RRS Discovery Cruise DY113, 3 February – 13 March 2020. Repeat hydrographic measurements on GO-SHIP lines SR1b and A23

Cruise DY113 comprised occupations of two repeat hydrographic sections, SR1b across Drake Passage from Burdwood Bank to Elephant Island, and A23 from the northern Weddell Sea across the Scotia Sea to South Georgia. Ocean physical measurements are made on these two sections annually funded by NERC as National Capability, currently through the ORCHESTRA (Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports) programme, in order to monitor and understand variability of Antarctic Circumpolar Current transports and Antarctic Bottom Water properties and volumes. In addition to the 62 CTD/LADCP casts on SR1b and A23, a CTD survey was made over 17 sites in Cumberland Bay, South Georgia, and a section along the North Scotia Ridge also occupied on cruise JR299 was revisited, bringing the total to 104 CTD/LADCP casts including one test cast and one other repeat. Water column samples were collected for calibration of CTD salinity and dissolved oxygen (most stations) as well as for measurements of oxygen isotopes (SR1b, A23, Cumberland Bay), nutrient (N and Si) isotopes (SR1b), nutrient (NO2+NO3, NO3, Si, P; SR1b, A23) concentrations, microplastics (SR1b, A23, Cumberland Bay), and environmental DNA (SR1b). Standard underway measurements including underway surface ocean and meteorological data and upper ocean vessel-mounted current measurements were collected throughout, while multibeam swath bathymetry data was recorded on the transit between SR1b and A23 (south of the South Orkney Islands), in Cumberland Bay, and on previously-unsurveyed parts of the North Scotia Ridge transect and between there and the Falkland Islands. Four standard Argo autonomous profiling floats were also deployed, two on SR1b and two on A23

RRS James Clark Ross Cruise JR15003, 17 Dec 2015 - 13 Jan 2016. Hydrographic measurements on GO-SHIP line SR1b and investigations of circulation and isotope cycles in coastal West Antarctica.

This cruise comprised work contributing to five projects. The twenty-first complete occupation of the Drake Passage GO-SHIP section SR1b obtained full-depth temperature, salinity, and lowered ADCP velocity profiles at 28 stations, along with underway measurements, with the objectives of investigating and monitoring interannual variability and trends in Antarctic Circumpolar Current structure and property transports and Southern Ocean water mass properties. Turnarounds of bottom pressure recorder (BPR) moorings contributed to the long time series of bottom pressure in Drake Passage. Biogeochemically-equipped Argo floats were deployed as part of the Southern Ocean Carbon and Climate Observations and Modelling (SOCCOM) project to increase climate-quality observations in the Southern Ocean. Gliders were deployed over the Western Antarctic Peninsula continental shelf to measure properties and circulation with the aim of understanding flow and mixing of warm waters onto the shelf. Hydrographic profiles and water column and sediment samples taken over the continental shelf will be used to investigate stable isotope nutrient cycling processes

CLASS and OSNAP report for JC238

The aim of this expedition was to recover and redeploy all moorings in the OSNAP array and to obtain a CTD section of hydrographic, nutrients, dissolved oxygen and dissolved organic carbon observations along the OSNAP section. The specific measurement objectives of the cruise were: 1. Recover and redeploy 4 existing mooring along the Ellett Array (EB1, WB1, WB2, RH ADCP) and 3 existing moorings in the Iceland Basin (IB5, IB4, IB3) 2. Deploy new drift-free pressure recorder (Sonardyne Fetch AZA) at EB1. 3. Conduct CTD stations and capture water samples for oxygen, total carbon, alkalinity and nutrients analysis. Data from the CTD stations was provided in near-real time (<12 hours) to the UK Met. Office to be assimilated into their short-range ocean forecast models: global 0.25 degree, North Atlantic 1/12th degree and AMM15 (European NW Shelf, ~ 1.5 km). 4. Recover and redeploy a sediment trap mooring on the Darwin Sea Mounds (DMLTM) as part of the UK Marine Protected Area habitat monitoring programme. 5. Deploy three BGC-Argo floats to maintain the UK contribution to the global ARGO network. 6. Using ship-based instrumentation to measure underway meteorology, sea-surface temperature and salinity, ocean currents from the surface to ~400m depth and waterdepth using a Kongsberg multibeam echo sounding system

Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of flow inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice are used to estimate basal melting of the ice shelf. The dissipation-rate-based melt rate estimates is sensitive to the stability correction parameter in the linear approximation of universal function of the Monin-Obukhov similarity theory for stratified boundary layers. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting

RRS James Cook Cruise JC159 28 February - 11 April 2018. Hydrographic sections from the Brazil to the Benguela Current across 24S in the Atlantic

A Hydrographic section was occupied at a nominal latitude of 24°S in the Atlantic Ocean during March and April 2018 on Cruise JC159 of RRS James Cook. The primary objective of this cruise was to measure ocean physical, chemical and biological parameters in order to establish regional budgets of heat, freshwater and carbon, and to infer decadal variability. In addition, 371 Niskin Bottles were sampled for microplastics, reflecting increasing awareness of plastics pollution in the oceans. A total of 121 CTD/LADCP stations were conducted, including one test station and two CFC bottle blank stations. In addition to temperature, salinity and oxygen profiles from the sensors on the CTD package, water samples from a 24 x 20 litre rosette were analysed for the following parameters at all stations: salinity; dissolved oxygen; inorganic nutrients; alkalinity and dissolved inorganic carbon; CFCs. Samples were collected for shore analysis for oxygen and carbon isotopes (del-18O, del13C and del-14C). Samples were collected and filtered for pigments (shore analysis) at 44 stations and for microplastics at 45 stations. 8 Argo floats were deployed, including two Bio-PROVOR floats and 2 Deep ARVORs. In addition, samples were collected from the ships’ underway system to calibrate and compliment the data continually collected by the TSG (thermosalinograph). Full depth velocity measurements were made at every station by LADCP (Lowered Acoustic Doppler Current Profiler) mounted on the frame of the rosette. Throughout the cruise, velocity data in the upper few hundred metres of the water column were collected by the ship’s VMADCP (Vessel Mounted Acoustic Doppler Current Profiler) transducers (75Hz and 150Hz) mounted on one of the two drop keels. Meteorological variables were monitored using the onboard surface water and meteorological sampling system (SURFMET). Bathymetric data were collected using the Kongsberg EM122 multibeam system and the EA640 echo sounder. This report describes the methods used to acquire and process the data on board the ship during cruise JC159

Antarctic circumpolar current transport through Drake Passage: What can we learn from comparing high‐resolution model results to observations?

Uncertainty exists in the time‐mean total transport of the Antarctic Circumpolar Current (ACC), the world's strongest ocean current. The two most recent observational programs in Drake Passage, DRAKE and cDrake, yielded transports of 141 and 173.3 Sv, respectively. In this paper, we use a realistic 1/12° global ocean simulation to interpret these observational estimates and reconcile their differences. We first show that the modeled ACC transport in the upper 1,000 m is in excellent agreement with repeat shipboard acoustic Doppler current profiler (SADCP) transects and that the exponentially decaying transport profile in the model is consistent with the profile derived from repeat hydrographic data. By further comparing the model results to the cDrake and DRAKE observations, we argue that the modeled 157.3 Sv transport, that is, approximately the average of the cDrake and DRAKE estimates, is actually representative of the time‐mean ACC transport through the Drake Passage. The cDrake experiment overestimated the barotropic contribution in part because the array undersampled the deep recirculation southwest of the Shackleton Fracture Zone, whereas the surface geostrophic currents used in the DRAKE estimate yielded a weaker near‐surface transport than implied by the SADCP data. We also find that the modeled baroclinic and barotropic transports are not correlated; thus, monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport

Bottom pressure torque and the vorticity balance from observations in Drake Passage

The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using 4 years of observations from current‐ and pressure‐recording inverted echo sounders. The time‐varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two‐dimensional array in the eddy‐rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50‐km scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second‐order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one‐third the size of the local depth‐integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth‐integrated vorticity balance. Observed near‐bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near‐bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to urn:x-wiley:21699275:media:jgrc21771:jgrc21771-math-0001 m s−2, exceeds the mean wind stress curl by a factor of 15–20

Macronutrient and carbon supply, uptake and cycling across the Antarctic Peninsi shelf during summer

The West Antarctic Peninsula shelf is a region of high seasonal primary production which supports a large and productive food web, where macronutrients and inorganic carbon are sourced primarily from intrusions of warm saline Circumpolar Deep Water. We examined the cross-shelf modification of this water mass during mid-summer 2015 to understand the supply of nutrients and carbon to the productive surface ocean, and their subsequent uptake and cycling. We show that nitrate, phosphate, silicic acid and inorganic carbon are progressively enriched in subsurface waters across the shelf, contrary to cross-shelf reductions in heat, salinity and density. We use nutrient stoichiometric and isotopic approaches to invoke remineralization of organic matter, including nitrification below the euphotic surface layer, and dissolution of biogenic silica in deeper waters and potentially shelf sediment porewaters, as the primary drivers of cross-shelf enrichments. Regenerated nitrate and phosphate account for a significant proportion of the total pools of these nutrients in the upper ocean, with implications for the seasonal carbon sink. Understanding nutrient and carbon dynamics in this region now will inform predictions of future biogeochemical changes in the context of substantial variability and ongoing changes in the physical environment

Wind-Driven Processes Controlling Oceanic Heat Delivery to the Amundsen Sea, Antarctica

Variability in the heat delivery by Circumpolar Deep Water (CDW) is responsible for modulating the basal melting of the Amundsen Sea ice shelves. However, the mechanisms controlling the CDW inflow to the region’s continental shelf remain little understood. Here, a high-resolution regional model is used to assess the processes governing heat delivery to the Amundsen Sea. The key mechanisms are identified by decomposing CDW temperature variability into two components associated with 1) changes in the depth of isopycnals [heave (HVE)], and 2) changes in the temperature of isopycnals [water mass property changes (WMP)]. In the Dotson–Getz trough, CDW temperature variability is primarily associated with WMP. The deeper thermocline and shallower shelf break hinder CDW access to that trough, and CDW inflow is regulated by the uplift of isopycnals at the shelf break—which is itself controlled by wind-driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, CDW temperature variability in the Pine Island–Thwaites trough is mainly linked to HVE. The shallower thermocline and deeper shelf break there permit CDW to persistently access the continental shelf. CDW temperature in the area responds to wind-driven modulation of the water mass on-shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-induced vertical displacement of isopycnals within the shelf. The western and eastern Amundsen Sea thus represent distinct regimes, in which wind forcing governs CDW-mediated heat delivery via different dynamics

The South Atlantic circulation between 34.5°S, 24°S and above the Mid‐Atlantic Ridge from an inverse box model

The South Atlantic Ocean plays a key role in the heat exchange of the climate system, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic Subtropical gyre at 34.5°S, 24°S, and 10°W, we identify water masses and compute absolute geostrophic circulation using inverse modeling. In the upper layers, the currents describe the South Atlantic anticyclonic gyre with the northwest flowing Benguela Current (26.3 ± 2.0 Sv at 34.5°S, and 21.2 ± 1.8 Sv at 24°S) flowing above the Mid-Atlantic Ridge (MAR) between 22.4°S and 28.4°S (−19.2 ± 1.4 Sv), and the southward flowing Brazil Current (−16.5 ± 1.3 Sv at 34.5°S, and −7.3 ± 0.9 Sv at 24°S); the deep layers feature the southward transports of Deep Western Boundary Current (−13.9 ± 3.0 Sv at 34.5°S, and −8.7 ± 3.8 Sv at 24°S) and Deep Eastern Boundary Current (−15.1 ± 3.5 Sv at 34.5°S, and −16.3 ± 4.7 Sv at 24°S), with the interbasin west-to-east flow close to 24°S (7.5 ± 4.4 Sv); the abyssal waters present northward mass transports through the Argentina Basin (5.6 ± 1.1 Sv at 34.5°S, and 5.8 ± 1.5 Sv at 24°S) and Cape Basin (8.6 ± 3.5 Sv at 34.5°S–3.0 ± 0.8 Sv at 24°S) before returning southward (−2.2 ± 0.7 Sv at 24°S to −7.9 ± 3.6 Sv at 34.5°S), without any interbasin exchange across the MAR. In addition, we compute the upper AMOC strength (14.8 ± 1.0 and 17.5 ± 0.9 Sv), the equatorward heat transport (0.30 ± 0.05 and 0.80 ± 0.05 PW), and the freshwater flux (0.18 ± 0.02 and −0.07 ± 0.02 Sv) at 34.5°S and 24°S, respectively