scholarly journals Temperature–Salinity Structure of the North Atlantic Circulation and Associated Heat and Freshwater Transports

2016 ◽  
Vol 29 (21) ◽  
pp. 7723-7742 ◽  
Author(s):  
Xiaobiao Xu ◽  
Peter B. Rhines ◽  
Eric P. Chassignet
Keyword(s):  
Heat Transport ◽  
North Atlantic ◽  
Sea Water ◽  
Potential Temperature ◽  
Meridional Overturning Circulation ◽  
Subtropical Gyre ◽  
Western Boundary ◽  
The North ◽  
Freshwater Transport ◽  
The North Atlantic

Abstract This study investigates the circulation structure and relative contribution of circulation components to the time-mean meridional heat and freshwater transports in the North Atlantic, using numerical results of a high-resolution ocean model that are shown to be in excellent agreement with the observations. The North Atlantic circulation can be separated into the large-scale Atlantic meridional overturning circulation (AMOC) that is diapycnal and the subtropical and subpolar gyres that largely flow along isopycnal surfaces but also include prominent gyre-scale diapycnal overturning in the Subtropical Mode Water and Labrador Sea Water. Integrals of the meridional volume transport as a function of potential temperature θ and salinity S yield streamfunctions with respect to θ and to S, and heat functions. These argue for a significant contribution to the heat transport by the southward circulation of North Atlantic Deep Water. At 26.5°N, the isopycnic component of the subtropical gyre is colder and fresher in the northward-flowing western boundary currents than the southward return flows, and it carries heat southward and freshwater northward, opposite of that of the diapycnal component. When combined, the subtropical gyre contributes virtually zero to the heat transport and the AMOC is responsible for all the heat transport across this latitude. The subtropical gyre however significantly contributes to the freshwater transport, reducing the 0.5-Sv (1 Sv ≡ 106 m3 s–1) southward AMOC freshwater transport by 0.13 Sv. In the subpolar North Atlantic near 58°N, the diapycnal component of the circulation, or the transformation of warm saline upper Atlantic water into colder fresher deep waters, is responsible for essentially all of the heat and freshwater transports.

Identification and quantification of the North Atlantic Deep Water pathways

2021 ◽  
Author(s):  
Philippe Miron ◽  
Maria J. Olascoaga ◽  
Francisco J. Beron-Vera ◽  
Kimberly L. Drouin ◽  
M. Susan Lozier
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Sea Water ◽  
Lower Layer ◽  
North Atlantic Deep Water ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Western Boundary ◽  
The North ◽  
The North Atlantic

<p>The North Atlantic Deep Water (NADW) flows equatorward along the Deep Western Boundary Current (DWBC) as well as interior pathways and is a critical part of the Atlantic Meridional Overturning Circulation. Its upper layer, the Labrador Sea Water (LSW), is formed by open-ocean deep convection in the Labrador and Irminger Seas while its lower layers, the Iceland–Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW), are formed north of the Greenland–Iceland–Scotland Ridge.</p><p>In recent years, more than two hundred acoustically-tracked subsurface floats have been deployed in the deep waters of the North Atlantic.  Studies to date have highlighted water mass pathways from launch locations, but due to limited float trajectory lengths, these studies have been unable to identify pathways connecting  remote regions.</p><p>This work presents a framework to explore deep water pathways from their respective sources in the North Atlantic using Markov Chain (MC) modeling and Transition Path Theory (TPT). Using observational trajectories released as part of OSNAP and the Argo projects, we constructed two MCs that approximate the lower and upper layers of the NADW Lagrangian dynamics. The reactive NADW pathways—directly connecting NADW sources with a target at 53°N—are obtained from these MCs using TPT.</p><p>Preliminary results show that twenty percent more pathways of the upper layer(LSW) reach the ocean interior compared to  the lower layer (ISOW, DSOW), which mostly flows along the DWBC in the subpolar North Atlantic. Also identified are the Labrador Sea recirculation pathways to the Irminger Sea and the direct connections from the Reykjanes Ridge to the eastern flank of the Mid–Atlantic Ridge, both previously observed. Furthermore, we quantified the eastern spread of the LSW to the area surrounding the Charlie–Gibbs Fracture Zone and compared it with previous analysis. Finally, the residence time of the upper and lower layers are assessed and compared to previous observations.</p>

scholarly journals A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

2011 ◽  
Vol 8 (1) ◽  
pp. 453-482 ◽  
Author(s):  
I. Núñez-Riboni ◽  
M. Bersch ◽  
H. Haak ◽  
J. H. Jungclaus
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Sea Water ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
The North ◽  
Meridional Displacement ◽  
The North Atlantic ◽  
Newfoundland Basin

Abstract. Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF) in the Newfoundland Basin (NFB) in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to Labrador Sea Water (LSW) volume, atmospheric forcing and intensities of the Subpolar Gyre (SPG) and Meridional Overturning Circulation (MOC). The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG), an inter-gyre gyre with a multi-decadal time scale. Contributions of wind stress curl (WSC) and LSW volume changes to the inter-gyre gyre are similar between 35 and 55° N (excluding the western boundary current). An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation

2008 ◽  
Vol 38 (9) ◽  
pp. 1913-1930 ◽  
Author(s):  
Armin Köhl ◽  
Detlef Stammer
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Special Focus ◽  
Western Boundary ◽  
Consistent Estimate ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.

scholarly journals Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change

2021 ◽  
Author(s):  
Daniel Santana-Toscano ◽  
M. Dolores Pérez-Hernández ◽  
Verónica Caínzos ◽  
Melania Cubas Armas ◽  
Cristina Arumí-Planas ◽  
...  
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Subtropical Gyre ◽  
North Equatorial Current ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
North Brazil ◽  
Different Seasons ◽  
Using Data

<p>The A20 is a meridional hydrographic section located at 52ºW on the western North Atlantic Subtropical Gyre that encloses the path of the water masses of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with LADCP-SADCP data and the velocities from an inverse box model, the circulation of the western North Atlantic Subtropical Gyre is estimated. The main poleward current of the AMOC is the Gulf Stream (GS) which carries 129.0±10.5 Sv in 2003 and 110.4±12.2 Sv in 2012. Due to the seasonality, the GS position is shifted southward in 2012 - relative to that of 2003 - as both cruises took place in different seasons. In opposite direction, the Deep Western Boundary Current (DWBC) crosses the section twice, first at 39.3-43.2ºN (-34.9±7.5 Sv in 2003 and -25.3±9.4 Sv in 2012) and then at 7.0-11.7ºN (42.0±8.0 Sv in 2003 and 48.0±8.1 Sv in 2012). Additionally, two zonal currents contribute with westward transport below 20ºN: the North Equatorial Current and the North Brazil Current; with a net value of -28.0±4.1 Sv in 2003 and -36.7±3.6 Sv in 2012.</p>

scholarly journals A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

Ocean Science ◽  
2012 ◽  
Vol 8 (1) ◽  
pp. 91-102 ◽  
Author(s):  
I. Núñez-Riboni ◽  
M. Bersch ◽  
H. Haak ◽  
J. H. Jungclaus ◽  
K. Lohmann
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Sea Water ◽  
Circulation Model ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Meridional Displacement ◽  
The North Atlantic ◽  
Newfoundland Basin

Abstract. Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF) in the Newfoundland Basin (NFB) in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to atmospheric forcing, intensities of the subpolar gyre (SPG) and Meridional Overturning Circulation (MOC), and Labrador Sea Water (LSW) volume. The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG), an inter-gyre gyre with a multi-decadal time scale. A sensitivity experiment indicates that both wind stress curl (WSC) and heat fluxes (which match LSW changes) contribute to the circulation anomalies in the frontal region and to the SPF displacement. An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB (a "SPF index") could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

scholarly journals How Much Has the North Atlantic Ocean Overturning Circulation Changed in the Last 50 Years?

2014 ◽  
Vol 27 (16) ◽  
pp. 6325-6342 ◽  
Author(s):  
Simon F. B. Tett ◽  
Toby J. Sherwin ◽  
Amrita Shravat ◽  
Oliver Browne
Keyword(s):  
North Atlantic ◽  
Mixed Layer Depth ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
The North ◽  
Ocean Reanalyses ◽  
The North Atlantic

Abstract Volume transports from six ocean reanalyses are compared with four sets of in situ observations: across the Greenland–Scotland ridge (GSR), in the Labrador Sea boundary current, in the deep western boundary current at 43°N, and in the Atlantic meridional overturning circulation (AMOC) at 26°N in the North Atlantic. The higher-resolution reanalyses (on the order of ¼° × ¼°) are better at reproducing the circulation pattern in the subpolar gyre than those with lower resolution (on the order of 1°). Simple Ocean Data Assimilation (SODA) and Estimating the Circulation and Climate of the Ocean (ECCO)–Jet Propulsion Laboratory (JPL) produce transports at 26°N that are close to those observed [17 Sv (1 Sv ≡ 106 m3 s−1)]. ECCO, version 2, and SODA produce northward transports across the GSR (observed transport of 8.2 Sv) that are 22% and 29% too big, respectively. By contrast, the low-resolution reanalyses have transports that are either too small [by 31% for ECCO-JPL and 49% for Ocean Reanalysis, system 3 (ORA-S3)] or much too large [Decadal Prediction System (DePreSys)]. SODA had the best simulations of mixed layer depth and with two coarse grid long-term reanalyses (DePreSys and ORA-S3) is used to examine changes in North Atlantic circulation from 1960 to 2008. Its results suggest that the AMOC increased by about 20% at 26°N while transport across the GSR hardly altered. The other (less reliable) long-term reanalyses also had small changes across the GSR but changes of +10% and −20%, respectively, at 26°N. Thus, it appears that changes in the overturning circulation at 26°N are decoupled from the flow across the GSR. It is recommended that transport observations should not be assimilated in ocean reanalyses but used for validation instead.

scholarly journals Mid-Pliocene Atlantic Meridional Overturning Circulation simulated in PlioMIP2

2020 ◽  
Author(s):  
Zhongshi Zhang ◽  
Xiangyu Li ◽  
Chuncheng Guo ◽  
Odd Helge Otterå ◽  
Kerim H. Nisancioglu ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Heat Transport ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ocean Heat Transport ◽  
Surface Warming ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. In the Pliocene Model Intercomparison Project phase 2 (PlioMIP2), coupled climate models have been used to simulate an interglacial climate during the mid-Piacenzian warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), poleward ocean heat transport and sea surface warming in the Atlantic simulated with these models. In PlioMIP2, all models simulate an intensified mid-Pliocene AMOC. However, there is no consistent response in the simulated Atlantic ocean heat transport, or the depth of the Atlantic overturning cell. The models show a large spread in the simulated AMOC maximum, the Atlantic ocean heat transport, as well as the surface warming in the North Atlantic. Although a few models simulate a surface warming of ~ 8–12 ° in the North Atlantic, similar to the reconstruction from Pliocene Research, Interpretation and Synoptic Mapping (PRISM), most models underestimate this warming. The large model-spread and model-data discrepancies in the PlioMIP2 ensemble does not support the hypothesis that an intensification of the AMOC, together with an increase in northward ocean heat transport, is the dominant forcing for the mid-Pliocene warm climate.

A Simple Model of the Response of the Atlantic to the North Atlantic Oscillation

2014 ◽  
Vol 27 (11) ◽  
pp. 4052-4069 ◽  
Author(s):  
Xiaoming Zhai ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Heat Content ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Western Boundary ◽  
Ekman Pumping ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract The response of an idealized Atlantic Ocean to wind and thermohaline forcing associated with the North Atlantic Oscillation (NAO) is investigated both analytically and numerically in the framework of a reduced-gravity model. The NAO-related wind forcing is found to drive a time-dependent “leaky” gyre circulation that integrates basinwide stochastic wind Ekman pumping and initiates low-frequency variability along the western boundary. This is subsequently communicated, together with the stochastic variability induced by thermohaline forcing at high latitudes, to the remainder of the Atlantic via boundary and Rossby waves. At low frequencies, the basinwide ocean heat content changes owing to NAO wind forcing and thermohaline forcing are found to oppose each other. The model further suggests that the recently reported opposing changes of the meridional overturning circulation in the Atlantic subtropical and subpolar gyres between 1950–70 and 1980–2000 may be a generic feature caused by interplay between the NAO wind and thermohaline forcing.

scholarly journals Size Matters: Another Reason Why the Atlantic Is Saltier than the Pacific

2017 ◽  
Vol 47 (11) ◽  
pp. 2843-2859 ◽  
Author(s):  
C. S. Jones ◽  
Paola Cessi
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Subpolar Gyre ◽  
Boundary Current ◽  
Surface Salinity ◽  
The North ◽  
The North Atlantic

AbstractThe surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation (MOC); the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific. Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing. This preference is linked to the details of the velocity and salinity fields in the “sinking” basin. The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin. It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin. The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation.

Potential Predictability of the North Atlantic Heat Transport Based on an Oceanic State Estimate

2012 ◽  
Vol 25 (24) ◽  
pp. 8475-8486 ◽  
Author(s):  
Bente Tiedje ◽  
Armin Köhl ◽  
Johanna Baehr
Keyword(s):  
Heat Transport ◽  
North Atlantic ◽  
Initial Conditions ◽  
Meridional Overturning Circulation ◽  
Lead Times ◽  
Meridional Heat Transport ◽  
Potential Predictability ◽  
Latitude Dependence ◽  
The North ◽  
The North Atlantic

Abstract This paper investigates the potential predictability of the meridional heat transport (MHT) in the North Atlantic on interannual time scales using hindcast ensembles based on an oceanic data assimilation product. The work analyzes the prognostic potential predictability (PPP), using the ocean synthesis of the German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) as initial conditions and as boundary conditions. The PPP of the MHT varies with latitude: local maxima are apparent within the subpolar and the subtropical gyres, and a minimum is apparent at the boundary between the gyres. This PPP minimum can also be seen in the PPP structure of the Atlantic meridional overturning circulation (AMOC), although it is considerably less pronounced. The decomposition of the MHT shows that within the subpolar gyre, the gyre component of the MHT influences the PPP structure of the MHT. Within the subtropical gyre, the overturning component of the MHT characterizes the PPP structure of the MHT. At the boundary between the subpolar and the subtropical gyres, the dynamics of the Ekman heat transport limit the predictable lead times of the MHT. At most latitudes, variations in the velocity field control the PPP structure of the MHT. The PPP structure of the AMOC can also be classified into gyre and gyre-boundary regimes, but the predictable lead times within the gyres are only similar to those of the overturning component of the MHT. Overall, the analysis provides a reference point for the latitude dependence of the MHT’s PPP structure and relates it to the latitude dependence of the AMOC’s PPP structure.

Sign in / Sign up

Export Citation Format

Share Document