Sensitivity of the Atlantic Thermohaline Circulation and Its Stability to Basin-Scale Variations in Vertical Mixing

2006 ◽  
Vol 19 (21) ◽  
pp. 5467-5478 ◽  
Author(s):  
Willem P. Sijp ◽  
Matthew H. England
Keyword(s):  
North Atlantic ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Vertical Mixing ◽  
Intermediate Water ◽  
Stable States ◽  
Atlantic Basin ◽  
Interhemispheric Competition ◽  
Basin Scale ◽  
The Stability

Abstract This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drastically increase North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied to the North Atlantic. This is contrary to the notion that the stability of the ocean’s thermohaline circulation simply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliant upon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable states where NADW is collapsed. Transitions between NADW “on” and “off” states are characterized by interhemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reverse cell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in vertical mixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell, resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cell is responsible for the stability of NADW collapsed states, and thereby plays a key role in maintaining multiple equilibria in the climate system. A global increase of vertical mixing in the model results in significantly enhanced NADW stability, as found in previous studies. However, an enhancement of vertical mixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded that the stability of NADW formation to freshwater perturbations depends critically on the basin-scale distribution of vertical mixing in the world’s oceans.

Can Isopycnal Mixing Control the Stability of the Thermohaline Circulation in Ocean Climate Models?

2006 ◽  
Vol 19 (21) ◽  
pp. 5637-5651 ◽  
Author(s):  
Willem P. Sijp ◽  
Michael Bates ◽  
Matthew H. England
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Climate Models ◽  
Thermohaline Circulation ◽  
Vertical Component ◽  
Coupled Model ◽  
General Circulation Models ◽  
Tracer Transport ◽  
Horizontal Diffusion ◽  
The Stability

Abstract Convective overturning arising from static instability during winter is thought to play a crucial role in the formation of North Atlantic Deep Water (NADW). In ocean general circulation models (OGCMs), a strong reduction in convective penetration depth arises when horizontal diffusion (HD) is replaced by Gent and McWilliams (GM) mixing to model the effect of mesoscale eddies on tracer advection. In areas of sinking, the role of vertical tracer transport due to convection is largely replaced by the vertical component of isopycnal diffusion along sloping isopycnals. Here, the effect of this change in tracer transport physics on the stability of NADW formation under freshwater (FW) perturbations of the North Atlantic (NA) in a coupled model is examined. It is found that there is a significantly increased stability of NADW to FW input when GM is used in spite of GM experiments exhibiting consistently weaker NADW formation rates in unperturbed steady states. It is also found that there is a significant increase in NADW stability upon the introduction of isopycnal diffusion in the absence of GM. This indicates that isopycnal diffusion of tracer rather than isopycnal thickness diffusion is responsible for the increased NADW stability observed in the GM run. This result is robust with respect to the choice of isopycnal diffusion coefficient. Also, the NADW behavior in the isopycnal run, which includes a fixed background horizontal diffusivity, demonstrates that HD is not responsible in itself for reducing NADW stability when simple horizontal diffusion is used. Our results suggest that care should be taken when interpreting the results of coarse grid models with regard to NADW sensitivity to FW anomalies, regardless of the choice of mixing scheme.

Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms

Science ◽  
2012 ◽  
Vol 337 (6090) ◽  
pp. 54-58 ◽  
Author(s):  
Amala Mahadevan ◽  
Eric D’Asaro ◽  
Craig Lee ◽  
Mary Jane Perry
Keyword(s):  
Density Gradient ◽  
North Atlantic ◽  
Climate Models ◽  
Light Exposure ◽  
Vertical Mixing ◽  
Three Dimensional ◽  
Biophysical Model ◽  
Phytoplankton Blooms ◽  
Near Surface ◽  
Basin Scale

Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.

scholarly journals On the Mediterranean Water Composition

2016 ◽  
Vol 46 (4) ◽  
pp. 1339-1358 ◽  
Author(s):  
L. I. Carracedo ◽  
P. C. Pardo ◽  
S. Flecha ◽  
F. F. Pérez
Keyword(s):  
North Atlantic ◽  
Sea Water ◽  
Source Region ◽  
Intermediate Water ◽  
Strait Of Gibraltar ◽  
Atlantic Basin ◽  
North Atlantic Basin ◽  
Water Composition ◽  
The Mediterranean ◽  
Mediterranean Water

AbstractThe Mediterranean Outflow Water (MOW) spills from the Mediterranean Sea (east North Atlantic basin) west off the Strait of Gibraltar. As MOW outflows, it entrains eastern North Atlantic Central Waters (ENACW) and Intermediate Waters to form the neutrally buoyant Mediterranean Water (MW) that can be traced over the entire North Atlantic basin. Its high salinity content influences the thermohaline properties of the intermediate–deep water column in the North Atlantic and its dynamics. Here, the composition of MW in its source region (the Gulf of Cádiz, west off Strait of Gibraltar) is investigated on the basis of an optimum multiparameter analysis. The results obtained indicate that mixing of MOW (34.1% ± 0.3%) occurs mainly with overlying ENACW (57.1% ± 0.8%) in a process broadly known as central water entrainment. A diluted form (80% of dilution) of the Antarctic Intermediate Water (AAIW) reaches the region and also takes part in MW formation (8.3% ± 0.5%). Finally, the underlying Labrador Sea Water (LSW) also contributes (0.4% ± 0.1%) to the characteristics of MW. From these results and considering 0.74 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) as the mean outflow of MOW, the MW exportation rate was inferred (2.2 Sv), which, decomposing MW, means that the MOW outflow is accompanied by 1.24 Sv of entrained ENACW, 0.18 Sv of AAIW, and <0.01 Sv of LSW.

Role of the Drake Passage in Controlling the Stability of the Ocean’s Thermohaline Circulation

2005 ◽  
Vol 18 (12) ◽  
pp. 1957-1966 ◽  
Author(s):  
Willem P. Sijp ◽  
Matthew H. England
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Deep Water ◽  
Bottom Water ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Drake Passage ◽  
Global Ocean ◽  
Intermediate Water

Abstract The role of a Southern Ocean gateway in permitting multiple equilibria of the global ocean thermohaline circulation is examined. In particular, necessary conditions for the existence of multiple equilibria are studied with a coupled climate model, wherein stable solutions are obtained for a range of bathymetries with varying Drake Passage (DP) depths. No transitions to a Northern Hemisphere (NH) overturning state are found when the Drake Passage sill is shallower than a critical depth (1100 m in the model described herein). This preference for Southern Hemisphere sinking is a result of the particularly cold conditions of the Antarctic Bottom Water (AABW) formation regions compared to the NH deep-water formation zones. In a shallow or closed DP configuration, this forces an exclusive production of deep/bottom water in the Southern Hemisphere. Increasing the depth of the Drake Passage sill causes a gradual vertical decoupling in Atlantic circulation, removing the influence of AABW from the upper 2000 m of the Atlantic Ocean. When the DP is sufficiently deep, this shifts the interaction between a North Atlantic Deep Water (NADW) cell and an AABW cell to an interaction between an (shallower) Antarctic Intermediate Water cell and an NADW cell. This latter situation allows transitions to a Northern Hemisphere overturning state.

Transport of heat, CO 2 and O 2 by the Atlantic’s thermohaline circulation

1995 ◽  
Vol 348 (1324) ◽  
pp. 133-142 ◽  
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Thermohaline Circulation ◽  
North Atlantic Deep Water ◽  
Return Flow ◽  
Intermediate Water ◽  
Biological Processes ◽  
Antarctic Intermediate Water ◽  
The North ◽  
The North Atlantic

We estimate transport of heat, CO 2 and O 2 by the Atlantic’s thermohaline circulation using an approach based on differences in the chemical and physical characteristics of North Atlantic Deep Water (NADW), Antarctic Intermediate Water (AAIW), and the northward return flow across the equator. The characteristics of the return-flow waters are constrained by imposing conservation of phosphate in the North Atlantic as a whole. Based on a total equatorial return flow of 13 x 10 6 m 3 s -1 , we find that the Atlantic north of the equator is a source of 7.7 ± 1.4 x 10 14 W to the atmosphere, a sink of 0.51 ± 0.21 x 10 14 mol of O 2 , and preindustrially was a sink of 0.33 ± 0.15 x 10 14 mol of CO 2 . Uptake of O 2 and CO 2 by the North Atlantic is driven mainly by thermal, as opposed to biological processes.

scholarly journals North Atlantic Basin-Scale Multi-Criteria Assessment Database to Inform Effective Management and Protection of Vulnerable Marine Ecosystems

2021 ◽  
Vol 8 ◽  
Author(s):  
Telmo Morato ◽  
Christopher K. Pham ◽  
Laurence Fauconnet ◽  
Gerald H. Taranto ◽  
Giovanni Chimienti ◽  
...  
Keyword(s):  
North Atlantic ◽  
Marine Ecosystems ◽  
Effective Management ◽  
Atlantic Basin ◽  
North Atlantic Basin ◽  
Basin Scale

scholarly journals The Effect of the Source of Deep Water in the Eastern Mediterranean on Western Mediterranean Intermediate and Deep Water

2021 ◽  
Vol 7 ◽  
Author(s):  
Yael Amitai ◽  
Yosef Ashkenazy ◽  
Hezi Gildor
Keyword(s):  
Mediterranean Sea ◽  
Deep Water ◽  
Multiple Equilibria ◽  
Eastern Mediterranean ◽  
Western Mediterranean ◽  
Intermediate Water ◽  
Stable States ◽  
Lagrangian Analysis ◽  
Water Characteristics ◽  
Deep Layers

The deep water in the western Mediterranean Sea was found to be significantly affected by a climatic event that took place in the eastern Mediterranean during the 1990s. Numerical simulations of the entire Mediterranean Sea showed that multiple equilibria states in the eastern Mediterranean can exist under present-day-like conditions. The two stable states that were found are associated with intermediate water exchange between the eastern Mediterranean's Aegean and Adriatic Basins. In the first state, the Adriatic acts as a source of deep water that flows into the deep layers of the eastern Mediterranean; in the second state, there is no source of deep water in the Adriatic and the eastern Mediterranean intermediate water is warmer and saltier. We studied the water pathways, in both stable states, into the western Mediterranean and found that the eastern Mediterranean water's properties signature can be seen as far as the Gulf of Lion, which is an important open-ocean deep water convection site. Meaning that, the eastern Mediterranean water characteristics are manifested in deep and intermediate water properties all over the Mediterranean Sea. The water propagating from the eastern to the western Mediterranean also has different flow regimes, in both states, through the Sicily Strait and in the Tyrrhenian Basin, as seen from a Lagrangian analysis.

Sign in / Sign up

Export Citation Format

Share Document