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The Indian Ocean Observing System (IndOOS), established in 2006, is a multinational network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. Almost one-third of humanity lives around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture that are vulnerable to climate variability and extremes. The Indian Ocean alone has absorbed a quarter of the global oceanic heat uptake over the last two decades and the fate of this heat and its impact on future change is unknown. Climate models project accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. In view of these new scientific challenges, a 3-yr international review of the IndOOS by more than 60 scientific experts now highlights the need for an enhanced observing network that can better meet societal challenges, and provide more reliable forecasts. Here we present core findings from this review, including the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) expansion into the western tropics to improve understanding of the monsoon circulation; 3) better-resolved upper ocean processes to improve understanding of air–sea coupling and yield better subseasonal to seasonal predictions; and 4) expansion into key coastal regions and the deep ocean to better constrain the basinwide energy budget. These goals will require new agreements and partnerships with and among Indian Ocean rim countries, creating opportunities for them to enhance their monitoring and forecasting capacity as part of IndOOS-2.

Given the major role of the Atlantic Ocean in the climate system, it is essential to characterize the temporal and spatial variations of its heat content. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean from satellite altimetry and gravimetry. The strategy developed in the frame of the ESA MOHeaCAN Project (https://eo4society.esa.int/projects/moheacan/) is pursued and refined at local scales both for the data generation and the uncertainty estimate. At two test sites, OHC derived from in situ data (RAPID and OVIDE-AR7W) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change. The Atlantic OHC product will be used to better understand the complexity of the Earth’s climate system. In particular, the project aims at better understanding the role played by the Atlantic Meridional Overturning Circulation (AMOC) in regional and global climate change, and the variability of the Meridional Heat transport in the North Atlantic. In addition, improving our knowledge on the Atlantic OHC change will help to better assess the global ocean heat uptake and thus estimate the Earth’s energy imbalance more accurately as the oceans absorb about 90% of the excess energy stored by the Earth system.The objectives of the 4DATLANTIC-OHC Project will be presented. The scientific requirements and data used to generate the OHC change products over the Atlantic Ocean and the first results in terms of development will be detailed. At a later stage, early adopters are expected to assess the OHC products strengths and limitations for the implementation of new solutions for Society. The project started in June 2021 for a 2-year duration.Visit https://www.4datlantic-ohc.org to follow the main steps of the project.

Sustained shipboard hydrography surveys along the A25-Ovide section (2002 – 2018) are combined with data from a regional pilot array of Deep Argo floats (2016 – 2021) to estimate the decadal variability and linear trends in the temperature of overflow-derived waters in the Irminger Sea. Removing local or remote dynamical influences (heave) enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures (spice). The latter took place in 2014 and interrupted a long-term warming of those water masses that was prevailing since the late 1990’s. Deep-Argo floats further reveal an overall acceleration of this cooling since 2014, with a mean rate of change estimated at -18 m°C yr-1 during 2016 – 2021, as well as a boundary-intensified pattern of change. This, along with the absence of apparent reversal in the Nordic Seas and with DSOW warming and cooling twice as fast as ISOW, points out the entrainment of subpolar intermediate signals within the overflow plumes near the Greenland-Iceland-Scotland sills as a most likely driver.

EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022.-- This work is distributed under the Creative Commons Attribution 4.0 License, About 30% of the carbon dioxide derived from human activities (CANTH) has been absorbed by the ocean (DeVries, 2014; Gruber et al., 2019; Friedlingstein et al., 2021), with the North Atlantic (NA) being one of the largest CANTH sinks per unit area (Khatiwala et al., 2013; Sabine et al., 2004). In the NA, oceanic CANTH uptake strongly relies on the meridional overturning circulation and the associated regional winter deep convection. In fact, the formation and deep spreading of Labrador Sea Water stands as a critical CANTH gateway to intermediate and abyssal depths. The NA CANTH uptake has fluctuated over the years according to changes in the North Atlantic Oscillation. Biennial observation of the marine carbonate system along the Go-Ship A25-OVIDE section has allowed us assessing the decadal and interannual variability of the CANTH storage in the subpolarNA from 2002 to 2021. In this study, we investigate 1) the trend of CANTH and 2) the relationship between the CANTH saturation, the apparent oxygen utilization, and the ventilation of the water masses between the A25-OVIDE section and the Greenland-Iceland-Scotland sills during 2002-2021. We divided the A25-OVIDE section into three main basins (Irminger, Iceland, and Eastern NA). Our results show that the Irminger Basin presents a more homogenous CANTH profile and higher CANTH saturation values at depth than the other two basins, which is related to the pronounced convective activity in the Irminger Basin. In contrast, the Eastern NA Basin has higher CANTH values at the surface due to its higher surface temperature, but its deep water masses show the lowest CANTH values since they are the less ventilated in the section. Our analysis also reveals that, overall, the NA CANTH storage has increased during 2002-2021, but varied according to the ventilation changes. While the Eastern NA water masses experienced a relatively constant, although shallower, average ventilation, the Irminger and Iceland Basins underwent a less steady CANTH uptake pattern characterized by alternating periods of strong and weak CANTH storage

EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022.-- This work is distributed under the Creative Commons Attribution 4.0 License, The OVIDE section, composed of a hundred top-to-bottom stations from Portugal to Greenland, has been visited biennially since 2002. Collected data show a strong variability of both the Atlantic Meridional Overturning Circulation (AMOC) and of the water mass properties. The OVIDE-AMOC timeseries built upon the Argo array and altimetry has been updated and validated with the in-situ cruise estimates. It shows a strong seasonal variability and, on longer time scales, significant transition in 2014, from moderate (19 Sv) to strong (23 Sv) amplitude, along with the development of a fresh and cold anomaly in the upper 800m over the eastern subpolar Atlantic, discussed in the literature and observed at the OVIDE section. Through a composite analysis of both transport and property data, we compare the 2002-2012 OVIDE average with the 2014-2018 average and analyze the evolutions of the transports of the different water masses with special attention to LSW, which has been largely renewed since 2014 through deep convection in the western subpolar gyre

Temperature and salinity seasonal to interannual variability of Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) is investigated by combining two in-situ datasets in the Irminger Sea for the period 1997-2020: 12-yr of repeated hydrography (1997-2018) provided by the FOUREX, OVIDE and RREX sections and 4-yr of data (2016-2020) from 8 Deep Argo floats deployed in the region between 2016 and 2018. In order to enable a consistent analysis of ocean temperature and salinity variability from unevenly distributed vertical profiles (both in space and time), it is necessary to estimate the appropriate regional climatology to be removed from every observation. Two independent procedures are followed to compute anomalies and quantify uncertainties related to the choice of climatology: First, the global 1°-resolution World Ocean Atlas 2018 (2005-2017 averages) climatology is retrieved from every observed profile (Deep Argo, hydrography). Second, the well-known and sampled OVIDE transect (2002-2018 average) is used to build a reference section of geographical anomalies that are subsequently propagated along potential vorticity contours in the Irminger Sea. Neutral density surfaces 28.02 kgm-3 and 28.12 kgm-3 are then chosen from mean OVIDE 2002-2018 gridded fields as representative of ISOW and DSOW levels, respectively. Significant decadal trends in water mass properties are revealed by repeated hydrography, whereas some striking boundary-interior spatial patterns are captured by Deep Argo floats. Property changes of ISOW and DSOW are discussed in terms of changes of source waters in the Nordic Seas, entrainment of Atlantic waters into the overflow waters and cascading events from the Greenland slope.

The study of energy flows in the Earth system is essential for understanding current climate change. To understand how energy is accumulating and being distributed within the climate system, an updated reconstruction of energy fluxes at the top of atmosphere, surface and within the atmosphere derived from observations is presented. New satellite and ocean data are combined with an improved methodology to quantify recent variability in meridional and ocean to land heat transports since 1985. A global top of atmosphere net imbalance is found to increase from 0.10 ± 0.61 W m−2 over 1985–1999 to 0.62 ± 0.1 W m−2 over 2000–2016, and the uncertainty of ± 0.61 W m−2 is related to the Argo ocean heat content changes (± 0.1 W m−2) and an additional uncertainty applying prior to 2000 relating to homogeneity adjustments. The net top of atmosphere radiative flux imbalance is dominated by the southern hemisphere (0.36 ± 0.04 PW, about 1.41 ± 0.16 W m−2) with an even larger surface net flux into the southern hemisphere ocean (0.79 ± 0.16 PW, about 3.1 ± 0.6 W m−2) over 2006–2013. In the northern hemisphere the surface net flux is of opposite sign and directed from the ocean toward the atmosphere (0.44 ± 0.16 PW, about 1.7 ± 0.6 W m−2). The sea ice melting and freezing are accounted for in the estimation of surface heat flux into the ocean. The northward oceanic heat transports are inferred from the derived surface fluxes and estimates of ocean heat accumulation. The derived cross-equatorial oceanic heat transport of 0.50 PW is higher than most previous studies, and the derived mean meridional transport of 1.23 PW at 26° N is very close to 1.22 PW from RAPID observation. The surface flux contribution dominates the magnitude of the oceanic transport, but the integrated ocean heat storage controls the interannual variability. Poleward heat transport by the atmosphere at 30° N is found to increase after 2000 (0.17 PW decade−1). The multiannual mean (2006–2013) transport of energy by the atmosphere from ocean to land is estimated as 2.65 PW, and is closely related to the ENSO variability.

The strength of the Atlantic meridional overturning circulation (AMOC) at 26∘ N has now been continuously measured by the RAPID array over the period April 2004–September 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic multidecadal variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy forcing by the atmosphere drove intense water-mass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45∘ N, observations suggest that the AMOC may already be increasing. Examining 26∘ N, we find that the AMOC is no longer weakening, though the recent transport is not above the long-term mean. Extending the record backwards in time at 26∘ N with ocean reanalysis from GloSea5, the transport fluctuations at 26∘ N are consistent with a 0- to 2-year lag from those at 45∘ N, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering nor how the AMOC at 26∘ N responds to intense buoyancy forcing.

Human-induced atmospheric composition changes cause a radiative imbalance at the top-of-atmosphere which is driving global warming. This Earth Energy Imbalance (EEI) is a fundamental metric of climate change. Understanding the heat gain of the Earth system from this accumulated heat – and particularly how much and where the heat is distributed in the Earth system – is fundamental to understanding how this affects warming oceans, atmosphere and land, rising temperatures and sea level, and loss of grounded and floating ice, which are fundamental concerns for society. This study is a Global Climate Observing System (GCOS) concerted international effort to update the Earth heat inventory, and presents an updated international assessment of ocean warming estimates, and new and updated estimates of heat gain in the atmosphere, cryosphere and land over the period 1960–2018. The study obtains a consistent long-term Earth system heat gain over the past 58 years, with a total heat gain of 393 ± 40 ZJ, which is equivalent to a heating rate of 0.42 ± 0.04 W m−2. The majority of the heat gain (89 %) takes place in the global ocean (0–700 m: 53 %; 700–2000 m: 28 %; > 2000 m: 8 %), while it amounts to 6 % for the land heat gain, to 4 % available for the melting of grounded and floating ice, and to 1 % for atmospheric warming. These new estimates indicate a larger contribution of land and ice heat gain (10 % in total) compared to previous estimates (7 %). There is a regime shift of the Earth heat inventory over the past 2 decades, which appears to be predominantly driven by heat sequestration into the deeper layers of the global ocean, and a doubling of heat gain in the atmosphere. However, a major challenge is to reduce uncertainties in the Earth heat inventory, which can be best achieved through the maintenance of the current global climate observing system, its extension into areas of gaps in the sampling, as well as to establish an international framework for concerted multi-disciplinary research of the Earth heat inventory. Earth heat inventory is published at DKRZ (https://www.dkrz.de/) under the doi: https://doi.org/10.26050/WDCC/GCOS_EHI_EXP (von Schuckmann et al., 2020).

Major changes are occurring across the North Atlantic climate system, including in the atmosphere, ocean and cryosphere, and many observed changes are unprecedented in instrumental records. As the changes in the North Atlantic directly affect the climate and air quality of the surrounding continents, it is important to fully understand how and why the changes are taking place, not least to predict how the region will change in the future. To this end, this article characterizes the recent observed changes in the North Atlantic region, especially in the period 2005–2016, across many different aspects of the system including: atmospheric circulation; atmospheric composition; clouds and aerosols; ocean circulation and properties; and the cryosphere. Recent changes include: an increase in the speed of the North Atlantic jet stream in winter; a southward shift in the North Atlantic jet stream in summer, associated with a weakening summer North Atlantic Oscillation; increases in ozone and methane; increases in net absorbed radiation in the mid‐latitude western Atlantic, linked to an increase in the abundance of high level clouds and a reduction in low level clouds; cooling of sea surface temperatures in the North Atlantic subpolar gyre, concomitant with increases in the western subtropical gyre, and a decline in the Atlantic Ocean's overturning circulation; a decline in Atlantic sector Arctic sea ice and rapid melting of the Greenland Ice Sheet. There are many interactions between these changes, but these interactions are poorly understood. This article concludes by highlighting some of the key outstanding questions.

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo’s global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities., Sí

Observations made on 21 occupations between 1993 and 2016 of GO-SHIP line SR1b in eastern Drake Passage show an average temperature of 0.53°C deeper than 2000 dbar, with no significant trend, but substantial year-to-year variability (standard deviation 0.08°C). Using a neutral density framework to decompose the temperature variability into isopycnal displacement (heave) and isopycnal property change components shows that approximately 95% of the year-to-year variance in deep temperature is due to heave. Changes on isopycnals make a small contribution to year-to-year variability but contribute a significant trend of -1.4±0.6 m°C per year, largest for density (?n)?>?28.1, south of the Polar Front (PF). The heave component is depth-coherent and results from either vertical or horizontal motions of neutral density surfaces, which trend upward and northward around the PF, downward for the densest levels in the southern section, and downward and southward in the Subantarctic Front and Southern Antarctic Circumpolar Current Front (SACCF). A proxy for the locations of the Antarctic Circumpolar Current (ACC) fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content, explaining 76% of deep temperature variance. The same frontal position proxy based on satellite altimeter-derived surface velocities explains 73% of deep temperature variance. The position of the PF plays the strongest role in this relationship between ACC fronts and deep temperature variability in Drake Passage, although much of the temperature variability in the southern half of the section can be explained by the position of the SACCF. This article is protected by copyright. All rights reserved.

We construct a novel framework to investigate the uncertainties and biases associated with estimates of deep ocean temperature change from hydrographic sections, and demonstrate this framework in an eddy‐permitting ocean model. Biases in estimates from observations arise due to sparse spatial coverage (few sections in a basin), low frequency of occupations (typically 5‐10 years apart), mismatches between the time period of interest and span of occupations, and from seasonal biases relating to the practicalities of sampling during certain times of year. Between the years 1990 and 2010, the modeled global abyssal ocean biases are small, although regionally some biases (expressed as a heat flux into the 4000 ‐ 6000 m layer) can be up to 0.05 W m−2. In this model, biases in the heat flux into the deep 2000 ‐ 4000 m layer, due to either temporal or spatial sampling uncertainties, are typically much larger and can be over 0.1 W m−2 across an ocean. Overall, 82% of the warming trend deeper than 2000 m is captured by hydrographic section‐style sampling in the model. At 2000 m, only half the model global warming trend is obtained from observational‐style sampling, with large biases in the Atlantic, Southern and Indian Oceans. Biases due to different sources of uncertainty can have opposing signs and differ in relative importance both regionally and with depth, revealing the importance of reducing temporal and spatial uncertainties in future deep ocean observing design. Plain Language Summary In recent decades, deep (below 2000 m) ocean temperature trends have been measured when scientific research vessels repeat the same lines across an ocean basin. Repeats typically happen once or twice a decade and there are only a few repeated lines across each basin. The sparsity of data in both space and time will result in errors in the multidecadal temperature trends calculated from this data. Here, we use a state‐of‐the‐art ocean model to show how trends calculated from observational‐style sampling compare to trends calculated using all model data. For the period 1990‐2010, we estimate the error that may exist in observed deep ocean trend estimates. Overall, around 80% of the below 2000 m warming trend was captured by observational‐style sampling in the model, so deep ocean warming in recent decades may have been underestimated. However, our results are based on only one model simulation. The largest sources of sampling error are found in the Atlantic, Southern and Indian Oceans. For each basin, we reveal whether limited sampling in time or space contributes most error to the temperature trend estimate, and therefore in which regions temperature trend estimates would benefit from additional deep ocean sampling.

andhighlatitudes(12Sv,1Sv=10 6 m 3 s –1 ) and a cellinternal to the subpolar gyre(4Sv). The decadal MOC variability is associated with synchronized transport changes of the subtropical and subpolar inflow within the North Atlantic Current (NAC). The varying strength of the MOC is further related to changes in the upper horizontal transport distribution. When the MOC is in a strong phase (early 1990s), the northern branch of the NAC in the Iceland Basin is strong while the southern branch at the Rockall Trough entrance is relatively weak. The inverse situation holds for a persistent weak MOC state (1970s). Contrary to the conclusions of earlier studies, variability in the strength and shape of the subpolar gyre does not stand as the main driver of the changing NAC structure, which is largely induced by the horizontal variability of the subtropical inflow. Additionally, the recently shown intrusion of subtropical waters into the northeastern Atlantic (late 1960s, early 1980s, and 2000s) are shown to primarily occur during periods of weak MOC circulation at A25-Ovide.

Global and regional ocean warming deeper than 2000 m is investigated using 35 years of sustained repeat hydrographic survey data starting in 1981. The global long-term temperature trend below 2000 m, representing the time period 1991–2010, is equivalent to a mean heat flux of 0.065 ± 0.040 W m?2 applied over the Earth's surface area. The strongest warming rates are found in the abyssal layer (4000–6000 m), which contributes to one third of the total heat uptake with the largest contribution from the Southern and Pacific Oceans. A similar regional pattern is found in the deep layer (2000–4000 m), which explains the remaining two thirds of the total heat uptake yet with larger uncertainties. The global average warming rate did not change within uncertainties pre-2000 versus post-2000, whereas ocean average warming rates decreased in the Pacific and Indian Oceans and increased in the Atlantic and Southern Oceans.

Since the early twenty-first century, improvements in understanding climate variability resulted from the growth of the ocean observing system. The potential for a closure of the Earth’s energy budget has emerged with the unprecedented coverage of Argo profiling floats, which now provide a decade (2006–2015) of invaluable information on ocean heat content changes above 2000 m. The expertise gained from Argo and repeat hydrography sections motivated the extension of the array toward the ocean bottom, which will progressively reveal the poorly known deep ocean and reduce the uncertainty of its presumed 10–15 % contribution to the global ocean warming trend of 0.65–0.80 W m?2. The sustainability and synergy of various observing systems helped to corroborate numerical models and decipher the internal variability of distinct ocean basins. Due to unique observations of the circulation in the North Atlantic, particular attention is paid to heat content changes and their relationship to dynamic variability in that region.

The meridional overturning circulation (MOC) in the North Atlantic transports heat from the subtropics to high latitudes and hence plays an important role in the Earth’s climate. A region crucial for the MOC is the northern North Atlantic and the adjacent Nordic Seas, where waters transported northward in the MOC upper limb progressively cool, gain density and eventually sink. Here we discuss the variability of the gyre circulation, the MOC and heat flux as quantified from a joint analysis of hydrographic and velocity data from six repeats of the Greenland to Portugal OVIDE section (1997–2010), satellite altimetry and Argo float measurements. For each repeat of the OVIDE section, the full-depth absolute circulation and transports were assessed using an inverse model constrained by ship-mounted Acoustic Doppler Current Profiler data and by an overall mass balance. The obtained circulation patterns revealed remarkable transport changes in the whole water column and evidenced large variations (up to 50% of the lowest value) in the magnitude of the MOC computed in density coordinates (MOCσ). The extent and time scales of the MOCσ variability in 1993–2010 were then evaluated using a monthly MOCσ index built upon altimetry and Argo. The MOCσ index, validated by the good agreement with the estimates from repeat hydrographic surveys, shows a large variability of the MOCσ at OVIDE on monthly to decadal time scales. The intra-annual variability is dominated by the seasonal component with peak-to-peak amplitude of 4.3 Sv (1 Sv = 106 m3 s–1). On longer time scales, the MOCσ index varies from less than 15 Sv to about 25 Sv. It averages to 18.1 ± 1.4 Sv and shows an overall decline of 2.5 ± 1.4 Sv (95% confidence interval) between 1993 and 2010. The heat flux estimates from repeat hydrographic surveys, which vary between 0.29 and 0.70 ± 0.05 PW, indicate that the heat flux across the OVIDE section is linearly related to the MOCσ intensity (0.054Sv/PW). Regional terms: North Atlantic Ocean; Subpolar gyre; Irminger Sea; Iceland Basin; West European Basin; Iberian Abyssal Plain

International audience; The vertical structure of temperature trends in the northeastern Atlantic (NEA) is investigated using a blend of Argo and hydrography data. The representativeness of sparse hydrography sampling in the basin mean is assessed using a numerical model. Between 2003 and 2013, the NEA underwent a strong surface cooling (0–450 m) and a significant warming at intermediate and deep levels (1000 m to 3000 m) that followed a strong cooling trend observed between 1988 and 2003. During 2003–2013, gyre-specific changes are found in the upper 1000 m (warming and cooling of the subtropical and subpolar gyres, respectively), while the intermediate and deep warming primarily occurred in the subpolar gyre, with important contributions from isopycnal heave and water mass property changes. The full-depth temperature change requires a local downward heat flux of 0.53 ± 0.06 W m−2 through the sea surface, and its vertical distribution highlights the likely important role of the NEA in the recent global warming hiatus.

The origin and formation of the Subpolar Mode Water (SPMW) located over the Reykjanes Ridge in the North-Atlantic Ocean and the variability of its properties over the period 1966-2004 are investigated through the use of a global eddy-permitting (1/4 degrees) ocean/sea-ice model and a Lagrangian analysis tool. The SPMW is fed by subtropical and subpolar waters advected by the branches of the North-Atlantic Current. The SPMW acquires its properties when its source waters enter the winter mixed layer in the Iceland Basin. The SPMW temperature variability is mainly explained by variations of the relative contributions of the subtropical and subpolar water transports to the total transport. Compared to the 1966-2004 mean, lower (higher) subtropical water relative transport contribution leads to colder (warmer) SPMW in the early 1990s (in the late 1960s and late 1990s). The intensity of the winter convection in the Iceland basin also influences the SPMW temperature through the amount of relatively cold intermediate waters of subtropical origin integrated in the SPMW layer. Strong convection partly explains the cold SPMW of the early 1990s. The large increase in the SPMW temperature in the late 1990s is due to both a decrease in the winter convection and an increase in the relative transport of the subtropical waters.

In a sensitivity experiment, an eddy-permitting ocean general circulation model is forced with realistic fresh- water fluxes from the Greenland Ice Sheet, averaged for the period 1991-2000. The fluxes are obtained with a mass bal- ance model for the ice sheet, forced with the ERA-40 reanal- ysis dataset. The freshwater flux is distributed around Green- land as an additional term in prescribed runoff, representing seasonal melting of the ice sheet and a fixed year-round ice- berg calving flux, for 8.5 model years. By adding Green- land freshwater fluxes with realistic geographical distribution and seasonality, the experiment is designed to investigate the oceanic response to a sudden and spatially/temporally uni- form amplification of ice sheet melting and discharge, rather than localized or gradual changes in freshwater flux. The impacts on regional hydrography and circulation are investi- gated by comparing the sensitivity experiment to a control experiment, without additional fluxes. By the end of the sensitivity experiment, the majority of additional fresh wa- ter has accumulated in Baffin Bay, and only a small frac- tion has reached the interior of the Labrador Sea, where win- ter mixed layer depth is sensitive to small changes in salin- ity. As a consequence, the impact on large-scale circulation is very slight. An indirect impact of strong freshening off the west coast of Greenland is a small anti-cyclonic compo- nent to the circulation around Greenland, which opposes the wind-driven cyclonic circulation and reduces net southward flow through the Canadian Archipelago by 10%. Implica- tions for the post-2000 acceleration of Greenland mass loss are discussed.

The Meridional Overturning Circulation (MOC) of the North Atlantic ocean is a key component of the global climate system, through its role in redistributing heat, freshwater and chemical properties between low and high latitude regions. In mid-high latitude regions, the North Atlantic Current (NAC) forms the upper limb of the MOC. It flows northeastward at the subtropical/subpolar boundary, and splits into two main branches in the eastern subpolar gyre: a northern branch that recirculates within the subpolar region and a southern branch that feed the Nordic Seas. A realistic eddy-permitting simulation (ORCA025-G70, 1/4°) is combined with a Lagrangian analysis tool (ARIANE) to investigate the MOC variability (1965-2004) across the A25-Ovide line, which joins Greenland to Portugal. Two vertical overturning cells are identified: a subtropical cell connecting low and high latitudes (12 Sv) and a cell internal to the subpolar gyre (4 Sv). The decadal MOC variability is associated with synchronized transport changes of the subtropical and subpolar inflow within the NAC. The latter undergoes important horizontal restructuring with opposed transport changes of its northern and southern branches. Those horizontal transport changes are largely induced by the horizontal variability of the subtropical inflow. Changes in oceanic heat transport across A25-Ovide are largely responsible for the observed heat content changes in the eastern subpolar gyre (1965-2004). Heat transport variability at A25-Ovide results from an imbalance between opposed changes in its velocity and temperature components. Both temperature and velocity anomalies are partly reflected in large scale heaves of isopycnals, and potentially relate to the varying proportion of warm subtropical waters and cold subpolar waters advected within the northern NAC branch. A 2000's mean full-depth circulation computed along the merged AR7W/A25-Ovide line from repeated hydrographic profile and altimetry data indicates a minor contribution of the Labrador Sea to the basin wide mean MOC. However, the strength of the diapycnal overturning at AR7W has almost halved between the 1990’s and the 2000’s, confirming the importance of the region for the low-frequency MOC variability., La circulation méridienne de retournement (MOC) de l'Atlantique Nord est une composante clé du système climatique global, via son rôle dans la redistribution de chaleur, d'eau douce et de propriétés chimiques entre hautes et basses latitudes. Aux moyennes et hautes latitudes, le Courant Nord-Atlantique (NAC) forme la branche haute de la MOC. Il s'écoule vers le nord-est à la frontière des gyres subpolaire et subtropical, et se divise en deux branches principales dans l'Est du gyre subpolaire: une branche nord qui recircule vers l'ouest dans le gyre subpolaire et une branche sud qui alimente les mers Nordiques. Une simulation réaliste haute résolution (ORCA025-G70, 1/4°) est combinée à un outil d'analyse Lagrangienne pour étudier la variabilité de la MOC (1965-2004) à travers la section A25-Ovide qui joint le Portugal au Groenland. Deux cellules de retournement vertical sont identifiées: une cellule subtropicale connectant les hautes et basses latitude, et une cellule interne au régions subpolaire. La variabilité décennale de la MOC est associée à des changements synchronisés des apports subtropical et subpolaire dans le NAC. Ce dernier subit d'importantes restructurations horizontales caractérisées par la variabilité opposée de ses deux branches. Ces modifications de la distribution horizontal du transport sont principalement régies par la variabilité de l'afflux subtropical. Les variations du transport de chaleur à travers A25-Ovide sont la cause principale de la variabilité du contenu de chaleur observée dans l'est du gyre subpolaire (1965-2004). La variabilité du transport de chaleur résulte d'un déséquilibre entre des changements opposés de ses composantes “vitesse” et “température”. Les anomalies de vitesse et température sont en partie refletées dans des deplacements verticaux d'isopycnes, potentiellement associés à la proportion changeante de masses d'eau subtropicales et subpolaires transportées par la branche nord du NAC. Une circulation surface-fond moyenne calculée depuis des mesures hydrographiques répétées et des mesures altimétriques indique une contribution mineure de la mer du Labrador pour la MOC global. Cependant, l'intensité du retournement diapycnal à AR7W a presque diminué de moitié entre les 1990's et les 2000's, confirmant l'importance de la région pour la variabilité basse-fréquence de la MOC.

The early twenty-first century’s warming trend of the full-depth global ocean is calculated by combining the analysis of Argo (top 2000 m) and repeat hydrography into a blended full-depth observing system. The surface-to-bottom temperature change over the last decade of sustained observation is equivalent to a heat uptake of 0.71 ± 0.09 W m−2 applied over the surface of Earth, 90% of it being found above 2000-m depth. The authors decompose the temperature trend pointwise into changes in isopycnal depth (heave) and temperature changes along an isopycnal (spiciness) to describe the mechanisms controlling the variability. The heave component dominates the global heat content increase, with the largest trends found in the Southern Hemisphere’s extratropics (0–2000 m) highlighting a volumetric increase of subtropical mode waters. Significant heave-related warming is also found in the deep North Atlantic and Southern Oceans (2000–4000 m), reflecting a potential decrease in deep water mass renewal rates. The spiciness component shows its strongest contribution at intermediate levels (700–2000 m), with striking localized warming signals in regions of intense vertical mixing (North Atlantic and Southern Oceans). Finally, the agreement between the independent Argo and repeat hydrography temperature changes at 2000 m provides an overall good confidence in the blended heat content evaluation on global and ocean scales but also highlights basin-scale discrepancies between the two independent estimates. Those mismatches are largest in those basins with the largest heave signature (Southern Ocean) and reflect both the temporal and spatial sparseness of the hydrography sampling.

The Atlantic Meridional Overturning Circulation (AMOC) is an essential component of the climate system due to its vital role in the global distribution of heat, carbon, and water masses. The downwelling of North Atlantic surface waters connecting the upper and lower AMOC limbs is an essential yet vulnerable partof this global circulation. This downwelling partly occurs along continental boundaries. This dissertation presents an extended observational investigation on the quantification of Eulerianmean downwelling along the continental slopes of the North Atlantic subpolar gyre (SPG) and an examination of the underlying mechanisms, with an emphasis on the role of mesoscale eddies. A volume budget of the SPG boundary reveals a total Eulerian-mean (2002-2019) downwelling of -4.41±0.96 Sv at 1300 m depth between Denmark Strait and Flemish Cap, with the barotropic transport (BT) contributing 2.66±0.40 Sv and the baroclinic transport (BC) contributing 1.75±0.43 Sv. To investigate the processes that cause the BC boundary downwelling, i.e., the boundary heat loss and associate along-boundary density gradient, the long-term mean heat budget of the boundary current system is studied. Both lateral heat fluxes, driven by the boundary current/mesoscale eddies, and air-sea heat flux play significant roles in the boundary heat loss. In a Lagrangian framework, it is found that cross-shore eddy propagation generally cools the SPG boundary.; La circulation méridienne de retournement de l'Atlantique (AMOC) est une composante essentielle du système climatique en raison de son rôle vital dans la distribution globale de la chaleur, du carbone et des masses d'eau. La descente des eaux de surface de l'Atlantique Nord reliant les branches supérieure et inférieure de l'AMOC est une composante essentielle mais vulnérable de cette circulation globale. Le transport vertical associé se produit en partie le long des frontières continentales. Cette thèse présente une étude observationnelle étendue sur la quantification du « downwelling » moyen eulérien le long des pentes continentales du gyre subpolaire de l'Atlantique Nord (SPG) et un examen des mécanismes sous-jacents, en mettant l'accent sur le rôle des tourbillons de méso-échelle. Un bilan de volume révèle un « downwelling » total moyen (2002-2019) de 4.41±0.96 Sv à 1300 m de profondeur entre le détroit du Danemark et le Cap de Flemish, le transport barotrope (BT) contribuant pour 2.66±0.40 Sv et le transport barocline (BC) pour 1.84±0.44 Sv. Afin d'étudier les processus à l'origine de la plongée BC le long des bords du gyre SPG, i.e. la perte de chaleur et le gradient de densité associé, le bilan de chaleur moyen du courant de bords est étudié. A la fois les flux de chaleur latéraux, induits par les courants de bord et les tourbillons à méso-échelle, et les flux de chaleur air-mer jouent un rôle important dans la perte de chaleur du courant de bord. Dans un cadre lagrangien basé, on constate que la propagation des tourbillons contribue à refroidir la région de bord du gyre SPG.