The lower limb of the Atlantic meridional overturning circulation (AMOC) is the equatorward flow of dense waters that have been transformed due to the cooling and freshening of the poleward-flowing upper limb. In the subpolar North Atlantic (SPNA), upper limb variability is primarily set by the North Atlantic Current, whereas lower limb variability is less well understood, particularly at subseasonal timescales. Using observations from a SPNA mooring array, we show that variability of the AMOC’s lower limb is connected to poleward flow in the interior Irminger Sea. We identify this flow as the northward branch of the Irminger Gyre (IG), accounting for 55% of the AMOC’s lower limb variability on monthly timescales. Further, wind stress curl fluctuations over the Labrador and Irminger Seas drives the IG and AMOC variability on monthly timescales. On interannual timescales, however, increasing thickness of intermediate water within the Irminger Sea coincides with decreasing IG recirculation.

Observations indicate that symmetric instability is active in the East Greenland Current during strong northerly wind events. Theoretical considerations suggest that baroclinic instability may also be enhanced during these events. An ensemble of idealised numerical ocean models, forced with northerly winds show that the short time-scale response (from two to four weeks) to the increased baroclinicity of the flow is the excitation of symmetric instability, which sets the potential vorticity of the flow to zero. The high latitude of the current means that the zero potential vorticity state has low stratification, and symmetric instability destratifies the water column. On longer time scales (greater than four weeks), baroclinic instability is excited and the associated slumping of isopycnals restratifies the water column. Eddy-resolving models that fail to resolve the submesoscale should consider using submesoscale parameterisations to prevent the formation of overly stratified frontal systems following down-front wind events. The mixed layer in the current deepens at a rate proportional to the square root of the time-integrated wind stress. Peak water mass transformation rates vary linearly with the time-integrated wind stress. The duration of a wind event leads to a saturation of mixing rates which means increasing the peak wind stress in an event leads to no extra mixing. Using ERA5 reanalysis data we estimate that between 1.5Sv and 1.8Sv of East Greenland Coastal Current Waters are produced by mixing with lighter surface waters during wintertime by down-front wind events. Similar amounts of East Greenland-Irminger Current water are produced at a slower rate.

We study the impacts of a continental slope on instability and mesoscale eddy fluxes in idealized 3-layer numerical model simulations. The simulations are inspired by and mimic the situation in the Arctic Ocean’s Beaufort Gyre where anti-cyclonic winds drive anti-cyclonic currents that are guided by the continental slope. The forcing and currents are retrograde with respect to topographic Rossby waves. The focus of the analysis is on eddy potential vorticity (PV) fluxes and eddy-mean flow interactions under the Transformed Eulerian Mean framework. Lateral momentum fluxes in the upper layer dominate over the actual continental slope where eddy form drag, i.e.\ vertical momentum flux, is suppressed due to the topographic PV gradient. The diagnosis also shows that while eddy momentum fluxes are up-gradient over parts of the slope, the total quasi-geostrophic PV flux is down-gradient everywhere. We then calculate the linearly unstable modes of the time-mean state and find that the most unstable mode contains several key features of the observed finite-amplitude fluxes over the slope, including down-gradient PV fluxes. When accounting for additional unstable modes, all qualitative features of the observed eddy fluxes in the numerical model are reproduced.

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. At 26.5oN, the meridional heat transport has traditionally been partitioned geometrically into vertical and horizontal circulation contributions; however, attributing these components to the AMOC and Subtropical Gyre (STG) flow structures remains widely debated. Using water parcel trajectories evaluated within an eddy-rich ocean hindcast, we present the first Lagrangian decomposition of the meridional heat transport at 26.5oN. We find that water parcels recirculating within the STG account for 37% (0.36 PW) of the total heat transport across 26.5oN, more than twice that of the classical horizontal gyre component (15%). Our findings indicate that STG heat transport cannot be meaningfully distinguished from that of the basin-scale overturning since water parcels cooled within the gyre subsequently feed the northward, subsurface limb of the AMOC.

Atlantic Water (AW) is the largest reservoir of heat in the Arctic Ocean, isolated from the surface and sea-ice by a strong halocline. In recent years AW shoaling and warming are thought to have had an increased influence on sea-ice in the Eurasian Basin. In this study we analyse 59000 profiles from across the Arctic from the 1970s to 2018 to obtain an observationally-based pan-Arctic picture of the AW layer, and to quantify temporal and spatial changes. The potential temperature maximum of the AW (the AW core) is found to be an easily detectable, and generally effective metric for assessments of AW properties, although temporal trends in AW core properties do not always reflect those of the entire AW layer. The AW core cools and freshens along the AW advection pathway as the AW loses heat and salt through vertical mixing at its upper bound, as well as via likely interaction with cascading shelf flows. In contrast to the Eurasian Basin, where the AW warms (by approximately 0.7°C between 2002 and 2018) in a pulse-like fashion and has an increased influence on upper ocean heat content, AW in the Canadian Basin cools (by approximately 0.1°C between 2008 and 2018) and becomes more isolated from the surface due to the intensification of the Beaufort Gyre. These opposing AW trends in the Eurasian and Canadian Basins of the Arctic over the last 40 years suggest that AW in these two regions may evolve differently over the coming decades.

Density staircases are observed in an idealised model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centres of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.

Translation of atmospheric forcing variability into the ocean interior via ocean ventilation is an important aspect of transient climate change. On a seasonal timescale, this translation is mediated by a so-called “Demon’ that prevents access to all except late-winter mixed-layer water. Here, we use an eddy-permitting numerical circulation model to investigate a similar process operating on longer timescales in the high-latitude North Atlantic, which we denote the “interannual Demon’. We find that interannual variations in atmospheric forcing are indeed mediated in their translation to the ocean interior. In particular, the signature of persistent strong atmospheric forcing driving deep mixed layers is preferentially ventilated to the interior when the forcing is ceased. Susceptibility to the interannual Demon depends on the location and density of subduction — with the rate at which newly ventilated water escapes its region of subduction being the crucial factor.

The volume, extent and age of Arctic sea ice is in decline, yet winter sea ice production appears to have been increasing, despite Arctic warming being most intense during winter months. Several negative feedback processes help to restore Arctic sea ice volume during the winter, though previous work suggests that these mechanisms will be overwhelmed by warming in the future, leading to a fall in ice production. Here, we analyse winter ice production in the Kara and Laptev seas–sometimes referred to as Arctic “ice factories”, for their outsized role in supplying the Arctic Ocean with young sea ice. Using the Community Earth System Model’s Large Ensemble (CESM-LE), we develop a simple linear model that can explain both the forced rise and forced fall of ice production, in terms of consistent physics. We apply our linear model to observation-based estimates of the same climate variables; our reconstruction of ice production suggests that–just as in CESM-LE–we are currently passing the peak of ice production in the Kara and Laptev seas.

Vast quantities of marine debris have beached at remote islands in the western Indian Ocean such as Seychelles, but little is known about where this debris comes from. To identify these sources and temporal patterns in accumulation rate, we carried out global Lagrangian particle tracking experiments incorporating surface currents, waves, and variable windage, beaching, and sinking rates, taking into account both terrestrial (coastal populations and rivers) and marine (fisheries and shipping) sources of debris. Our results show that, whilst low-buoyancy terrestrial debris may originate from the western Indian Ocean (principally Tanzania, Comoros, and Seychelles), most terrestrial debris beaching at remote western Indian Ocean islands drifts from the eastern and northern Indian Ocean, primarily Indonesia and, to a lesser extent, India and Sri Lanka. Purse-seine fragments beaching at Seychelles are likely associated with fishing activity in the western Indian Ocean, but longline fragments may also be swept from the southeastern Indian Ocean. The entire of Seychelles is at very high risk from waste discarded from shipping routes transiting the Indian Ocean, and comparison with observations suggests that many bottles washing up on beaches may indeed originate from these routes. Our analyses indicate that marine debris accumulation at Seychelles (and the Outer Islands in particular) is likely to be strongly seasonal, peaking during February-April, and this pattern is driven by local monsoonal winds. This seasonal cycle may be amplified during positive Indian Ocean Dipole phases and El-Ni\~{n}o events. These results underline the vulnerability of small island developing states to marine plastic pollution, and are a crucial first step towards improved management of the issue. The Lagrangian trajectories used in this study are available for download, and our analyses can be rerun under different parameters using the associated scripts.

Diapycnal mixing shapes the distribution of climatically-important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation. The rate of water mass transformation in the Atlantic Ocean’s interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW), with a diapycnal circulation of up to 4 Sv between 24N and 32S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to hundreds of metres in the vertical. This result is confirmed with a zonally-averaged numerical model of the AMOC and indicates that tracer mixing can lead to divergent global pathways and ventilation timescales following the upwelling of tracers in the Southern Ocean. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

The subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large-scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water and triggers slantwise convection. We estimate that this down-front wind forcing is four times stronger than air--sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large-scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.