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KEYNOTE POINTS 1. Thermal expansion from a warming ocean and land ice melt are the main causes of the accelerating global rise in the mean sea level. 2. Global warming is also affecting many circulation systems. The Atlantic Meridional Overturning Circulation has already weakened and will most likely continue to do so in the future. The impacts of ocean circulation changes include a regional rise in sea levels, changes in the nutrient distribution and carbon uptake of the ocean and feedbacks with the atmosphere, such as altering the distribution of precipitation. 3. More than 90 percent of the heat from global warming is stored in the global ocean. Oceans have exhibited robust warming since the 1950s from the surface to a depth of 2,000 m. The proportion of ocean heat content has more than doubled since the 1990s compared with long-term trends. Ocean warming can be seen in most of the global ocean, with a few regions exhibiting long-term cooling. 4. The ocean shows a marked pattern of salinity changes in multidecadal observations, with surface and subsurface patterns providing clear evidence of water cycle amplification over the ocean. That is manifested in enhanced salinities in the near-surface, high-salinity subtropical regions and freshening in the low-salinity regions such as the West Pacific Warm Pool and the poles. 5. An increase in atmospheric CO2 levels, and a subsequent increase in carbon in the oceans, has changed the chemistry of the oceans to include changes to pH and aragonite saturation. A more carbon-enriched marine environment, especially when coupled with other environmental stressors, has been demonstrated through field studies and experiments to have negative impacts on a wide range of organisms, in particular those that form calcium carbonate shells, and alter biodiversity and ecosystem structure. 6. Decades of oxygen observations allow for robust trend analyses. Long-term measurements have shown decreases in dissolved oxygen concentrations for most ocean regions and the expansion of oxygen-depleted zones. A temperature-driven solubility decrease is responsible for most near-surface oxygen loss, though oxygen decrease is not limited to the upper ocean and is present throughout the water column in many areas. 7. Total sea ice extent has been declining rapidly in the Arctic, but trends are insignificant in the Antarctic. In the Arctic, the summer trends are most striking in the Pacific sector of the Arctic Ocean, while, in the Antarctic, the summer trends show increases in the Weddell Sea and decreases in the West Antarctic sector of the Southern Ocean. Variations in sea ice extent result from changes in wind and ocean currents. Versión del editor

The upper ocean circulation of the tropical Atlantic experiences long-term changes associated with different climate modes, but is at the same time expected to adjust to changes in the meridional overturning forced by climate warming. While observations of decadal variability of the surface circulation are generally based on satellite altimetry, direct observations of subsurface circulation mostly rely on very few long-term mooring sites typically covering energetic currents such as the Atlantic Equatorial Undercurrent (EUC). Here we focus on the period 2006 to 2018 that was covered by an intense field program including oxygen and circulation observations in the equatorial and tropical North Atlantic. During the observational period, a strengthening of the EUC of about 20% was detected based on data of an equatorial current meter mooring at 23°W. The EUC strengthening is related to a similar strengthening of the subtropical cells (STC). These STC changes were forced by a trade wind intensification in both hemispheres, however, more pronounced in the north and in the western basin. The STC strengthening is found to be consistent with the observed 12-year oxygen increase in the equatorial band (i.e. south of about 5°N) in the upper 400m obtained from repeat ship sections along 23°W. Such strongly enhanced oxygen levels relative to climatological mean were also observed in the upper 300-400m during a recent cruise along the whole Atlantic equator from Africa to South America. Our results are discussed with regard to the superposition of internal climate variability likely associated to a recent phase shift in the Atlantic multidecadal variability and changes due to global warming including ocean deoxygenation and enhanced thermocline stratification.

A strong oxygen-deficient layer is located in the upper layers of the tropical Pacific Ocean and deeper in the North Pacific. Processes related to climate change (upper-ocean warming, reduced ventilation) are expected to change ocean oxygen and nutrient inventories. In most ocean basins, a decrease in oxygen (“deoxygenation”) and an increase in nutrients have been observed in subsurface layers. Deoxygenation trends are not linear and there could be multiple influences on oxygen and nutrient trends and variability. Here oxygen and nutrient time series since 1950 in the Pacific Ocean were investigated at 50 to 300 m depth, as this layer provides critical pelagic habitat for biological communities. In addition to trends related to ocean warming the oxygen and nutrient trends show a strong influence of the Pacific Decadal Oscillation (PDO) in the tropical and the eastern Pacific, and the North Pacific Gyre Oscillation (NPGO) in particular in the North Pacific. In the Oyashio Region the PDO, the NPGO, the North Pacific Index (NPI) and an 18.6-year nodal tidal cycle overlay the long-term trend. In most eastern Pacific regions oxygen increases and nutrients decrease in the 50 to 300 m layer during the negative PDO phase, with opposite trends during the positive PDO phase. The PDO index encapsulates the major mode of sea surface temperature variability in the Pacific, and oxygen and nutrients trends throughout the basin can be described in the context of the PDO phases. El Niño and La Niña years often influence the oxygen and nutrient distribution during the event in the eastern tropical Pacific but do not have a multi-year influence on the trends.

The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres they are characterized by equatorward transport at thermocline level, upwelling at the equator and poleward Ekman transport in the surface layer. This study uses recent data from Argo oats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward ow and the thermocline equatorward ow. The latter is bounded by the 26.0 kg m‐3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ±1.1 Sv from the southern hemisphere compared to 2.9 ±1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ±3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ±0.8 Sv at 10°S and 6.4 ±1.4 Sv at 10°N). A diapycnal transport of about 4 Sv across the 26.0 kg m‐3 isopycnal is required in order to maintain the mass balance in the STC circulation.

The scientific program of the MARIA S. MERIAN MSM74 expedition was dedicated to studies on the intensity of water mass transformation and the southward transport of water masses in the boundary current systems off Labrador and at the southern tip of Greenland. During the expedition we recovered 17/deployed 8 deep sea moorings. Measurements of the vertical structure of temperature, salinity, density, oxygen, optical properties and the flow along selected sections have been surveyed during the MSM74 expedition. Close to the surface, permanent registrations are carried out with the thermosalinograph (temperature, salinity) and meteorological data are continuously collected. Flow measurements up to 1000m depth are performed with the ships installed ADCPs. The expedition is a contribution to national (RACE) and international projects (OSNAP, AtlantOS).

Summary • The global oxygen inventory has decreased by ~2% over the period 1960 to 2010, this finding is supported by regional time series data that indicate a continuous decrease in oceanic dissolved oxygen. • Ocean model simulations predict a decline in the dissolved oxygen inventory of the global ocean of 1 to 7% by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean. • Open-ocean deoxygenation is resulting mainly from a warming ocean, increased stratification and changing circulation which interact with eutrophication-induced hypoxia (oxygen concentration below ~60 to 120 μmol O2 kg-1) and biological activity in shelf regions. • Climate change related longer-term oxygen trends are masked by oxygen variability on a range of different spatial and temporal scales. • The decline in the oceanic oxygen content can affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries, ecosystems and coastal economies. • Oxygen loss is closely related to ocean warming and acidification caused by CO2 increase driven by CO2 emissions as well as biogeochemical consequences related to anthropogenic fertilization of the ocean; hence a combined effort investigating the different stressors will be most beneficial to understand future ocean changes.

Cruise M135 was a contribution to the DFG Collaborative Research Project (SFB) 754: “Climate-Biogeochemistry Interactions in the Tropical Ocean” with the main goal to better understand the the role of diffusive and advective pathways connecting water within the bottom boundary layer (i.e. the water directly affected by sediment processes) to the pelagic and surface ocean. To achieve this, we have injected a conservative tracer (CF3SF5) within the bottom boundary layer at three different sites along the Peruvian coast at a depth of about 300 m in October 2015 that was mapped during M135. Tracer sampling was carried out by measuring water samples from the CTD-rosette water bottles. In total 144 CTD casts were carried out. From 132 CTD profiles 2828 samples for CF3CF5 investigations were gained and on most stations the tracer could be found. In addition 48 trace metal CTD’s were recorded and trace metal and chemical samples taken from the rosette bottles. On 166 of the CTD profiles oxygen samples were taken and on 94 CTD profiles nutrient samples were collected. Microstructure measurements were made on 24 stations and 2 gliders were deployed. For geological investigations at 5 locations multicorer and long gravity cores were taken. Continuous underway measurements of CO2,N2O and CO as well as continuous ADCP and thermosalinograph recording was made on 37 days. The cruise M135 was very successful; most systems on METEOR worked well and all planned objectives were reached.

Special Supplement to the Bulletin of the American Meteorological Society Vol. 97, No. 8, August 2016; International audience