On the variability of the DWBC transport between 26.5°N and 16°N in an eddy-rich ocean model and its implications for meridionally coherent changes.

2021 ◽  
Author(s):  
Tobias Schulzki ◽  
Klaus Getzlaff ◽  
Arne Biastoch
Keyword(s):  
North Atlantic ◽  
Time Distribution ◽  
Ocean Model ◽  
High Variability ◽  
Western Boundary ◽  
Barotropic Instability ◽  
Boundary Current ◽  
Deep Western Boundary Current ◽  
Transit Time Distribution ◽  
Southward Flow

<p>The southward flow of North Atlantic Deep Water makes up the major component of the AMOC's deepwater limb. In the subtropical North Atlantic, it's flow is concentrated along the continental slope, forming a coherent Deep Western Boundary Current (DWBC). Both, observations and models show a high variability of the flow in this region.<br>We use an eddy-rich ocean model to show that this variability is mainly caused by eddies and meanders that are generated by barotropic instability. They occur along the entire DWBC pathway and introduce several reciruculation gyres that result in a decorrelation of DWBC transport at 26.5°N and 16°N, despite the fact that a considerable mean transport of 20 Sv connects the two latitudes. Water in the DWBC at 26.5°N is partly returned northward. Because the amount of water returned depends on the DWBC transport itself, a stronger DWBC does not necessarily lead to an increased amount of water that reaches 16°N. <br>Along the pathway to 16°N, the transport signal is altered by a broad and temporally variable transit time distribution. Thus, advection in the DWBC cannot account for coherent AMOC changes on interannual timescales seen in the model.</p>

scholarly journals Atlantic transport variability at 25° N in six hydrographic sections

Ocean Science ◽  
2012 ◽  
Vol 8 (4) ◽  
pp. 497-523 ◽  
Author(s):  
C. P. Atkinson ◽  
H. L. Bryden ◽  
S. A. Cunningham ◽  
B. A. King
Keyword(s):  
North Atlantic ◽  
Water Mass ◽  
Decadal Variability ◽  
Potential Temperature ◽  
Deep Ocean ◽  
Velocity Shear ◽  
Western Boundary ◽  
Boundary Current ◽  
Annual Average ◽  
Deep Western Boundary Current

Abstract. In January and February 2010, a sixth transatlantic hydrographic section was completed across 25° N, extending the hydrographic record at this latitude to over half a century. In combination with continuous transport measurements made since 2004 at 26.5° N by the Rapid-WATCH project, we reassess transport variability in the 25° N hydrographic record. Past studies of transport variability at this latitude have assumed transport estimates from each hydrographic section to represent annual average conditions. In this study the uncertainty in this assumption is assessed through use of Rapid-WATCH observations to quantify sub-seasonal and seasonal transport variability. Whilst in the upper-ocean no significant interannual or decadal transport variability are identified in the hydrographic record, in the deep ocean transport variability in both depth and potential temperature classes suggests some interannual or decadal variability may have occurred. This is particularly striking in the lower North Atlantic Deep Water where southward transports prior to 1998 were greater than recent transports by several Sverdrups. Whilst a cooling and freshening of Denmark Straits Overflow Water has occurred which is coincident with these transport changes, these water mass changes appear to be density compensated. Transport changes are the result of changing velocity shear in the vicinity of the Deep Western Boundary Current.

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.

The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

2020 ◽  
Vol 50 (9) ◽  
pp. 2735-2758
Author(s):  
Tiago Carrilho Biló ◽  
William E. Johns
Keyword(s):  
North Atlantic ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Deep Western Boundary Current ◽  
Mean Structure ◽  
The Mean ◽  
Theoretical Evidence ◽  
Anticyclonic Eddies ◽  
Geostrophic Velocities

AbstractThe mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

Dynamics of Deep Recirculation Cells Offshore of the Deep Western Boundary Current in the Subtropical North Atlantic (15°–30°N)

2021 ◽  
Vol 51 (1) ◽  
pp. 131-145
Author(s):  
Tiago Carrilho Biló ◽  
William E Johns ◽  
Jian Zhao
Keyword(s):  
North Atlantic ◽  
Primary Source ◽  
Water Layer ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Study Region ◽  
Deep Western Boundary Current ◽  
Deep Water Layer ◽  
Circulation Patterns

AbstractThe dynamics of the deep recirculation offshore of the deep western boundary current (DWBC) between 15° and 30°N within the upper North Atlantic Deep Water layer (1000 ≤ z ≤ 3000 m) is investigated with two different eddy-resolving numerical simulations. Despite some differences in the recirculation cells, our assessment of the modeled deep isopycnal circulation patterns (36.77 ≤ σ2 ≤ 37.06 kg m−3) shows that both simulations predict the DWBC flowing southward along the continental slope, while the so-called Abaco Gyre and two additional cyclonic cells recirculate waters northward in the interior. These cells are a few degrees wide, located along the DWBC path, and characterized by potential vorticity (PV) changes occurring along their mean streamlines. The analysis of the mean PV budget reveals that these changes result from the action of eddy forcing that tends to erode the PV horizontal gradients. The lack of a major upper-ocean boundary current within the study region, and the fact that the strongest eddy forcing is constrained within a few hundreds of kilometers of the western boundary, suggest that the DWBC is the primary source of eddy forcing. Finally, the eddies responsible for forcing the recirculation have dominant time scales between 100 and 300 days, which correspond to the primary observed variability scales of the DWBC transport at 26.5°N.

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