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Changes in the Atlantic Meridional Overturning Circulation (AMOC) represent a crucial component of Northern Hemisphere climate variability. In modelling studies decadal overturning variability has been attributed to the intensity of deep winter convection in the Labrador Sea. This linkage is challenged by transport observations at sections across the subpolar gyre. Here we report simulations with an eddy-rich ocean model which captures the observed concentration of downwelling in the northeastern Atlantic and the negligible impact of interannual variations in Labrador Sea convection during the last decade. However, the exceptionally cold winters in the Labrador Sea during the first half of the 1990s induced a positive AMOC anomaly of more than 20%, mainly by augmenting the downwelling in the northeastern North Atlantic. The remote effect of excessive Labrador Sea buoyancy forcing is related to rapid spreading of mid-depth density anomalies into the Irminger Sea and their entrainment into the deep boundary current off Greenland. Using high-resolution model experiments, the authors identify the rapid spreading of mid-depth density anomalies from the Labrador to the Irminger Sea as a prime mechanism in the generation of decadal changes in the Atlantic overturning circulation.

Vast amounts of methane hydrates are potentially stored in sediments along the continental margins, owing their stability to low temperature – high pressure conditions. Global warming could destabilize these hydrates and cause a release of methane (CH4) into the water column and possibly the atmosphere. Since the Arctic has and will be warmed considerably, Arctic bottom water temperatures and their future evolution projected by a climate model were analyzed. The resulting warming is spatially inhomogeneous, with the strongest impact on shallow regions affected by Atlantic inflow. Within the next 100 years, the warming affects 25% of shallow and mid‐depth regions containing methane hydrates. Release of methane from melting hydrates in these areas could enhance ocean acidification and oxygen depletion in the water column. The impact of methane release on global warming, however, would not be significant within the considered time span. Key Points Arctic bottom water temperatures will rise under climate warming Methane hydrates will be released in the coming decades The release will be limited in the next decades and lead to ocean acidification

High primary productivity in the equatorial Atlantic and Pacific oceans is one of the key features of tropical ocean biogeochemistry and fuels a substantial flux of particulate matter towards the abyssal ocean. How biological processes and equatorial current dynamics shape the particle size distribution and flux, however, is poorly understood. Here we use high-resolution size-resolved particle imaging and Acoustic Doppler Current Profiler data to assess these influences in equatorial oceans. We find an increase in particle abundance and flux at depths of 300 to 600 m at the Atlantic and Pacific equator, a depth range to which zooplankton and nekton migrate vertically in a daily cycle. We attribute this particle maximum to faecal pellet production by these organisms. At depths of 1,000 to 4,000 m, we find that the particulate organic carbon flux is up to three times greater in the equatorial belt (1° S–1° N) than in off-equatorial regions. At 3,000 m, the flux is dominated by small particles less than 0.53 mm in diameter. The dominance of small particles seems to be caused by enhanced active and passive particle export in this region, as well as by the focusing of particles by deep eastward jets found at 2° N and 2° S. We thus suggest that zooplankton movements and ocean currents modulate the transfer of particulate carbon from the surface to the deep ocean. Vertical migration of organisms and deep currents control the transport and characteristics of particles at the equator, according to an analysis of current and particle measurements. Particles fluxes are an important part of the ocean carbon cycle.

Clash of currents The possibility of future decline in the Atlantic meridional overturning circulation (AMOC) is central to discussions of climate change. Attention focuses on the effects of freshening of the subpolar North Atlantic, for example via increases in high-latitude precipitation and melting ice. A new high-resolution ocean model study shows that the North Atlantic is already experiencing an opposite effect from the south. In response to a southward shift of the westerly winds — probably anthropogenically influenced — the transport of warm and salty Indian Ocean waters around the tip of Africa is strengthening. Known as the Agulhas leakage, this flow may offset possible reductions in Atlantic deep-water circulation from freshwater inputs in the North Atlantic, possibly contributing to the stability of the AMOC system, including the Gulf Stream. The Agulhas leakage allows the transport of warm and salty Indian Ocean waters into the Atlantic Ocean and provides the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation. The results of a high-resolution ocean general circulation model now show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to a change in wind forcing. The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has a crucial role in the global oceanic circulation 1 and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC) 2 . There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages 3 and model studies 4 that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies 5 . A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing 6 , but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model 7 , 8 to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification 9 of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.

Interocean exchange: south of Africa The Meridional Overturning Circulation (MOC) mixes heat and salinity among ocean basins and between deep and shallow waters; global climate effects, including moderation of the climate of Western Europe, are extensive. Changes in the strength of the MOC could thus have dramatic impacts for local to regional climate change, but evidence for systematic changes is, to date, equivocal. Decadal variation in the MOC, though, is accepted and was thought to be largely controlled by freshwater inputs from the Labrador Sea. By analysing the differences between a coarse resolution ocean model and a model with a localized fine-resolution grid, Biastoch et al . show that the input of warm and salty water transferred around the southern tip of Africa — termed the Agulhas leakage — contributes significantly to variations in the strength of the MOC. The Meridional Overturning Circulation (MOC) mixes heat and salinity among ocean basins and between deep and shallow waters. Global climate effects, including moderation of the climate of western Europe, are extensive. Changes in the strength of the MOC could thus have dramatic impacts for local to regional climate change, but evidence for systematic changes is, to date, equivocal. Analysis of the residuals between a coarse resolution ocean model and a model with a localized fine-resolution grid shows that the input of warm and salty water transferred around the southern tip of Africa (termed the Agulhas leakage) contributes strongly to the strength of the MOC. Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC) 1 . Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic 2 , 3 (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades 4 , 5 . Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of ±1.5–3 Sv (1 Sv = 10 6  m 3  s -1 ) on decadal timescales in the subtropics 6 . Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water 7 . Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.

How far do marine larvae disperse in the ocean? Decades of population genetic studies have revealed generally low levels of genetic structure at large spatial scales (hundreds of kilometres). Yet this result, typically based on discrete sampling designs, does not necessarily imply extensive dispersal. Here, we adopt a continuous sampling strategy along 950 km of coast in the northwestern Mediterranean Sea to address this question in four species. In line with expectations, we observe weak genetic structure at a large spatial scale. Nevertheless, our continuous sampling strategy uncovers a pattern of isolation by distance at small spatial scales (few tens of kilometres) in two species. Individual-based simulations indicate that this signal is an expected signature of restricted dispersal. At the other extreme of the connectivity spectrum, two pairs of individuals that are closely related genetically were found more than 290 km apart, indicating long-distance dispersal. Such a combination of restricted dispersal with rare long-distance dispersal events is supported by a high-resolution biophysical model of larval dispersal in the study area, and we posit that it may be common in marine species. Our results bridge population genetic studies with direct dispersal studies and have implications for the design of marine reserve networks.

Analyses of ocean observations and model simulations suggest that there have been considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and are driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead, a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weakening of the THC will remain within the range of natural variability during the next several decades.

Mesoscale dynamics of the Agulhas Current system determine the exchange between the Indian and Atlantic oceans, thereby influencing the global overturning circulation. Using a series of ocean model experiments compared to observations, we show that the representation of mesoscale eddies in the Agulhas ring path improves with increasing resolution of submesoscale flows. Simulated submesoscale dynamics are validated with time‐mean horizontal‐wavenumber spectra from satellite sea surface temperature measurements and mesoscale dynamics with spectra from sea surface height. While the Agulhas ring path in a nonsubmesoscale‐resolving (1/20)° configuration is associated with too less power spectral densities on all scales and too steep spectral slopes, the representation of the mesoscale dynamics improves when the diffusion and the dissipation of the model are reduced and some small‐scale features are resolved. Realistic power spectral densities over all scales are achieved when additionally the horizontal resolution is increased to (1/60)° and a larger portion of the submesoscale spectrum is resolved. Results of an eddy detection algorithm applied to the model outputs as well as to a gridded sea surface height satellite product show that in particular strong cyclones are much better represented when submesoscale flows are resolved by the model. The validation of the submesoscale dynamics with sea surface temperature spectra provides guidance for the choice of advection schemes and explicit diffusion and dissipation as well as for further subgrid‐scale parameterizations. For the Agulhas ring path, the use of upstream biased advection schemes without explicit diffusion and dissipation is found to be associated with realistically simulated submesoscales. Key Points Submesoscale‐permitting ocean models can be validated with wavenumber spectra from satellite observations Simulated mesoscale Agulhas eddies strengthen with the increasing resolution of submesoscale dynamics Simulated mesoscale dynamics in the Agulhas ring path converge to observations in a (1/60)° ocean model

Time series of transports in the Agulhas region have been constructed by simulating Lagrangian drifter trajectories in a 1/10 degree two‐way nested ocean model. Using these 34 year long time series it is shown that smaller (larger) Agulhas Current transport leads to larger (smaller) Indian‐Atlantic inter‐ocean exchange. When transport is low, the Agulhas Current detaches farther downstream from the African continental slope. Moreover, the lower inertia suppresses generation of anti‐cyclonic vorticity. These two effects cause the Agulhas retroflection to move westward and enhance Agulhas leakage. In the model a 1 Sv decrease in Agulhas Current transport at 32°S results in a 0.7 ± 0.2 Sv increase in Agulhas leakage.

Howfar domarine larvae disperse in the ocean? Decades of population genetic studies have revealed generally low levels of genetic structure at large spatial scales (hundreds of kilometres). Yet this result, typically based on discrete sampling designs, does not necessarily imply extensive dispersal. Here, we adopt a continuous sampling strategy along 950 km of coast in the northwestern Mediterranean Sea to address this question in four species. In line with expectations, we observe weak genetic structure at a large spatial scale. Nevertheless, our continuous sampling strategy uncovers a pattern of isolation by distance at small spatial scales (few tens of kilometres) in two species. Individual- based simulations indicate that this signal is an expected signature of restricted dispersal. At the other extreme of the connectivity spectrum, two pairs of individuals that are closely related genetically were found more than 290 km apart, indicating long-distance dispersal. Such a combination of restricted dispersal with rare long-distance dispersal events is supported by a high-resolution biophysical model of larval dispersal in the study area, and we posit that it may be common in marine species. Our results bridge population genetic studies with direct dispersal studies and have implications for the design of marine reserve networks