-3) and weakening velocities. This also results in a 27% decrease in transport of the densest water mass of the DWBC, Denmark Strait Overflow Water (σθ > 27.88 kg m-3). Overall, the eight-year transport mean of the DWBC is 8.4 Sv. This presentation will also consider alternative methods for evaluating the DWBC that find a transport reduction of only 12-17% over the eight-year observation period.

In the subpolar North Atlantic, the strength of the Meridional Overturning Circulation is linked to rates of North Atlantic Deep Water formation, a water mass partially composed of Nordic Seas Overflow Waters. While Denmark Strait Overflow Water takes a relatively direct route out of the Irminger basin via the cyclonic boundary current, exit pathways of Iceland-Scotland Overflow Water (ISOW) from the Iceland Basin are less understood and more complex. Here, ISOW pathways and their interannual variability are explored in a Lagrangian framework using particles seeded within the 45-year 1/12° eddy-resolving North Atlantic HYCOM simulation. Our analysis reveals significant depth-dependent variability in ISOW pathways. Upper layers preferentially cross into the Irminger Basin through gaps in the Reykjanes Ridge while deeper layers take the more traditional route to the Charlie-Gibbs Fracture Zone (CGFZ). At the CGFZ, we observe a strong anticorrelation in the percentage of particles that end up in the western vs. eastern basin which varies on a timescale of ~2.5 years and is likely associated with the position of the North Atlantic Current (NAC). This anticorrelation however is much stronger in the upper layers as the influence of the NAC appears to decrease with depth. Of the approximately 55% of particles that translate through the CGFZ, those in the upper layers are more likely to follow the cyclonic boundary current while lower layer particles diffuse northwestward towards the Labrador Sea. These depth-dependent patterns, identified from simulated particle trajectories, are corroborated by observations from RAFOS floats deployed during the OSNAP campaign. These findings illustrate the importance of depth-dependent dynamics and interannual variability of the NAC in shaping ISOW pathways, with implications for deep circulation patterns in the subpolar North Atlantic and the rate of large-scale overturning.