Geostrophic Velocity Research Articles

The Fiordland Current, southwest New Zealand: mean, variability, and trends

ABSTRACT The mean, interannual variability, decadal variability, and longer-term trends in the across-track geostrophic velocity of the poleward-flowing boundary current along the Fiordland coast (the Fiordland Current, FC) are examined over the altimeter record (September 1992–May 2017). In the mean, the FC is southwestward, with stronger velocities in an inner current near the coast and weaker velocities in an outer current further offshore. Examination of the alongshore momentum balance suggests that the mean flow is driven by a poleward downsloping alongshore pressure gradient. At interannual time-scales, variability in the FC was correlated with the alongshore wind stress, indicating that an equatorward (poleward) wind stress drives an equatorward (poleward) velocity anomaly. At decadal time-scales, variability in the FC was correlated with the South Pacific wind stress curl, indicating that as the wind stress curl increases (decreases) the poleward flow in the FC weakens (strengthens), consistent with the response of a traditional eastern boundary current. Long-term trends show a weakening outer current and strengthening inner current. Increasing flow in the inner current may result in increased transport of subtropical water around southern New Zealand.

The Use of a Jet Reference Frame to Analyze Drifter Trajectories in the Agulhas Current

AbstractStudies based on drifters that are deployed using fixed geographical locations can alias the variability in the Agulhas Current. Numerical model simulations have shown that tracking particles using jet coordinate systems will improve our understanding of the variability in western boundary currents. In this study we use in situ observations to show the potential of quasi‐Lagrangian measurements with an investigation into the relationship of the upstream surface velocity configuration and the trajectories surface drifters follow. Additionally, we use these drifters, along with ship‐based measurements, to expose biases in satellite‐derived geostrophic velocities in the Agulhas Current. Between September 1992 and October 2017, 49 surface drifters crossed the altimeter track #096 in a nonmeandering state. Of the 49 surface drifters, 16 crossed inshore of the surface velocity maxima, 3 of which leaked into the South Atlantic Ocean. Biases between altimetry‐derived geostrophic velocities and absolute velocities from ship acoustic Doppler current profiler and drifters measurements have pointed toward surface drifters leaking from inshore of the Agulhas Current core in a region of high shear. However, the bias between these velocities is inconsistent, with the highest range in bias found inshore of the Agulhas Current core. Due to the lack of data in the Agulhas Current and various sources of error, much work remains to be done and results presented here may provide motivation for further targeted drifter deployments in the future.

Propagation of the Azov Sea waters in the Black sea under impact of variable winds, geostrophic currents and exchange in the Kerch Strait

Satellite and hydrological data were used to study the propagation of the Azov waters (Azov plume - AP) in the Black Sea, its relation to wind forcing, eddy dynamics, and water balance in the Azov and Black Seas. Strong north winds increase the inflow of the Azov waters in the basin and their further transport to the west along the Crimean coast. AP usually extends from the Kerch Strait to the west up to cape Chauda and Feodosiya Bay over the shallow areas less than 30 m deep. In winter-early spring, when northeast storms and the Rim Current are the most intense, they penetrate further up to cape Meganom or even to the southwestern Crimean coast. Near the capes, AP may detach from the coast and get trapped in the mesoscale anticyclones formed beyond the capes. Salinity gradients between the AP and the Black Sea waters induce the formation of frontal geostrophic currents, reaching 10 cm/s, which can constitute half of the total geostrophic velocity in this part of the basin. During the strong south winds, AP is transported both to the east and west of the Kerch Strait, or the inflow of the Azov waters is completely blocked. These storms are the reason of the highest turbidity in the vicinity of the Kerch Strait, which is related to the wave-induced coastal erosion and resuspension of bottom sediments over the shallow Kerch-Taman shelf.Altimetry and in-situ data on sea level difference in the two basins were used to study the variability of the barotropic transport near the Kerch Strait, and its dependence on the water balance in the two basins, wind direction and magnitude. The greatest difference between mean sea level in the Azov Sea and the Black Sea in winter increases the inflow of the Azov waters in cold season. Rise of the dynamic sea level over the coastal part of the Black Sea in winter caused by the intensification of cyclonic circulation partly compensates this difference and decreases the exchange. North, and northwest winds with magnitude 12–16 m/s increase the barotropic velocity in the strait by 30–40 cm/s, while south, and southwest winds with the same magnitude decrease it by 20–30 cm/s. Turbidity near the strait that is an indicator of AP has significant interannual variability. In 2003–2015 it declines over the whole Kerch-Taman shelf, which is related to the decrease of the inflow of the Azov waters into the Black Sea in this period.

The exact geostrophic streamfunction for neutral surfaces

McDougall (1989) proved that neutral surfaces possess an exact geostrophic streamfunction, but its form has remained unknown. The exact geostrophic streamfunction for neutral surfaces is derived here. It involves a path integral of the specific volume along a neutral trajectory. On a neutral surface, the specific volume is a multivalued function of the pressure on the surface, p∼. By decomposing the neutral surface into regions where the specific volume is a single-valued function of p∼, the path integral is simply a sum of integrals of these single-valued functions. The regions are determined by the Reeb graph of p∼, and the neutral trajectory is encoded by a walk on this graph. Islands, and other holes in the neutral surface, can create cycles in the Reeb graph, causing the exact geostrophic streamfunction on a neutral surface to be multivalued. Moreover, neutral surfaces are ill-defined in the real ocean. Hence, the topobaric geostrophic streamfunction is presented: a single-valued approximation of the exact geostrophic streamfunction for neutral surfaces, for use on any well-defined, approximately neutral surface. Numerical tests on several approximately neutral surfaces reveal that the topobaric geostrophic streamfunction estimates the geostrophic velocity with an error that is about an order of magnitude smaller than that for any previously known geostrophic streamfunction. Also, the Montgomery potential is generalized, leading to an alternate form of the exact geostrophic streamfunction for neutral surfaces. This form is used to construct the orthobaric Montgomery potential, an easily computable geostrophic streamfunction that estimates the geostrophic velocity more accurately than any previously known geostrophic streamfunction, but less so than the topobaric geostrophic streamfunction.

Extracting Lagrangian coherent structures in the Kuroshio current system

In this paper, we propose to extract Lagrangian coherent structures (LCSs) based on MADT data to describe the structure of the Kuroshio Current System, and this method helps study the monthly variation in the intensity. By exploiting the improved variational principles, we extract LCSs through a geostrophic velocity vector field derived from the absolute dynamic topography. Specifically, we address hyperbolic LCSs and parabolic LCSs, with the former as the outer bounds of the jet flow and the latter as a more robust depiction of the jet axis. These LCSs delineate the jet with theoretical underpinnings, as opposed to common contours identified in an anecdotal manner. We employ the top 20% most repelling and attracting hyperbolic LCSs to approximate the outer bounds of the Kuroshio Current System, and these structures are apt to enclose the jet. Compared with the results of FTLEs and FSLEs, our results are more reliable. A comparison in the intensity based on the jet axis of months in 2012 demonstrates the effectiveness of our method in extracting LCSs. In situ data, such as drifter tracks and temperature-salinity profiles, are applied to validate the reliability of the axis and outer bounds extracted from the Kuroshio.

Horizontal circulation and volume transports in the Weddell Gyre derived from Argo float data

Argo floats provide both hydrographic and trajectory data, affording the opportunity to investigate surface to mid-depth ocean dynamics. Here, Argo float data are used to determine the absolute geostrophic velocity field of the upper 50–2000 m of the Weddell Gyre, from which the overall circulation pattern is investigated. The Weddell Gyre plays a pivotal role in the modification of climate by advecting heat towards the Antarctic ice shelves and by modifying the water masses that feed into the lowest limb of the global ocean overturning circulation. Warm Deep Water, the source water mass that delivers heat to the Weddell Gyre, is conveniently located within the upper 2000 m domain covered by the floats; we investigate its volume transport as it circulates the gyre. Full depth volume transports are estimated by applying a quadratic function to extrapolate the relative dynamic height field component to the full ocean depth, using CTD profiles from ship-based surveys to determine an extrapolation error. Major new insights are provided by this study. There is an established double-gyre structure to the circulation, with a strong eastern cell and a weaker western cell. Regional variation of the baroclinic component of the flow field is revealed, indicating a northeast-to-southwest reduction in the baroclinic flow, along with a strong meridional gradient of baroclinic flow along the northern limb of the gyre, especially east of ∼25°W. The zonal mean gyre strength away from the shelf edge is 32 ± 5 Sv (1 Sv = 1 × 106 m3 s−1), of which 13 ± 3 Sv are associated with the advection of Warm Deep Water. There is a considerable amount of recirculation within the gyre interior, where water does not traverse the full zonal extent of the gyre. The recirculation is stronger in the eastern cell of the observed double-gyre structure. The interior circulation cells partly explain the large variations in previous gyre strength estimates. We provide an extensive review of previous estimates in context of the new results obtained.

Water Masses and Ocean Currents Over the Continental Slope off Northern Baja California

AbstractOcean currents and thermohaline properties over the continental slope off northern Baja California are analyzed with data collected between 2008 and 2012. The data set is composed of 3‐year‐long time series of two moorings with current meter profilers and one chain of conductivity‐temperature‐depths, as well as data collected during 13 synoptic hydrographic surveys. The upper layer (<100 m) was characterized by low salinity Subarctic Waters. Geostrophic velocities suggest that waters of the lighter layer are advected equatorward by near‐surface flows. Below 50 m, poleward flow dominates, so the denser Subarctic Waters recirculates back north along the slope. The combination of geostrophic velocities derived from the hydrographic surveys and the current meter data confirm the presence of the California Undercurrent (CUC), a narrow subsurface poleward flow with a high‐velocity core confined to the continental slope. Mean core speeds were 12–15 cm/s at an average depth of ~150 m. Below the CUC, the core of spicy North Pacific Equatorial Water (NPEW) was found at a mean depth of ~200 m. The analysis of the continuous time series provides strong evidence of the seasonal signal in speed, spiciness, and depth of the CUC and NPEW cores, with a time resolution unprecedented to date. Most of the variance of water mass and flow properties is explained by the annual cycle. The phase shift between the depth of the NPEW core and the strength of the CUC suggests that remotely generated Kelvin waves are responsible for the seasonal variability observed off northern Baja California.

Modulation of Lateral Transport by Submesoscale Flows and Inertia‐Gravity Waves

AbstractWe investigate the role of small‐scale, high‐frequency motions on lateral transport in the ocean, by using velocity fields and particle trajectories from an ocean general circulation model (MITgcm‐llc4320) that permits submesoscale flows, inertia‐gravity waves, and tides. Temporal averaging/filtering removes most of the submesoscale turbulence, inertia‐gravity waves, and tides, resulting in a largely geostrophic flow, with a rapid drop‐off in energy at scales smaller than the mesoscales. We advect two types of Lagrangian particles: (a) 2‐D particles (surface restricted) and (b) 3‐D particles (advected in full three dimensions) with the filtered and unfiltered velocities and calculate Lagrangian diagnostics. At large length/time scales, Lagrangian diffusivity is comparable for filtered and unfiltered velocities, while at short scales, unfiltered velocities disperse particles much faster. We also calculate diagnostics of Lagrangian coherent structures:rotationally coherent Lagrangian vortices detected from closed contours of the Lagrangian‐averaged vorticity deviation and material transport barriers formed by ridges of maximum finite‐time Lyapunov exponent. For temporally filtered velocities, we observe strong material coherence, which breaks down when the level of temporal filtering is reduced/removed, due to vigorous small‐scale mixing. In addition, for the lowest temporal resolution, the 3‐D particles experience very little vertical motion, suggesting that degrading temporal resolution greatly reduces vertical advection by high‐frequency motions. Our study suggests that Lagrangian diagnostics based on satellite‐derived surface geostrophic velocity fields, even with higher spatial resolutions as in the upcoming Surface Water and Ocean Topography mission, may overestimate the presence of mesoscale coherent structures and underestimate dispersion.

The interactions between surface Kuroshio transport and the eddy field east of Taiwan using satellite altimeter data

Based on the daily sea surface height and absolute geostrophic velocity data from 1993 to 2015 provided by the AVISO Center of French Space Agency, the surface Kuroshio transport east of Taiwan and its adjacent eddy field (sea surface height anomaly) were analyzed. Four main periods of the surface Kuroshio transport and eddy field east of Taiwan were obtained, which were used to reveal their interactions. The main conclusions are as follows: (1) Based on the wavelet analysis, the surface Kuroshio transport east of Taiwan and its nearby eddy field showed significant seasonal, annual and interannual periods. In addition to the obvious periods of 182 days (0.5 year) and 365 days (1 year), there were also more obvious periods of about 860 days (2.35 years) and 2 472 days (6.8 years) for the surface Kuroshio transport. There were also four more obvious periods corresponding to the eddy field of 200 days (0.55 year), 374 days (1 year), 889 days (2.43 years) and 2 374 days (6.5 years), although there were latitudinal variations. (2) Based on both the correlation and causal analysis, the correlation between the surface Kuroshio transport and the nearby eddy field over the above four periods was analyzed, and different Kuroshioeddy interactions, with period and latitudinal variability, were revealed.

Characterizing the Transition From Balanced to Unbalanced Motions in the Southern California Current

AbstractAs observations and models improve their resolution of oceanic motions at ever finer horizontal scales, interest has grown in characterizing the transition from the geostrophically balanced flows that dominate at large‐scale to submesoscale turbulence and waves that dominate at small scales. In this study we examine the mesoscale‐to‐submesoscale (100 to 10 km) transition in an eastern boundary current, the southern California Current System (CCS), using repeated acoustic Doppler current profiler transects, sea surface height from high‐resolution nadir altimetry and output from a (1/48)° global model simulation. In the CCS, the submesoscale is as energetic as in western boundary current regions, but the mesoscale is much weaker, and as a result the transition lacks the change in kinetic energy (KE) spectral slope observed for western boundary currents. Helmholtz and vortex‐wave decompositions of the KE spectra are used to identify balanced and unbalanced contributions. At horizontal scales greater than 70 km, we find that observed KE is dominated by balanced geostrophic motions. At scales from 40 to 10 km, unbalanced contributions such as inertia‐gravity waves contribute as much as balanced motions. The model KE transition occurs at longer scales, around 125 km. The altimeter spectra are consistent with acoustic Doppler current profiler/model spectra at scales longer than 70/125 km, respectively. Observed seasonality is weak. Taken together, our results suggest that geostrophic velocities can be diagnosed from sea surface height on scales larger than about 70 km in the southern CCS.

Understanding altimetry signals in the Northeastern Ligurian sea using a multi-platform approach

During the Long-Term Glider Mission for Environmental Characterization 2016 sea trial, carried out in the eastern Ligurian Sea (Northwestern Mediterranean Sea), two gliders rated to a maximum depth of 1000 m operated continuously from 3 May 2016–27 June 2016. When possible, glider tracks were synchronized with the contemporaneous footprints of the Jason-2, SARAL/AltiKa and CryoSat-2 altimeters. Temperature and salinity measured by the gliders that were co-localised with the altimeter passages were used to calculate along-track dynamic heights (DH). The latter were then compared with the altimeters’ near real-time absolute dynamic topography (ADT) measurements. ADT and DH values showed very similar, but shifted patterns, suggesting that gliders had sampled the same structures but at different times. Average surface absolute geostrophic velocities at the time of glider transit were used in a novel relocation technique to reposition glider measurements where they would have been at the time of the altimeter passage. The relocation increases the correlation between datasets to a value close to 1 and reduces the RMSE of an order of magnitude, also when the time-difference between measurements was greater than 3 days. The sea level spatial and temporal variability is attributed to the presence of meanders generated by the baroclinic instability of the Liguro- Provençal Current (LPC). These meanders are characterized by a 38 km amplitude and 0.12 m s−1 average propagation speed that are confirmed by the larger scale ocean colour measurements and are in agreement with those previously described in the literature. We found that the presence of LPC meanders in along-track ADT and DH measurements is evidenced by a decrease in sea level when profiles sampled the Modified Atlantic Water, and an increase in sea level when measurements sampled the adjacent waters of Tyrrhenian Sea origin. The relocation technique introduced here is expected to improve future altimetry-glider comparisons, as similar validation experiments were considered possible only when glider and altimetry measurements were near synoptic, and collected up to one day from each other.

Seasonality of the Kuroshio intensity east of Taiwan modulated by mesoscale eddies

Abstract The intensity of the Kuroshio Current east of Taiwan shows a clear seasonality, and the Kuroshio is affected by mesoscale eddies east of Taiwan. Sea surface geostrophic velocity and mesoscale eddies derived from satellite altimeter data and gridded Argo-only geostrophic velocity data were used to analyze the mesoscale eddy contribution to the Kuroshio intensity seasonality in the East Taiwan Channel. The low potential vorticity water stacked at the northern side of the subtropical countercurrent (STCC) and eddy generation favored the cyclonic eddy. Between August and January, the STCC near the western boundary area (125°E-135°E) showed two branches and a significant negative zonal velocity meridional gradient between 19°N and 21°N from October to December, which led to a higher occurrence of cyclonic eddies. These eddies reached the area east of Taiwan in 100 days and resulted in the dominance of cyclonic eddies between January and April and a weakened Kuroshio Current.

Disassociation of the Kuroshio Current with the Pacific Decadal Oscillation Since 1999

Variability of the Kuroshio Current along the margin of the East China Sea was examined using principal component analysis of 21-year (1993–2013) multi-satellite geostrophic velocity data. Its seasonal zonal migration agrees with previous observations and model simulations. Beyond seasonal time scales, the influence of the Pacific Decadal Oscillation (PDO) on the Kuroshio decreased considerably since 1999. Wind patterns over the North Pacific varied primarily in the meridional direction before 1999, but exhibited strong zonal variation thereafter. Post-1998 climate variability was particularly evident over the central and eastern Pacific. The strong association between PDO and Kuroshio variability also deteriorated after that time.

The relationship between zooplankton distribution and hydrography in oceanic waters of the Southern Gulf of Mexico

Zooplankton was sampled in the Southern Gulf of Mexico, during autumn 2010, summer 2011 and winter 2013. Hydrographic data, dynamic topography and geostrophic velocities showed the presence of alternating cyclonic and anticyclonic gyres in the area, and a quasi-permanent gyre in the Bay of Campeche, with higher surface temperatures and lower salinities during autumn and summer, than in winter. Stronger rainfall was present during summer, which coincided with a strong salps “bloom” that considerably increased biovolumes. A significant difference in biovolumes was found for the three seasons, with highest zooplankton biovolumes during summer, and lowest during autumn. A shoaling of higher fluorescence and higher nutrients (Nitrate + Nitrite) was found in the BOC, in coincidence with higher biovolumes during the three seasons sampled. Multivariate analysis showed that the variables controlling the productivity were mainly fluorescence and nutrients for summer 2011 and winter 2013. Higher biovolumes were present in autumn, summer and winter at the same stations, being higher during summer. Data obtained show that the Bay of Campeche is a high productivity area, all year round, particularly inside the quasi-permanent gyre where high biovolumes were present in autumn, summer and winter, but with considerable seasonal variations in primary and secondary productivity, probably dependent in great measure from river discharge and rainfall.

Does the East Greenland Current exist in the northern Fram Strait?

Abstract. Warm Atlantic Water (AW) flows around the Nordic Seas in a cyclonic boundary current loop. Some AW enters the Arctic Ocean where it is transformed to Arctic Atlantic Water (AAW) before exiting through the Fram Strait. There the AAW is joined by recirculating AW. Here we present the first summer synoptic study targeted at resolving this confluence in the Fram Strait which forms the East Greenland Current (EGC). Absolute geostrophic velocities and hydrography from observations in 2016, including four sections crossing the east Greenland shelf break, are compared to output from an eddy-resolving configuration of the sea ice–ocean model FESOM. Far offshore (120 km at 80.8∘ N) AW warmer than 2 ∘C is found in the northern Fram Strait. The Arctic Ocean outflow there is broad and barotropic, but gets narrower and more baroclinic toward the south as recirculating AW increases the cross-shelf-break density gradient. This barotropic to baroclinic transition appears to form the well-known EGC boundary current flowing along the shelf break farther south where it has been previously described. In this realization, between 80.2 and 76.5∘ N, the southward transport along the east Greenland shelf break increases from roughly 1 Sv to about 4 Sv and the proportion of AW to AAW also increases fourfold from 19±8 % to 80±3 %. Consequently, in the southern Fram Strait, AW can propagate into the Norske Trough on the east Greenland shelf and reach the large marine-terminating glaciers there. High instantaneous variability observed in both the synoptic data and the model output is attributed to eddies, the representation of which is crucial as they mediate the westward transport of AW in the recirculation and thus structure the confluence forming the EGC.

The Vertical Structure of a Loop Current Eddy

AbstractThe vertical structure of a recently detached Loop Current Eddy (LCE) is studied using in situ data collected with an underwater glider from August to November 2016. Altimetry and Argo data are analyzed to discuss the context of the eddy shedding and evolution as well as the origin and transformation of its thermohaline properties. The LCE appeared as a large body of nearly homogeneous water between 50 and 250 m confined between the seasonal and main thermoclines. A temperature anomaly relative to surrounding Gulf's water of up to 9.7∘ C was observed within the eddy core. The salinity structure had a double core pattern. The subsurface fresh core had a negative anomaly of 0.27 practical salinity unit, while the deeper saline core's positive anomaly reached 1.22 practical salinity unit. Both temperature and salinity maxima were stronger than previously reported. The saline core, of Caribbean origin, was well conserved during its journey from the Yucatan Basin to the Loop Current and at least 7 months after eddy detachment. The fresher homogeneous core resulted from surface diabatic transformations including surface heat fluxes and mixing within the top 200 m during the winter preceding eddy detachment. Heat and salt excess carried by the LCE were large and require important negative heat fluxes and positive fresh water input to be balanced. The geostrophic velocity structure had the form of a subsurface intensified vortex ring.

Surface Salinity Balance in the Tropical Pacific Off Mexico

AbstractA simplified analytical model was used to solve the salinity balance equation and study the relative contributions of freshwater flux and salt advection to the salinity tendency between the Gulf of Tehuantepec and Cabo Corrientes (14–22°N). The model used climatological mixed‐layer salinity, satellite‐derived geostrophic velocity and evaporation minus precipitation data in a four‐domain scheme. The mean circulation derived from the model largely coincides with previous reports and suggests that, in the steady state, the Mexican Coastal Current is decoupled from the Tehuantepec region. The analysis of the time‐dependent variability showed that the salt advection can be largely explained by the annual variations of the alongshore velocity (corresponding with the Mexican Coastal Current) and an average salinity. At the seasonal scale, the salt advection and the freshwater flux have similar contributions to the resulting salinity tendency. Near Tehuantepec, due to a local salinity regime, the analytical model does not represent well the observations, while in Manzanillo and Zihuatanejo the agreement between the model and the observations is good in both amplitude and phase. The accuracy of the model in representing the observations is improved in the first third of the year by inclusion of the semiannual scale and the nonlinear processes in the equation of salt advection.

Examining the Accuracy of GlobCurrent Upper Ocean Velocity Data Products on the Northwestern Atlantic Shelf

This study provides a regional coastal ocean assessment of global upper ocean current data developed by the GlobCurrent (GC) project. These gridded data synthesize multiple satellite altimeter and wind model inputs to estimate both Geostrophic and Ekman-layer velocities. While the GC product was mostly devised and intended for open ocean studies, the present objective is to assess whether its data quality nearer the coast is suitable for other applications. The key ground truth sources are long-term mean and time series observations on the Northwestern Atlantic (NWA) shelf derived from Acoustic Doppler Current Profilers (ADCP) and high frequency (HF) radar networks in both the Mid-Atlantic Bight (MAB) and the Gulf of Maine (GoM). Results indicate that mean geostrophic currents across the MAB and the offshore GoM agree to roughly 10% in speed and 10 degree in direction with the in situ depth-averaged currents, with correlation levels of 0.5–0.8 at seasonal and longer time scales. Interior GoM comparisons at 5 coastal buoys show much less agreement. One likely source of GoM error is shown to be the GC mean dynamic topography near the coast. Comparison to near-surface MAB HF radar current measurements on the MAB shelf shows significant GC data improvement when including the surface Ekman term. Overall, the study results imply that application of GlobCurrent data may prove useful in coastal seas with broad continental shelves such as the MAB or Scotian shelf, but that large inaccuracies inside the GoM diminish its utility there.

Malvinas Current at 40°S–41°S: First Assessment of Temperature and Salinity Temporal Variability

AbstractThe temporal variability of 11 months of in situ velocity, temperature, and salinity data collected at five moorings deployed at 40°S–41°S across the shelf‐break in the Southwestern Atlantic is analyzed. Two distinct regimes characterized by strong and weak along‐slope velocities are present. During the strong regime the Malvinas Current flows northward through the moorings while during the nearly 5 months long weak regime reversals of the along‐slope velocities are frequently observed. Comparison with the previous in situ time series obtained in the same region shows that such an extended period of weak flow has not been previously observed. During the weak regime, Sub‐Antarctic Mode Water is observed over the 1,800 m isobath at an average depth of 500 m. Water masses occupying the upper 1,600 m during the strong regime deepen and shift eastward during the weak period. Satellite geostrophic velocities and sea surface temperature clearly show that the weak regime is due to a deflection to the east of the Malvinas Current, upstream of the mooring position. Analysis of the vertical structure of the currents indicate that during the weak regime the flow weakens mostly at the surface and presents a very small vertical shear. In contrast, during the strong regime currents are surface‐intensified. The change in the structure of the currents at the mooring location impacts the relationship between in situ and altimetry‐derived currents: during the weak regime altimetry adequately represents (rmsd 12 cm/s) in situ currents in the whole water column, while during strong regime rmsd are larger than 15 cm/s below 600 m depth.

Structure and Transport of the North Atlantic Current in the Eastern Subpolar Gyre From Sustained Glider Observations

AbstractRepeat glider sections obtained during 2014–2016, as part of the Overturning in the Subpolar North Atlantic Program, are used to quantify the circulation and transport of North Atlantic Current (NAC) branches over the Rockall Plateau. Using 16 glider sections collected along 58°N and between 21°W and 15°W, absolute geostrophic velocities are calculated, and subsequently the horizontal and vertical structure of the transport are characterized. The annual mean northward transport (± standard deviation) is 5.1 ± 3.2 Sv over the Rockall Plateau. During summer (May to October), the mean northward transport is stronger and reaches 6.7 ± 2.6 Sv. This accounts for 43% of the total NAC transport of upper‐ocean waters (σO<27.55 kg/m3) estimated by Sarafanov et al. (2012, https://doi.org/10.1029/2011JC007572) along 59.5°N, between the Reykjanes Ridge and Scotland. Two quasi‐permanent northward flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 ± 2.1 Sv) over the eastern flank of the Iceland Basin (20.5°W to 18.5°W) and (ii) the Rockall Bank Jet (1.5 ± 0.7 Sv) over the eastern flank of the Hatton‐Rockall Basin (16°W to 15°W). Transport associated with the Rockall Bank Jet is mostly depth independent during summer, while 30% of the Hatton Bank Jet transport is due to vertical geostrophic shear. Uncertainties are estimated for each individual glider section using a Monte Carlo approach, and mean uncertainties of the absolute transport are less than 0.5 Sv. Although comparisons with altimetry‐based estimates indicate similar large‐scale circulation patterns, altimetry data do not resolve small mesoscale current bands in the Hatton‐Rockall Basin which are strongly needed for the right transport estimates.