North Atlantic Current Transport Research Articles

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.

Flow paths and variability of the North Atlantic Current: A comparison of observations and a high‐resolution model

AbstractThe 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.

Long‐term observations of North Atlantic Current transport at the gateway between western and eastern Atlantic

AbstractIn the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four inverted echo sounders with high precision pressure sensors (PIES) were moored between 47°40′N and 52°30′N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central, and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid‐Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21 years is 27 ± 5 Sv, consisting of 60% warm water of subtropical origin and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4–9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the southern segment.

Circulation and transports in the Newfoundland Basin, western subpolar North Atlantic

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.

Predicting a Decadal Shift in North Atlantic Climate Variability Using the GFDL Forecast System

Abstract Decadal prediction experiments were conducted as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5) using the GFDL Climate Model, version 2.1 (CM2.1) forecast system. The abrupt warming of the North Atlantic Subpolar Gyre (SPG) that was observed in the mid-1990s is considered as a case study to evaluate forecast capabilities and better understand the reasons for the observed changes. Initializing the CM2.1 coupled system produces high skill in retrospectively predicting the mid-1990s shift, which is not captured by the uninitialized forecasts. All the hindcasts initialized in the early 1990s show a warming of the SPG; however, only the ensemble-mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG. Examination of the physical mechanisms responsible for the successful retrospective predictions indicates that initializing the ocean is key to predicting the mid-1990s warming. The successful initialized forecasts show an increased Atlantic meridional overturning circulation and North Atlantic Current transport, which drive an increased advection of warm saline subtropical waters northward, leading to a westward shift of the subpolar front and, subsequently, a warming and spindown of the SPG. Significant seasonal climate impacts are predicted as the SPG warms, including a reduced sea ice concentration over the Arctic, an enhanced warming over the central United States during summer and fall, and a northward shift of the mean ITCZ. These climate anomalies are similar to those observed during a warm phase of the Atlantic multidecadal oscillation, which is encouraging for future predictions of North Atlantic climate.

Structure of the North Atlantic current in stream-coordinates and the circulation in the Newfoundland basin

Accurate knowledge of the absolute North Atlantic Current (NAC) transport has been limited in the past by the difficulty of obtaining absolute velocity measurements. For this study, historical hydrography was combined with time series of round trip travel time from inverted echo sounders, bottom pressure sensor measurements, and measured velocities from deep current meters, via a “Gravest Empirical Mode” technique, to yield time series of temperature and absolute velocity sections. These sections were used to determine the mean NAC stream-coordinates structure of temperature and absolute velocity for the period from August 1993 to February 1995. This structure is narrower and stronger than the corresponding Eulerian average, with peak speeds greater than 80 cm s −1 and bottom velocities exceeding 10 cm s −1 . The calculated mean NAC transport was 131±14 (×10 6 m 3 s −1 ); temporal variations resulted in a 41×10 6 m 3 s −1 standard deviation. The stream-coordinates estimate of mean absolute transport was found to be about 10% smaller than the corresponding Eulerian mean transport of 146×10 6 m 3 s −1 as a result of partially summing the north and south transports within the Mann Eddy, a large semi-permanent anticyclonic eddy normally located just offshore of the NAC. Most transport sections across the NAC in this region include the northward transport of the inshore side of the Mann Eddy because it is difficult to discriminate between the two flows. Historical estimates place the Mann Eddy absolute transport around 50–60×10 6 m 3 s −1 ; thus, the transport crossing this transect that can be identified with the NAC is about 90×10 6 m 3 s −1 . The barotropic component of the transport (defined as the bottom velocity multiplied by the water depth) is found to contribute at least 30% of the total absolute transport, depending on the choice of baroclinic reference level. Because a time series of transport measurements was made in this study, the accuracy of the mean transport estimate is better than previous estimates, which have all been based on snapshot measurements. From a combination of these measurements with other published measurements, a scheme for the overall circulation and transport in the Newfoundland Basin is suggested.

Seasonal variation of the North Atlantic Current

The seasonal circulation of the upper 1000 m of the North Atlantic between 40°–55°N and 20°–40°W is calculated using the traditional dynamic method and a circulation model with a density field that evolves with the flow. The model is of finite difference form and is based on dynamics that describe the nonlinear evolution of the ocean at low Rossby number. The model is controlled by initial and boundary conditions that include air‐sea buoyancy and momentum fluxes. The model is run in two ways: with controls specified directly from observations and with controls inferred by the assimilation of all available data. These data include surface drifter trajectories, sea levels from the TOPEX/Poseidon altimeter, Bunker air‐sea fluxes, and the Levitus climatological monthly means of temperature and salinity. We conclude that the North Atlantic Current transport is 40±18 Sv with seasonal variations of the order of 2 Sv. The mean vertical transport out of the region is 2±9 Sv and is subject to seasonal variations of 2 Sv. Overall, these estimates are in good agreement with integral North Atlantic Current features derived from independent long‐term measurements made in the region over the past decade. The optimal ocean state has a volume transport across the western boundary of 51±3 Sv with a maximum transport of 61±5 Sv in April‐May and a minimum of 42±3 Sv in October‐November. This western inflow is compensated by mean outflows of 28±2 (east), 16±2 (north), 5±2 (south), and 1.8±0.4 Sv out of the domain at 1000 m. Sensitivity studies show that nonlinear mixing and seasonality are important in determining the overall circulation. Specifically, steady boundary forcing leads to annual mean transports that are 15–25% smaller than transports obtained with seasonal forcing. Winter convection is also shown to play a significant role in determining the overall circulation pattern.

Absolutely referenced geostrophic velocity and transport on a section across the North Atlantic Current

A transect of CTD profiles crossing the North Atlantic Current (NAC) along WOCE line ACM6 near 42.5°N during August 1–7, 1993, provides geostrophic shear velocity profiles, which were absolutely referenced using simultaneous POGO transport float measurements and velocity measurements from a ship-mounted acoustic doppler current profiler (ADCP). The NAC absolute transport was 112±23×10 6 m 3 s −1, which includes a portion of the transport of the Mann Eddy, a large permanent anticyclonic eddy commonly adjacent to the NAC. The NAC transport estimated relative to a level of no motion at the bottom would have underestimated the true total absolute transport by 20%. A surprisingly large 58×10 6 m 3 s −1 flowed southward just inshore of the NAC. This flow, centered near 1500 dbars about 200 km offshore of the shelf-break, was fairly barotropic with a peak velocity of greater than 20 cm s −1, and the water mass characteristics were of Labrador Sea Water. These absolute transport observations suggest southward recirculation inshore of the NAC at 42.5°N and a stronger NAC than has previously been observed.

The North Atlantic Current and surrounding waters: At the crossroads

The North Atlantic Current is a well‐defined western boundary current that flows north along the east side of the Grand Banks from 40° to 51°N, where it turns sharply to the east and begins its journey across the ocean. The current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the European subcontinent. The North Atlantic Current originates in the Gulf Stream when the latter curves north around the Southeast Newfoundland Rise, a major submarine ridge that stretches SE from the Grand Banks. A well‐defined front delineates the path of the current as long as it flows north as a western boundary current. After the current turns east in the north, it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing Gulf Stream after it separates from Cape Hatteras. The North Atlantic Current transports more than 40 Sv (1 Sv = 106 m³ s−1) in the south and about 20 Sv by the time it flows east across the Mid‐Atlantic Ridge. The currents along the northward flowing front are quite swift, with typical maximum average speeds in the upper 300 m near 1 m s−1 (= 2 knots). The current meanders almost as wildly as a “snaking” river, but unlike steep meanders in the Gulf Stream these meanders appear to be stable, and with one exception have not been observed to break off to form pools of warm and/or cold waters as frequently occurs in the Gulf Stream. The meanders appear to be induced by major topographic features along the path of the current, namely, the Southeast Newfoundland Rise, the Newfoundland Seamounts, and Flemish Cap. Strong recirculations develop on the concave side of the meanders. One of these, the “Mann eddy” at the first meander crest of the North Atlantic Current, should be regarded as a permanent feature of the North Atlantic circulation. Other meanders also contain recirculations that can persist for months. Under certain conditions these can merge together to form an extended SW flow (recirculation) just east of the North Atlantic Current.