A Transport Hiatus in the North Atlantic Current in Early 2014 Led to the Coincident Trans-Atlantic Heat and Salt Anomaly Dipoles of 2015

The North Atlantic Current (NAC) is a major source of heat toward the subpolar gyre and northern seas. However, its variability and drivers are not well understood. Here, we evaluated 8 years of continuous daily measurements as part of the international program Overturning in the Subpolar North Atlantic Program to investigate the NAC in the Iceland Basin. We found that the NAC volume and freshwater anomaly transport and heat content (HC) were highly variable with significant variability at timescales of 16–120 days to annual. Intraseasonal to short interannual variability was associated with mesoscale and intermittent mesoscale features abundant in the region. Composites analysis revealed that strong NAC periods were associated with less eddy kinetic energy in the Iceland Basin, which was consistent with the presence of frontal‐like structures instead of eddy‐like structures. On longer timescales, the westward migration of the eastern boundary of the subpolar North Atlantic (SPNA) gyre favors a stronger NAC volume transport and HC in the region. Stronger zonal wind stress triggers a fast response that piles water up between the SPNA and subtropical gyres, which increases the sea surface height gradient and drives the acceleration of the NAC. The strengthening of the NAC increases the heat and salt transport northward. During our study period, both heat and salt increased across the moorings. These observations are important for understanding the heat and freshwater variability in the SPNA, which ultimately impacts the Atlantic meridional overturning circulation.

North Atlantic sea surface temperature (SST) distributions derived from observations and a coupled model from NOAA's Geophysical Fluid Dynamics Laboratory, CM2.1, are compared to evaluate the model's ability to simulate recent (1900 to the present) oceanic surface characteristics. The North Atlantic focus will limit our analyses to spatial scales less than gyre, scales usually not addressed in previous model‐observation comparisons. Identifying model differences from observations at these scales will assist modelers in identifying problems to be considered and remedies to be applied. The properties compared are the mean annual SST, standard deviation, amplitude of the annual and semiannual harmonic, decadal meridional movements of the axis of the Gulf Stream, propagation of SST anomalies along the axis of the Gulf Stream, and 100‐year trends in SST records. Because of the dependence of SST on surface currents, observed flow from surface drifters and simulated flow from 15 m fields are also compared. The model simulates the large‐scale properties of all the variables compared. However, there are areas of differences in some variables that can be related to inadequacies in the simulated current fields. For example, the model Gulf Stream (GS) axis after separation from the western boundary is located some 100 km north of the observed axis, which contributes to an area of warmer simulated SSTs. The absence of a slope current in the same region that advects colder water from the Labrador Sea in the observations also contributes to this area of higher model SSTs. The model North Atlantic Current (NAC) is located to the east of the observed NAC contributing to a large area of SST discrepancy. The patterns of the amplitude of the annual harmonic are similar with maximum amplitude off the east coast of northern North America. The semiannual harmonic exhibits relatively large amplitudes (>1°C) north of about 55°N, a signal not found in the observations. In both the model and observations, a region of increased standard deviations encompasses the GS and NAC. The model simulates north‐south migrations of the GS core but at a longer period (20 years) than observed. The model does not simulate the SST anomalies that propagate along the observed GS and NAC. The model captures both the spatial and temporal characteristics of the Atlantic Multidecadal Oscillation. Both model and observations exhibit a dipole in trends, with positive trends in the subtropical Atlantic and negative trends in the subpolar gyre. The modeled region of negative trends is limited to the western subpolar Atlantic. The observed trends extend farther to the east.

The North Atlantic Current (NAC) is a major source of heat towards the subpolar gyre and northern seas. However, its variability and drivers are not well understood. Here, we evaluated 8 years of continuous daily measurements as part of the international programme Overturning in the Subpolar North Atlantic Program (OSNAP) to investigate the NAC in the Iceland Basin. We found that the NAC volume and freshwater anomaly transport and heat content were highly variable, with significant variability at time scales of 16-120 days to annual. The shorter time scales were associated with mesoscale features abundant in the region. Composites analysis revealed that strong NAC periods were associated with a westward migration of the eastern boundary of the subpolar North Atlantic (SPNA) gyre and less eddy kinetic energy in the Iceland Basin, which was consistent with the presence of frontal-like structures instead of eddy-like structures. Stronger zonal wind stress triggers a fast response that piles water up between the SPNA and subtropical gyres which increases the sea surface height gradient and drives the acceleration of the NAC. The strengthening of the NAC increases the heat and salt transport northward. During our study period, both heat and salt increased across the moorings. These observations are important for understanding the heat and freshwater variability in the SPNA, which ultimately impact the Atlantic Meridional Overturning Circulation.

The North Atlantic Current (NAC) supplies the subpolar gyre with warm and saline water from the subtropics as part of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). In the context of climate changes, the North Atlantic Ocean is one of the key regions to investigate the variability of the overturning circulation in which salinity and freshwater variability are playing a central role. Through the gravest empirical mode (GEM) method, we reconstructed salinity and velocity fields in the same spatiotemporal resolution as the sea surface height (SSH) product. The time series of salinity, freshwater, and volume transport are characterized by strong interannual variability related to most of the recently developed subpolar gyre indices. The variability of the freshwater transport in the study area is as high or even higher than the freshwater fluxes from the Arctic Ocean and thus needs to be considered for the impact of freshwater on the subpolar North Atlantic. Our analysis also revealed that the NAC import into the subpolar gyre is not only occurring in the western Atlantic close to the western boundary current. One o the NAC branches that forms at the Mann Eddy contributes about 15% of the volume transport and 28% of the freshwater transport crossing from the western into the eastern Atlantic north of 45°N. This branch has not been subject to the strong lateral mixing with freshwater at the western boundary, and thus, the salinity of this NAC pathway is higher than the one at the western boundary.

A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge (RR; core LO09-14; 59°12.30′N, 31°05.94′W) and the Faroe–Shetland Channel (FSC; core HM03-133-25; 60°06.55′N, 06°04.18′W) to document hydrographical changes of the North Atlantic Current (NAC) during the Holocene. Dinocyst and coccolith assemblages were analyzed, and quantitative reconstructions of sea surface temperatures (SSTs) and sea surface salinities (SSSs) were conducted based on dinocyst assemblages. Both proxies suggest a major reorganization of surface circulation patterns in the northeastern North Atlantic between 7 and 5.4 ka BP. At both sites, SSSs before 6.5–7 ka BP were lower than during the mid-late Holocene, suggesting dispersal of meltwater through the NAC. Long term trends of SSTs, however, show higher than present summer SSTs on the RR from 9.3 to ∼6 ka BP, and lower than present SSTs in the FSC until ca. 5.4 ka BP. The contrasted SST trends at the two sites suggest that decreasing summer insolation was not the only forcing behind hydrographical changes in the region. Decoupling of the NAC and the Slope Current (SC), which both influence the FSC, is proposed as a possible mechanism. We hypothesize that a strong NAC during the early to middle Holocene resulted in a SST increase on the RR and decrease in the FSC. Inversely, a weaker NAC after 5–6 ka BP, leading to decreased SSTs on the RR, would have enhanced the relative contribution of the warmer, saltier SC in the FSC, thus resulting in a regional SST and SSS increase.

The long-term North Atlantic Cold Anomaly (Cold Blob, CB) was largely defined by three major episodes of low sea surface temperature (SST) in the subpolar North Atlantic in 1972–1974, 1984–1985 and 1991–1994. Without these cold periods, there would have been no CB. Each of these episodes correlated with unusually low SST at the Flemish Cap (a subsurface island of the Canadian continental shelf) and with periods of high sea ice cover over the deep basin of the Labrador Sea a year earlier. These cold periods at the Flemish Cap and the CB were associated with the advance of sea ice and icebergs to the Flemish Cap, high iceberg counts off the coast of Newfoundland and the encroachment of icebergs on the path of the North Atlantic Current (NAC). Studies of SST anomalies in high iceberg years provided evidence for surface connections between the Flemish Cap and the CB utilizing part of the NAC pathway. We propose that in the cold periods, residual meltwater from sea ice and icebergs conveyed in the Labrador Current to the Flemish Cap was relayed via the NAC to the subpolar North Atlantic to form the CB. After 1995, anomalous ice expansion in the Labrador Sea basin greatly diminished, sea ice and icebergs did not reach the Flemish Cap and cold meltwater was no longer transmitted to the subpolar North Atlantic to sustain the CB. These observations make it difficult to see how the CB could be relevant to mooted changes in the Atlantic Meridional Overturning Circulation and associated impacts on regional climate in the twenty-first century.

The southwestern part of the subpolar North Atlantic east of the Grand Banks of Newfoundland and Flemish Cap is a crucial area for the Atlantic Meridional Overturning Circulation. Here the exchange between subpolar and subtropical gyre takes place, southward flowing cold and fresh water is replaced by northward flowing warm and salty water within the North Atlantic Current (NAC). As part of a long-term experiment, the circulation east of Flemish Cap has been studied by seven repeat hydrographic sections along 47 degrees N (2003-2011), a 2 year time series of current velocities at the continental slope (2009-2011), 19 years of sea surface height, and 47 years of output from an eddy resolving ocean circulation model. The structure of the flow field in the measurements and the model shows a deep reaching NAC with adjacent recirculation and two distinct cores of southward flow in the Deep Western Boundary Current (DWBC): one core above the continental slope with maximum velocities at mid-depth and the second farther east with bottom-intensified velocities. The western core of the DWBC is rather stable, while the offshore core shows high temporal variability that in the model is correlated with the NAC strength. About 30 Sv of deep water flow southward below a density of sigma=27.68 kg m(-3) in the DWBC. The NAC transports about 110 Sv northward, approximately 15 Sv originating from the DWBC, and 75 Sv recirculating locally east of the NAC, leaving 20 Sv to be supplied by the NAC from the south.

[ES] El estudio de los eventos climáticos abruptos y de los mecanismos implicados representan uno de los mayores retos a los que se enfrenta la investigación climática hoy en día. La identificación de los mecanismos de retroalimentación entre los sistemas océano- atmósfera-criosfera son de vital importancia para la comprensión de la variabilidad climática presente en registros sedimentarios del fondo oceánico. Entender cómo y por qué ocurrieron los cambios climáticos del pasado nos puede ayudar a predecir futuros escenarios relacionados con cambios climáticos. En esta Tesis se aportan nuevos datos y enfoques sobre los cambios climáticos y oceanográficos que se registraron entre aproximadamente 1,050,000 y 400,000 mil años en las latitudes altas del Atlántico Norte y que se relacionan con el clima y la circulación oceánica global.
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\nEl Atlántico Norte subpolar está considerado como una de las regiones climáticas más sensibles de la Tierra y es la llave fundamental para la compresión de la de la circulación de retorno (AMOC) en el Atlántico. Las variaciones de los parámetros superficiales influyen en la circulación y en la formación de masas de agua profundas, explicando por qué las variaciones entre periodos glaciares e interglaciares afectan a la formación de agua profunda, a cómo el calor se distribuye en el océano y a la distribución vertical de nutrientes que afectan a las comunidades planctónicas.
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\nEl intervalo de tiempo estudiado en esta Tesis Doctoral está caracterizado por la asimetría en los ciclos climáticos y contiene la llamada “transición del Pleistoceno medio”, entre los 1.1 y 0.6 Ma, caracterizados por una reorganización climática que se ha registrado en sedimentos marinos y continentales y se caracteriza por cambios a nivel global, entre los cuáles se incluyen cambios en temperatura del mar, nivel del mar y masas de hielo y afectó a la distribución y evolución de la vida en la Tierra, incluyendo a los ancestros de los seres humanos modernos.
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\nDurante la primera parte del intervalo de estudio, predominan los ciclos de ~100 ky que habían surgido entre los 950-900 ka mostrando un aumento progresivo en la intensidad de los periodos glaciales, produciéndose grandes acumulaciones de hielo en el hemisferio norte. Sin embargo, no es hasta aproximadamente los 400 ka (durante el periodo llamando “Evento Mid-Brunhes”) cuando los ciclos de ~100 ka adquieren su mayor amplitud, caracterizándose por periodos glaciales muy fríos e interglaciales muy cálidos. Dada la estrecha relación entre hidrosfera y criosfera, esta reconfiguración del volumen de hielo global tuvo gran repercusión en los patrones de circulación oceánica superficial y profunda a nivel global y muy especialmente en la región del Atlántico Norte. Establecer cuáles son los mecanismos de retroalimentación implicados en esta reconfiguración del sistema climático a escala orbital y suborbital representa uno de los grandes retos de la paleoclimatología moderna que se trataran en esta Tesis.
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\nEl trabajo presentado se basa en el análisis de una secuencia sedimentaria del sondeo marino profundo U1314, perforado en la formación Gardar Drift (sur de Islandia), durante la Expedición 306 del Integrated Ocean Drilling Program (IODP) en el Atlántico Norte subpolar. El material recuperado proporciona un registro de alta resolución del intervalo comprendido entre 1.050.000 y 400.000 años.

The Atlantic Meridional Overturning Circulation (AMOC) is important for the climate in Western Europe and models predict a decrease in the overturning circulation due to climate change. It is the fundamental mechanism for the transports of heat, freshwater, and dissolved gases. In the region east of the Grand Banks of Newfoundland and Flemish Cap, the Deep Western Boundary Current (DWBC) is part of the lower limb of the AMOC and brings cold and fresh water to the south, which is balanced by the North Atlantic Current (NAC), which brings warm and salty water to the north. In a long-term study we investigated the transport variability along a transect at 47°N that includes DWBC and NAC and lies in the transition zone between the subpolar and subtropical gyres, where decadal changes in the AMOC are transferred southward. The interactions between the DWBC and the NAC and its influence to the AMOC variability are subject to current research. We use 6 years of moored current meter observations between 2014 and 2020 within the DWBC as well as shipboard hydrographic and current meter measurements from 15 cruises between 2003 and 2020. The shipboard and mooring data were used to calculate volume transports. The shipboard data show that the DWBC consists of two cores, one in close proximity to the continental slope with maximum velocities at mid-depth and a bottom-intensified core further offshore. The correlation of both the hydrographic properties and absolute current measurements with sea surface height was used to reconstruct time series of geostrophic transport variability from satellite altimetry alone. The sea surface height and moored current meter volume transport time series are compared to estimate the reliability of the sea surface height time series. For the offshore core a higher correlation between the time series is found than for the slope core. To estimate the relation between the DWBC cores and the NAC, the correlation between their volume transport time series is examined. The two DWBC cores are not correlated, while a comparison with a NAC time series shows that the offshore core is significantly correlated with the NAC. A combination of these two DWBC time series was constructed to cover the entire DWBC. The entire DWBC is significantly anti-correlated with the NAC, which leads to larger volume transport of the NAC when the transport of the DWBC is smaller and vice versa. Overall, the sea surface height time series show no long-term trend in DWBC volume transport. When comparing the reconstructed monthly mean DWBC transports with a time series of AMOC variability at 47°N, a significant anti-correlation is found. This indicates that AMOC variability could be characterized to a large extent by the variability of the DWBC-NAC system.

In the subpolar North Atlantic, the strength of the Meridional Overturning Circulation is linked to rates of North Atlantic Deep Water formation, a water mass partially composed of Nordic Seas Overflow Waters. While Denmark Strait Overflow Water takes a relatively direct route out of the Irminger basin via the cyclonic boundary current, exit pathways of Iceland-Scotland Overflow Water (ISOW) from the Iceland Basin are less understood and more complex. Here, ISOW pathways and their interannual variability are explored in a Lagrangian framework using particles seeded within the 45-year 1/12° eddy-resolving North Atlantic HYCOM simulation. Our analysis reveals significant depth-dependent variability in ISOW pathways. Upper layers preferentially cross into the Irminger Basin through gaps in the Reykjanes Ridge while deeper layers take the more traditional route to the Charlie-Gibbs Fracture Zone (CGFZ). At the CGFZ, we observe a strong anticorrelation in the percentage of particles that end up in the western vs. eastern basin which varies on a timescale of ~2.5 years and is likely associated with the position of the North Atlantic Current (NAC). This anticorrelation however is much stronger in the upper layers as the influence of the NAC appears to decrease with depth. Of the approximately 55% of particles that translate through the CGFZ, those in the upper layers are more likely to follow the cyclonic boundary current while lower layer particles diffuse northwestward towards the Labrador Sea. These depth-dependent patterns, identified from simulated particle trajectories, are corroborated by observations from RAFOS floats deployed during the OSNAP campaign. These findings illustrate the importance of depth-dependent dynamics and interannual variability of the NAC in shaping ISOW pathways, with implications for deep circulation patterns in the subpolar North Atlantic and the rate of large-scale overturning.

The North Atlantic Current (NAC) is subject to variability on multiannual to decadal time scales, influencing the transport of volume, heat, and freshwater from the subtropical to the eastern subpolar North Atlantic (NA). Current observational time series are either too short or too episodic to study the processes involved. Here we compare the observed continuous NAC transport time series at the western flank of the Mid‐Atlantic Ridge (MAR) and repeat hydrographic measurements at the OVIDE line in the eastern Atlantic with the NAC transport and circulation in the high‐resolution (1/20°) ocean model configuration VIKING20 (1960–2008). The modeled baroclinic NAC transport relative to 3400 m (24.5 ± 7.1 Sv) at the MAR is only slightly lower than the observed baroclinic mean of 27.4 ± 4.7 Sv from 1993 to 2008, and extends further north by about 0.5°. In the eastern Atlantic, the western NAC (WNAC) carries the bulk of the transport in the model, while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern NAC (ENAC). The model is able to simulate the main features of the subpolar NA, providing confidence to use the model output to analyze the influence of the North Atlantic Oscillation (NAO). Model based velocity composites reveal an enhanced NAC transport across the MAR of up to 6.7 Sv during positive NAO phases. Most of that signal (5.4 Sv) is added to the ENAC transport, while the transport of the WNAC was independent of the NAO.

The northern North Atlantic Ocean mean circulation in the early 21st century

Expansion of fresh and sea‐ice loaded surface waters from the Arctic Ocean into the sub‐polar North Atlantic is suggested to modulate the northward heat transport within the North Atlantic Current (NAC). The Reykjanes Ridge south of Iceland is a suitable area to reconstruct changes in the mid‐ to late Holocene fresh and sea‐ice loaded surface water expansion, which is marked by the Subarctic Front (SAF). Here, shifts in the location of the SAF result from the interaction of freshwater expansion and inflow of warmer and saline (NAC) waters to the Ridge. Using planktic foraminiferal assemblage and concentration data from a marine sediment core on the eastern Reykjanes Ridge elucidates SAF location changes and thus, changes in the water‐mass composition (upper ˜200 m) during the last c. 5.8 ka BP. Our foraminifer data highlight a late Holocene shift (at c. 3.0 ka BP) in water‐mass composition at the Reykjanes Ridge, which reflects the occurrence of cooler and fresher surface waters when compared to the mid‐Holocene. We document two phases of SAF presence at the study site: from (i) c. 5.5 to 5.0 ka BP and (ii) c. 2.7 to 1.5 ka BP. Both phases are characterized by marked increases in the planktic foraminiferal concentration, which coincides with freshwater expansions and warm subsurface water conditions within the sub‐polar North Atlantic. We link the SAF changes, from c. 2.7 to 1.5 ka BP, to a strengthening of the East Greenland Current and a warming in the NAC, as identified by various studies underlying these two currents. From c. 1.5 ka BP onwards, we record a prominent subsurface cooling and continued occurrence of fresh and sea‐ice loaded surface waters at the study site. This implies that the SAF migrated to the southeast of our core site during the last millennium.

<p>The North Atlantic Subpolar Gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between different indices of SPG strength based on Sea Surface Height (SSH) observations have raised questions about the active role SPG strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing SSH-based and various other SPG-strength indices derived from observations and a global coupled model, we show that the interpretation of SPG strength-salinity relationship is dictated by the choice of the SPG index. Our results emphasize that SPG indices should be interpreted cautiously because they represent variability in different regions of the subpolar North Atlantic. Idealized Lagrangian trajectory experiments illustrate that zonal shifts of main current pathways in the ENA and meridional shifts of the North Atlantic Current (NAC) in the western intergyre region during strong and weak SPG circulation regimes are manifestations of variability in the size and strength of the SPG. Such shifts in advective pathways modulate the proportions of subpolar and subtropical water reaching the ENA, and thus impact salinity. Inconsistency among SPG indices arises due to the inability of some indices to capture the meridional shifts of the NAC in the western intergyre region. Overall, our results imply that salinity variability in the ENA is not exclusively sourced from the subtropics, instead the establishment of a dominant subpolar pathway also points to redistribution within the SPG.</p>

[1] The sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow is investigated for the first time using a high resolution eddy-permitting global coupled ocean-atmosphere model (GFDL CM2.5). The Nordic Sea overflow is perturbed through the change of the bathymetry in GFDL CM2.5. We analyze the Atlantic Meridional Overturning Circulation (AMOC) adjustment process and the downstream oceanic response to the perturbation. The results suggest that north of 34°N, AMOC changes induced by changes in the Nordic Sea overflow propagate on the slow tracer advection timescale, instead of the fast Kelvin wave timescale, resulting in a time lead of several years between subpolar and subtropical AMOC changes. The results also show that a stronger and deeper-penetrating Nordic Sea overflow leads to stronger and deeper AMOC, stronger northward ocean heat transport, reduced Labrador Sea deep convection, stronger cyclonic Northern Recirculation Gyre (NRG), westward shift of the North Atlantic Current (NAC) and southward shift of the Gulf Stream, warmer sea surface temperature (SST) east of Newfoundland and colder SST south of the Grand Banks, stronger and deeper NAC and Gulf Stream, and stronger oceanic eddy activities along the NAC and the Gulf Stream paths. A stronger/weaker Nordic Sea overflow also leads to a contracted/expanded subpolar gyre (SPG). This sensitivity study points to the important role of the Nordic Sea overflow in the large scale North Atlantic ocean circulation, and it is crucial for climate models to have a correct representation of the Nordic Sea overflow.