Model-observation and reanalyses comparison at key locations for heat transport to the Arctic: Assessment of key lower latitude influences on the Arctic and their simulation

Blue-Action Work Package 2 (WP2) focuses on lower latitude drivers of Arctic change, with a focus on the influence of the Atlantic Ocean and atmosphere on the Arctic. In particular, warm water travels from the Atlantic, across the Greenland-Scotland ridge, through the Norwegian Sea towards the Arctic. A large proportion of the heat transported northwards by the ocean is released to the atmosphere and carried eastward towards Europe by the prevailing westerly winds. This is an important contribution to northwestern Europe's mild climate. The remaining heat travels north into the Arctic. Variations in the amount of heat transported into the Arctic will influence the long term climate of the Northern Hemisphere. Here we assess how well the state of the art coupled climate models estimate this northwards transport of heat in the ocean, and how the atmospheric heat transport varies with changes in the ocean heat transport. We seek to improve the ocean monitoring systems that are in place by introducing measurements from ocean gliders, Argo floats and satellites. These state of the art computer simulations are evaluated by comparison with key trans-Atlantic observations. In addition to the coupled models ‘ocean-only’ evaluations are made. In general the coupled model simulations have too much heat going into the Arctic region and the transports have too much variability. The models generally reproduce the variability of the Atlantic Meridional Ocean Circulation (AMOC) well. All models in this study have a too strong southwards transport of freshwater at 26°N in the North Atlantic, but the divergence between 26°N and Bering Straits is generally reproduced really well in all the models. Altimetry from satellites have been used to reconstruct the ocean circulation 26°N in the Atlantic, over the Greenland Scotland Ridge and alongside ship based observations along the GO-SHIP OVIDE Section. Although it is still a challenge to estimate the ocean circulation at 26°N without using the RAPID 26°N array, satellites can be used to reconstruct the longer term ocean signal. The OSNAP project measures the oceanic transport of heat across a section which stretches from Canada to the UK, via Greenland. The project has used ocean gliders to great success to measure the transport on the eastern side of the array. Every 10 days up to 4000 Argo floats measure temperature and salinity in the top 2000m of the ocean, away from ocean boundaries, and report back the measurements via satellite. These data are employed at 26°N in the Atlantic to enable the calculation of the heat and freshwater transports. As explained above, both ocean and atmosphere carry vast amounts of heat poleward in the Atlantic. In the long term average the Atlantic ocean releases large amounts of heat to the atmosphere between the subtropical and subpolar regions, heat which is then carried by the atmosphere to western Europe and the Arctic. On shorter timescales, interannual to decadal, the amounts of heat carried by ocean and atmosphere vary considerably. An important question is whether the total amount of heat transported, atmosphere plus ocean, remains roughly constant, whether significant amounts of heat are gained or lost from space and how the relative amount transported by the atmosphere and ocean change with time. This is an important distinction because the same amount of anomalous heat transport will have very different effects depending on whether it is transported by ocean or the atmosphere. For example the effects on Arctic sea ice will depend very much on whether the surface of the ice experiences anomalous warming by the atmosphere versus the base of the ice experiencing anomalous warming from the ocean. In Blue-Action we investigated the relationship between atmospheric and oceanic heat transports at key locations corresponding to the positions of observational arrays (RAPID at 26°N, OSNAP at ~55N, and the Denmark Strait, Iceland-Scotland Ridge and Davis Strait at ~67N) in a number of cutting edge high resolution coupled ocean-atmosphere simulations. We split the analysis into two different timescales, interannual to decadal (1-10 years) and multidecadal (greater than 10 years). In the 1-10 year case, the relationship between ocean and atmosphere transports is complex, but a robust result is that although there is little local correlation between oceanic and atmospheric heat transports, Correlations do occur at different latitudes. Thus increased oceanic heat transport at 26°N is accompanied by reduced heat transport at ~50N and a longitudinal shift in the location of atmospheric flow of heat into the Arctic. Conversely, on longer timescales, there appears to be a much stronger local compensation between oceanic and atmospheric heat transport i.e. Bjerknes compensation

Circulation at the western boundary of the South and Equatorial Atlantic: Exchanges with the ocean interior.

International audienceData from a hydrographic section carried out in January-March 1994 offshore from the eastern coast of South America from 50S to 10N, are used to quantify the full-depth exchanges of water between the western boundary currents and the ocean interior. In the upper and intermediate layers, the westward transport associated with the southern branch of the South Equatorial Current was 49 Sv at the time of the cruise. The transports of the central and northern branches in the upper 200 m were 17 Sv and 12 Sv, respectively. After subtraction of the parts that recirculate in the subtropical, subequatorial, and equatorial domains, the fraction of the South Equatorial Current that effectively contributes to the warm water export to the North Atlantic is estimated at 18 Sv. The poleward boundary of the current southern branch is at 31S through the whole thickness of the subtropical gyre, but the latitude of the northern boundary varies from 7°30primeS at the surface to 27S at 1400 m depth. The estimated latitude of its bifurcation into the Brazil Current and North Brazil Undercurrent also varies downward from about 14S at the surface to 28S at a depth of 600 m. In the North Atlantic Deep Water, eastward flows exceeding 10 Sv are observed at 3°-4° of latitude in both hemispheres, at 10S, and at 34S-30S. Between 4S and 17S, a net westward flow with an estimated transport of 19 Sv reinforces the southward deep western boundary current. Cyclonic circulations of Antarctic Bottom Water along the western boundaries of the Argentine and Brazil basins have amplitudes of 15 Sv and 13 Sv, respectively, exceeding those of the interbasin exchanges. The net alongshore transport of this water mass between the hydrographic section and the continental slope reverses to a southward direction from 13S to 27S, probably in relation with an eastward shift of the equatorward near-bottom boundary current at these latitudes

Upper-layer circulation in the eastern Equatorial and South Atlantic Ocean in January–March 1995.

International audienceThe upper-layer circulation in the eastern basin of the South Atlantic was studied from hydrographic and direct velocity measurements along WOCE lines A11, A13 and A14. A13 and A14 provide quasi-meridional samplings of the equatorial, subequatorial and subtropical circulation regimes. A13 was carried out along the African coast at about 600 km offshore from it, and A14 along the nominal longitude 9°W. A11 intersects the Cape Basin between 46°S in the west and 30°S in the east. Transport estimates were derived from direct velocity measurements and a box inverse model. In the equatorial eastern Atlantic, the Equatorial Undercurrent (EUC) transport decreases from 25×106 m3 s−1 at 9°W to 13×106 m3 s−1 at 5°E. Re-circulations of the EUC into the northern and equatorial branches of the South Equatorial Current (SEC) are evidenced at 5°E and quantified. In the tropical Atlantic, we estimate 7.5×106 and 4.2×106 m3 s−1 for the transports at 9°W of the South Equatorial Undercurrent (SEUC) and South Equatorial Countercurrent (SECC), respectively. Both the SEUC and SECC extend vertically down to intermediate depths and contribute to the northern limb of the Angola Gyre. The Angola Current transport is estimated to be 16±5×106 m3 s−1 for σ1<32.1 at 13°S. South of the Angola Gyre, the transports show an apparent cyclonic circulation, developed mostly at the intermediate level. The water mass properties suggest that it is, at least partially, a re-circulation of the Benguela Current. Further south and for the subtropical gyre, we estimate 10±5×106 m3 s−1 for the transport of the South Atlantic Current across 9°W and 28±4×106 m3 s−1 for the transport of the Benguela Current at 10°E for σ1<32.1

POMME. Reconstruction des champs 4D. Analyse basée sur les profils et mesures eulériennes - Expériences 7 et 9

We present here the synthesis of in-situ observations and satellite data analyzed with Soprane, as obtained with the Kalman filter.Ce document présente une synthèse combinant différents types de mesures in situ avec les analyses de données altimétriques faites par SOPRANE. Le modèle diagnostique avec filtre de kalman (KA_meso_V1.5, Kermabon et al, 2003), reconstruit les champs de température, salinité et courants sur un pavé d’océan. Nous présentons deux expériences, basées sur le même jeu de données. L’expérience 7 utilise les modes verticaux définis pour l’expérience de référence (Gaillard et al. 2002). L’expérience 9 utilise un jeu de modes plus adapté à la zone POMME et prend en compte des contraintes supplémentaires, introduites sous la forme de pseudo-données

A complex North Atlantic permanent pycnocline revealed by Argo data

In the North Atlantic subtropical gyre, the oceanic vertical structure of density is characterized by a region of rapid increase with depth. This layer is called the permanent pycnocline. The pycnocline is the transition layer between light, low-latitude, surface water masses which are ventilated every winter when penetrated locally by the mixed layer and dense, deeper water masses whose properties are set in the high latitudes. Assessing the structure and variability of the permanent pycnocline is of a major interest in the under- standing of the climate system because the pycnocline embeds the warm water sphere and most of the wind-forced horizontal circulation. We characterized the large scale structure of the permanent pycnocline with in-situ data from the Argo ar- ray. We developed a new method to objectively characterize its properties (depth, thickness, temperature, salinity, density, potential vorticity). Results reveal a surprisingly complex structure with inhomogeneous properties. In the Gulf Stream recirculation region the pycnocline is deep, thick, the maximum of stratification is found in the middle on the layer and follow an isopycnal surface. But away from this textbook regional description, the pycnocline is characterized by vertical asymmetries and gradients in thermohaline properties. T/S distribution along the permanent pycnocline depth reveals a diversity of water masses. We will present the mean observed structure and properties of the permanent pycnocline and relate them to physical processes that constraint them

RREX 2015. S-ADCP data processing report

The Reykjanes Ridge is a major topographic feature of the North-Atlantic Ocean. It lies in a central position along the main paths followed by the upper and lower limbs of the Meridional Overturning Cell (MOC), which contributes at moderating the European climate by transporting heat northward. The objective of the RREX project was to conduct a process study in order to better understand the role of the Reykjanes Ridge on the dynamics and water mass transformation in the subpolar gyre and ultimately on the MOC. The RREX2015 cruise, carried out from 5 June to 10 July 2015 on the R/V Thalassa, was the first of the two cruises carried by the RREX project. During this cruise, we realized 133 surface-bottom CTDO2-LADCP stations along 4 sections. Current measurements were continuously acquired by two Shipboard ADCPs (RD Instrument) operating at 38 kHz (OS38) and at 150 kHz (OS150). This report details the processing of those two S-ADCPs datasets using the software Cascade. The processing consisted in validating, correcting, filling gaps in, filtering, and selecting final S-ADCP data. Considering the mean vertical velocity averaged over the cruise, we estimated an attitude corrections of 0.3° for OS38 and 0.1° for OS150. We also estimated the misalignment (α) and amplitude (a) corrections in comparing the ship velocity, determined by GPS, to the ship velocity estimated from the S-ADCP bottom ping in shallow water. Minimizing the bias between OS38 and OS150 further refined the misalignement correction. For the OS38, we found α = 0.05° and a = 1.0067 cm s−1. For the OS150, we found α = -0.04° and a = 1.0027 cm s−1. We also estimated the total instrumental error on the absolute ocean velocity calculated from errors on the flow velocity relative to the ship velocity estimated by the ADCP data, and errors on the ship velocity relative to the bottom measured by GPS. For OS38 in Narrow Band mode, the total instrumental error on absolute ocean velocity during the cruise is 4.39 cm s−1. For OS150 in Broad Band mode, the total instrumental error during the cruise is 2.40 cm s-1.La dorsale de Reykjanes est une structure topographique majeure de l'océan Atlantique Nord. Elle est située au cœur de la gyre subpolaire le long des chemins suivis par les branches hautes et basses de la cellule méridienne de retournement (MOC, Meridional Overturning Cell). Cette dernière transporte de la chaleur vers le nord de l’Atlantique Nord et contribue à modérer le climat européen. L'objectif du projet RREX est de réaliser une étude de processus afin de mieux comprendre le rôle de la dorsale de Reykjanes sur la dynamique et la transformation des masses d'eau dans le gyre subpolaire et, in fine, sur la MOC. La campagne RREX2015 réalisée du 5 juin au 10 juillet 2015 est la première des 2 campagnes prévues du projet RREX. Au cours de cette campagne, nous avons réalisé 133 stations CTDO2-LADCP le long de 4 radiales. Des données de courant ont été continuellement acquises par deux ADCPs: un S-ADCP opérant à 38 kHz appelé OS38 et un S-ADCP opérant à 150 kHz appelé OS150 (RD Instrument). Ce rapport détaille le traitement des données des ces deux S-ADCPs à l'aide du logiciel Cascade. Le traitement consistait à valider, corriger, interpoler, filtrer et sélectionner les données S-ADCP finales. Considérant la vitesse verticale moyennée sur toute la campagne, nous avons estimé une correction d'attitude de 0,3 ° pour l’OS38 et de 0,1 ° pour l’OS150. Nous avons également estimé les corrections de désalignement (α) et d'amplitude (a) en comparant la vitesse du navire, déterminée par GPS, à la vitesse du navire estimée à partir des données S-ADCP acquises en eaux peu profondes et en minimisant le biais entre l’OS38 et l’OS150. Pour l'OS38, nous avons trouvé α = 0,05 ° et a = 1,0067 cm s-1. Pour l'OS150, nous avons trouvé α = -0,04 ° et a = 1,0027 cm s-1. Nous avons également estimé l'erreur instrumentale totale sur la vitesse océanique absolue calculée à partir des erreurs sur la vitesse de l’écoulement relative à la vitesse du bateau estimée par les données ADCP, et sur la vitesse du navire par rapport au fond mesurée par GPS. Pour l’OS38 en mode Narrow Band, l'erreur instrumentale totale sur la vitesse absolue pendant la campagne est de 4,39 cm s-1. Pour l’OS150 en mode Broad Band, l'erreur instrumentale totale pendant la campagne est de 2,40 cm s-1

POMME - Programme Océan Multidisciplinaire Méso Echelle. CAMPAGNE POMME T0. Rapport de Données ADCP (Volume 2)

One of the key processes of the ocean circulation and large scale air-sea interaction is water mass formation. In the north Atlantic, the upper ocean circulation brings warm waters to higher latitudes, where vigorous cooling leads, during the winter, to the formation of deep mixed layers (up to several hundred meters deep). As these newly formed water masses are advected and spread southward they are overlaid during the spring and summer by warmer water. Thus they lose contact with the atmosphere and become part of ocean circulation as intermediate water. This succession of events constitute subduction. The newly formed water mass is typically of low potential vorticity (nearly homogeneous), with characteristics imparted by the local prevailing air sea fluxes during the cooling phase. The program POMME is a major experiment (several cruises, various observational techniques, modelling) whose aims include the understanding of the role of air sea fluxes and mesoscale processes in the formation, subduction and spreading of the subpolar mode water in the north east Atlantic. During the cruise POMME0, several fixed points moorings have been set to observe the current, temperature and salinity structure, with currentmeters, T/S sensors and acoustic tomography. A surface meteorological mooring has also been set at the centre of the study area. A network of hydrographic stations has been covered to observe the physical and biological properties at the initial conditions for the studies to be conducted during the subsequent cruises. The report (volume 1) shows the temperature, salinity and current serials obtained on the 5 moorings deployed for 13 months. This report (volume 2) shows the ADCP measurements.Un des éléments essentiels de la circulation de l’océan et de ses interactions avec l’atmosphère est la formation d’eaux modales. Dans l’Atlantique Nord, les courants chauds se dirigent vers le Nord (dérive Nord-Atlantique) cèdent leur chaleur à l’atmosphère et se refroidissent vigoureusement. Il en résulte, l’hiver, la formation de couches superficielles homogènes, dites «couches de mélange», qui peuvent atteindre plusieurs centaines de mètres. Ces masses d’eau étant entraînées par le courant vers le Sud se retrouvent enfouies au cours du printemps et de l’été sous des eaux plus chaudes; elles perdent ainsi contact avec l’atmosphère : c’est le processus de subduction et de formation de masses d’eau, dont les caractéristiques en température et en salinité dépendent des échanges air-mer au moment de leur formation. L’objectif global de POMME (Programme Océan Multidisciplinaire Méso Echelle) est de comprendre le rôle de la méso-échelle sur le processus de la subduction des eaux modales et de la floraison printanière, ainsi que de déterminer les processus régulant les caractéristiques physiques et biochimiques des masses d’eau modales et le devenir de la matière biogène. L’intégration forte entre les mesures dynamiques et biogéochimiques constitue un des points forts du projet. POMME est un programme national soutenu par plusieurs organismes (IFREMER, CNRS, SHOM, Météo) et impliquant de nombreux laboratoires francais et étrangers. L’objectif de la composante physique du programme POMME est d’étudier, de quantifier et de modéliser le rôle des flux air-mer et des phénomènes de moyenne échelle dans la formation des couches de mélange et dans leur épanchement (advection) ultérieur à des profondeurs intermédiaires. POMME implique plusieurs campagnes et diverses techniques d’observation. La campagne POMME0 était la première d’une série. Elle a pour objet la mise en place de mouillages de courantométrie, de tomographie, d’émetteurs acoustiques et d’observations météorologiques ainsi que différents types de flotteurs autonomes qui constituent les mesures physiques du programme. Des mesures d’hydrologie et de biologie ont également été réalisées pour évaluer les conditions initiales dans la zone concernée. Le précédent rapport (volume 1) présente les séries de température, salinité et de courant obtenues sur les 5 mouillages de courantométrie qui ont été maintenus sur zone pendant une durée de 13 mois. Ce rapport (volume 2) présente les mesures de courant issues des ADCP (Acoustic Doppler Current Profiler)

Ekman transport as the driver of extreme interannual formation rates of Eighteen Degree Water

In the North Atlantic subtropical gyre, the Eighteen Degree Water (EDW) is a voluminous heat reservoir, submerged under a seasonal pycnocline that can be progressively removed through the winter, allowing EDW ventilation in the early spring. We target the EDW formation extremes, namely 2004-2005, 2009-2010, and 2012-2013 for the strong years, and 2007-2008, 2008-2009, 2011-2012, and 2013-2014 for the weak years. We employ gridded hydrographic datasets mainly measured by Argo floats over the last 20 years, and provide a synthetic study on the extreme events of strong and weak EDW formation of this time period. We found that the Ekman transport is the indicator and driving mechanism explaining these extremes. Strong (Weak) EDW formation years correspond with atmospheric patterns resembling NAO- (NAO+), attributed to a strong (weak) winter air-sea surface heat loss, and a strong (weak) winter heat loss due to Ekman transport. Further, we show that such extreme Ekman advection patterns can be linked to mid-latitude storms, of which both intensity and duration have an impact on the extreme of EDW ventilation in the western subtropical North Atlantic. To yield a strong EDW formation, it requires a large winter heat deficit due to Ekman divergence, which can be sufficiently represented by numbers of strong winter storms, most notably, remnants of hurricanes and US east coast snowstorms. Meanwhile, to yield a weak EDW formation, apart from weak atmospheric forcings, a remnant positive heat content anomaly carried through from previous years would serve as an unfavorable preconditioning, hindering the EDW formation. Plain Language Summary The EDW is the most voluminous water body in the North Atlantic subtropical region. It is critical in the biology cycle and the ocean dynamics. For most of the year, EDW is buried underneath the sea surface. In winter, when sea surface loses enough heat, sinking cold water reaches the EDW bulk, forming fresh EDW. In this research, we target the EDW formation extremes, namely 2004-2005, 2009-2010, and 2012-2013 for the strong years, and 2007-2008, 2008-2009, 2011-2012, and 2013-2014 for the weak years. Using modern observational datasets, we found that the remnant hurricanes and US east coast snowstorms have an impact on the extreme interannual formation rate of EDW. To have a strong EDW formation, it is sufficient to have several strong winter storms passing by the EDW formation region, where the ocean loses more heat to the atmosphere than average over the winter. These winter-long sustained forcings have a cumulative effect on the ocean, and promote strong EDW formation. Conversely, when fewer winter storms pass, the ocean loses less heat to the atmosphere, promoting weak EDW formation. Meanwhile, the extra heat carried through from the previous years can also result in a weak EDW formation

Subtropical Mode Waters and Permanent Pycnocline properties in the World Ocean

A global reference state of the subtropical mode waters and permanent pycnoclines properties for the 2000‐2015 period is presented. The climatology is obtained from a pattern recognition algorithm applied to stratification profiles from the Argo global array. The stratification features are identified as permanent upper ocean pycnostad and pycnocline even when the seasonal pycnocline is developed. The climatology shows that both Northern Hemisphere subtropical gyres have a qualitatively very similar stratification structure. The permanent pycnocline in the North Atlantic and North Pacific show two deep centers colocated with thick subtropical and subpolar mode waters. These centers coincide with modes in the density and stratification space. These deep pycnocline centers are separated by a region with a shallower and thinner permanent pycnocline that is located downstream of Western Boundary Current Extensions and upstream of Eastern Subtropical Fronts. This feature creates a remarkable double‐bowl pattern at the basin scale. In the subtropical gyres of the South Atlantic and South Pacific Oceans, the mode water and permanent pycnocline structures are characterized by two modes in the density and stratification space that, unlike in the Northern Hemisphere, do not necessarily correspond to deep and thick permanent pycnocline regions. In the subtropical gyre of the South Indian Ocean, a single mode is found to correspond to a single center in the western part of the gyre. This study also shows that away from these deep centers where the pycnocline depth almost follows isopycnals, the permanent pycnocline experiences significant thermohaline gradients that are not density compensated. Plain Language Summary The vertical structure of the ocean is of great importance to understand the impact of climate changes on the global ocean because a larger density differences (i.e. increased stratification) between the upper and the deeper ocean can prevent anthropogenic excess of heat, and carbon, from reaching the abyssal ocean. Using data from autonomous Argo floats that sample the ocean properties (e.g. temperature and salinity) from the surface down to 2000m, we describe the vertical structure of the ocean's surface layers at mid‐latitudes, where heat is mostly stored in the upper 1000m. This new study shows for the first time: (i) a strong dependence of the ocean properties of the surface layer with the ocean properties of the transition layer with the abyss and (ii) that this transition layer exhibits rapid changes in temperature and density when rising to the surface as well as in the center of the each oceans. These new results are a significant refinement to the classic depiction of the mid‐latitude ocean as a simple bowl of warm water separated from the abyss by a layer of constant density and thus provide an accurate benchmark for climate models to detect long‐term changes in the ocean vertical structure