scholarly journals Variability of Shallow and Deep Western Boundary Currents off the Bahamas during 2004–05: Results from the 26°N RAPID–MOC Array

2008 ◽  
Vol 38 (3) ◽  
pp. 605-623 ◽  
Author(s):  
W. E. Johns ◽  
L. M. Beal ◽  
M. O. Baringer ◽  
J. R. Molina ◽  
S. A. Cunningham ◽  
...  
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
The Bahamas ◽  
Meridional Overturning ◽  
Baroclinic Transport

Abstract Data from an array of six moorings deployed east of Abaco, Bahamas, along 26.5°N during March 2004–May 2005 are analyzed. These moorings formed the western boundary array of a transbasin observing system designed to continuously monitor the meridional overturning circulation and meridional heat flux in the subtropical North Atlantic, under the framework of the joint U.K.–U.S. Rapid Climate Change (RAPID)–Meridional Overturning Circulation (MOC) Program. Important features of the western boundary circulation include the southward-flowing deep western boundary current (DWBC) below 1000 m and the northward-flowing “Antilles” Current in the upper 1000 m. Transports in the western boundary layer are estimated from direct current meter observations and from dynamic height moorings that measure the spatially integrated geostrophic flow between moorings. The results of these methods are combined to estimate the time-varying transports in the upper and deep ocean over the width of the western boundary layer to a distance of 500 km offshore of the Bahamas escarpment. The net southward transport of the DWBC across this region, inclusive of northward deep recirculation, is −26.5 Sv (Sv ≡ 106 m3 s−1), which is divided nearly equally between upper (−13.9 Sv) and lower (−12.6 Sv) North Atlantic Deep Water (NADW). In the top 1000 m, 6.0 Sv flows northward in a thermocline-intensified jet near the western boundary. These transports are found to agree well with historical current meter data in the region collected between 1986 and 1997. Variability in both shallow and deep components of the circulation is large, with transports above 1000 m varying between −15 and +25 Sv and deep transports varying between −60 and +3 Sv. Much of this transport variability, associated with barotropic fluctuations, occurs on relatively short time scales of several days to a few weeks. Upon removal of the barotropic fluctuations, slower baroclinic transport variations are revealed, including a temporary stoppage of the lower NADW transport in the DWBC during November 2004.

scholarly journals Sensitivity of Atlantic Meridional Overturning Circulation Variability to Parameterized Nordic Sea Overflows in CCSM4

2012 ◽  
Vol 25 (6) ◽  
pp. 2077-2103 ◽  
Author(s):  
Stephen Yeager ◽  
Gokhan Danabasoglu
Keyword(s):  
Deep Ocean ◽  
Atlantic Meridional Overturning Circulation ◽  
Order Effect ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Frequency Space ◽  
Meridional Overturning

Abstract The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in a much improved representation of the North Atlantic tracer and velocity distributions compared to a control CCSM4 simulation without this parameterization. As a consequence, the variability of the Atlantic meridional overturning circulation (AMOC) on decadal and longer time scales is generally lower, but the reduction is not uniform in latitude, depth, or frequency–space. While there is dramatically less variance in the overall AMOC maximum (at about 35°N), the reduction in AMOC variance at higher latitudes is more modest. Also, it is somewhat enhanced in the deep ocean and at low latitudes (south of about 30°N). The complexity of overturning response to overflows is related to the fact that, in both simulations, the AMOC spectrum varies substantially with latitude and depth, reflecting a variety of driving mechanisms that are impacted in different ways by the overflows. The usefulness of reducing AMOC to a single index is thus called into question. This study identifies two main improvements in the ocean mean state associated with the overflow parameterization that tend to damp AMOC variability: enhanced stratification in the Labrador Sea due to the injection of dense overflow waters and a deepening of the deep western boundary current. Direct driving of deep AMOC variance by overflow transport variations is found to be a second-order effect.

scholarly journals Inferring the zonal distribution of measured changes in the meridional overturning circulation

Ocean Science ◽  
2007 ◽  
Vol 3 (1) ◽  
pp. 55-57 ◽  
Author(s):  
A. M. de Boer ◽  
H. L. Johnson
Keyword(s):  
Dynamic Method ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Zonal Distribution ◽  
Deep Western Boundary Current ◽  
Meridional Overturning ◽  
Florida Straits ◽  
Southward Flow

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics) is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region immediately outside of the Florida Straits.

scholarly journals Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

2013 ◽  
Vol 26 (18) ◽  
pp. 7167-7186 ◽  
Author(s):  
Carl Wunsch ◽  
Patrick Heimbach
Keyword(s):  
Ocean Circulation ◽  
Extreme Values ◽  
Statistical Significance ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Gaussian Stochastic Process ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

scholarly journals The Observed North Atlantic Meridional Overturning Circulation: Its Meridional Coherence and Ocean Bottom Pressure

2014 ◽  
Vol 44 (2) ◽  
pp. 517-537 ◽  
Author(s):  
Shane Elipot ◽  
Eleanor Frajka-Williams ◽  
Chris W. Hughes ◽  
Josh K. Willis
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Overturning Circulation ◽  
Ocean Bottom Pressure ◽  
Meridional Overturning

Abstract Analyses of meridional transport time series from the Rapid Climate Change–Meridional Overturning Circulation (RAPID MOC) array at 26°N and from Argo float and altimetry data at 41°N reveal that, at semiannual and longer time scales, the contribution from the western boundary dominates the variability of the North Atlantic meridional overturning circulation (MOC), defined as the transport in the upper 1000 m of the ocean. Because the variability of the western boundary contribution is associated with a geostrophic overturning, it is reflected in independent estimates of transports from gradient of ocean bottom pressure (OBP) relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes (26°, 39°, and 42.5°N). Time series of western meridional transports relative to and below 1000 m derived from the OBP gradient, or equivalently derived from the transport shear profile, exhibit approximately the same phase relationship between 26° and 39°–42.5°N as the western contribution to the geostrophic MOC time series do: the western geostrophic MOC at 41°N precedes the MOC at 26°N by approximately a quarter of an annual cycle, resulting in a zero correlation at this time scale. This study therefore demonstrates how OBP gradients on basin boundaries can be used to monitor the MOC and its meridional coherence.

scholarly journals Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 265-284
Author(s):  
Josefine Herrford ◽  
Peter Brandt ◽  
Torsten Kanzow ◽  
Rebecca Hummels ◽  
Moacyr Araujo ◽  
...  
Keyword(s):  
Seasonal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Bottom Pressure ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Western Basin ◽  
Meridional Overturning

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11∘ S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The characteristics of the observed seasonal cycles of the AMOC and its components are compared to results from an ocean general circulation model, which is known to reproduce the variability of the Western Boundary Current on longer timescales. The observed seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, especially at the western boundary, which translates into the estimate of the upper-ocean geostrophic transport. In the model, seasonal AMOC variability at 11∘ S is governed, besides the Ekman transport, by the geostrophic transport variability in the eastern basin. The geostrophic contribution of the western basin to the seasonal cycle of the AMOC is instead comparably weak, as transport variability in the western basin interior related to local wind curl forcing is mainly compensated by the Western Boundary Current. Our analyses indicate that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes not being properly represented in the model, uncertainties in the wind forcing are particularly relevant for the resulting uncertainties of AMOC estimates at 11∘ S.

scholarly journals The wind-driven overturning circulation of the World Ocean

2005 ◽  
Vol 2 (5) ◽  
pp. 473-505 ◽  
Author(s):  
K. Döös
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
World Ocean ◽  
Conveyor Belt ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Boundary ◽  
Overturning Circulation ◽  
The World ◽  
Meridional Overturning

Abstract. The wind driven aspects of the meridional overturning circulation of the world ocean and the Conveyor Belt is studied making use of a simple analytical model. The model consists of three reduced gravity layers with an inviscid Sverdrupian interior and a western boundary layer. The net north-south exchange is made possible by setting appropriate western boundary conditions, so that most of the transport is confined to the western boundary layer, while the interior is the Sverdrupian solution to the wind stress. The flow across the equator is made possible by the change of potential vorticity by the Rayleigh friction in the western boundary layer, which is sufficient to permit water and the Conveyor Belt to cross the equator. The cross-equatorial flow is driven by a weak meridional pressure gradient in opposite direction in the two layers on the equator at the western boundary. The model is applied to the World Ocean with a realistic wind stress. The amplitude of the Conveyor Belt is set by the northward Ekman transport in the Southern Ocean and the outcropping latitude of the NADW. It is in this way possible to set the amount of NADW that is pumped up from the deep ocean and driven northward by the wind and converted in the surface layer into less dense water by choosing the outcropping latitude and the depth of the layers at the western boundary. The model has proved to be able to simulate many of the key features of the Conveyor Belt and the meridional overturning cells of the World Ocean. This despite that there is no deep ocan mixing and that the water mass conversions in the this model are made at the surface.

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 Inferring the zonal distribution of measured changes in the meridional overturning circulation

2006 ◽  
Vol 3 (5) ◽  
pp. 1653-1660 ◽  
Author(s):  
A. de Boer ◽  
H. L. Johnson
Keyword(s):  
Dynamic Method ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Zonal Distribution ◽  
Deep Western Boundary Current ◽  
Meridional Overturning ◽  
Florida Straits ◽  
Southward Flow

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here, we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics), is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region, immediately outside of the Florida Straits.

scholarly journals Wind-Forced Variability of the Remote Meridional Overturning Circulation

2020 ◽  
Vol 50 (2) ◽  
pp. 455-469 ◽  
Author(s):  
Michael A. Spall ◽  
David Nieves
Keyword(s):  
Wind Stress ◽  
Rossby Wave ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Overturning Circulation ◽  
Deep Western Boundary Current ◽  
Wave Basin ◽  
Meridional Overturning

AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.

scholarly journals Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
F. Li ◽  
M. S. Lozier ◽  
S. Bacon ◽  
A. S. Bower ◽  
S. A. Cunningham ◽  
...  
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Minimal Impact ◽  
Water Formation ◽  
Meridional Overturning

AbstractChanges in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.

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