A bifurcation study of the three-dimensional thermohaline ocean circulation: the double-hemispheric case

Within a low-resolution primitive-equation model of the three-dimensional ocean circulation, a bifurcation analysis is performed of double-hemispheric basin flows. Main focus is on the connection between results for steady two-dimensional flows in a non-rotating basin and those for three-dimensional flows in a rotating basin. With the use of continuation methods, branches of steady states are followed in parameter space and their linear stability is monitored. There is a close qualitative similarity between the bifurcation structure of steady-state solutions of the two- and three dimensional flows. In both cases, symmetry-breaking pitchfork bifurcations are central in generating a multiple equilibria structure. The locations of these pitchfork bifurcations in parameter space can be characterized through a zero of the tendency of a particular energy functional. Although balances controlling the steady-state ?ows are quantitatively very di?erent, the zonally averaged patterns of the perturbations associated with symmetry-breaking are remarkably similar for two-dimensional and three-dimensional ?ows, and the energetics of the symmetry-breaking mechanism is in essence the same

Retroflection from a double-slanted coastline: a model for the Agulhas leakage variability

The Agulhas leakage to the South Atlantic exhibits a strong anti-correlation with the mass flux of the Agulhas Current. When the Agulhas retroflection is in its normal position near Cape Agulhas, leakage is relatively high and the nearby South African coastal slant (angle of derivation from zonal) is very small and relatively invariant alongshore. During periods of strong incoming flux (low leakage), the retroflection shifts upstream to Port Elizabeth or East London, where the coastline shape has a "kink", i.e., the slant changes abruptly from small on the west side, to large (about 55°) on the east side. Here, we show that the variability of rings shedding and anti-correlation between Agulhas mass flux and leakage to the South Atlantic may be attributed to this kink. <br><br> To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, a zonal western section and a strongly slanted eastern section. The principal difference between this and the model of a straight slanted coast (discussed in our earlier papers) is that, here, free purely westward propagation of eddies along the zonal coastline section is allowed. This introduces an interesting situation in which strong slant of the coast east of the kink prohibits the formation and shedding of rings, while the almost zonal coastal orientation west of the kink encourages shedding. Therefore, the kink "locks" the position of the retroflection, forcing it to occur just downstream of the kink. Rings are necessarily shed from the retroflection area in our kinked model, regardless of the degree of eastern coast slant. In contrast, a no-kink model with a coastline of intermediate slant indicates that shedding is almost completely arrested by that slant. <br><br> We suggest that the observed difference in ring-shedding intensity during times of normal retroflection position and times when the retroflection is shifted eastward is due to the change in the retroflection location with respect to the kink. When the incoming flux detaches from the coast north of the kink, ring transport is small; when the flux detaches south of the kink, transport is large. Simple process-oriented numerical simulations are in fair agreement with our analytical results

Stability of the Atlantic overturning circulation: competition between Bering Strait freshwater flux and Agulhas heat and salt sources

In this study we examine the role that is played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, the Drake Passage and via Agulhas Leakage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resolution ocean model (OCCAM). When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the northern hemisphere (NPP circulation), and resembles the present-day Atlantic overturning circulation. The Southern Ocean sources appear to stabilize this circulation, whereas Bering Strait freshwater input tends to destabilize it. Already a small buoyancy input at southerly latitudes is enough to prohibit the existence of a southern sinking circulation (SPP),lea ving the NPP circulation as a unique and stable solution. A large, factor three increase in Bering Strait freshwater import would be necessary to bring the SPP circulation back into existence. Especially the Indian-Atlantic transfer of heat and salt, brought about by Agulhas Leakage,c ontributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model,sh utting off Agulhas Leakage, and consequently the so-called warm water route for North Atlantic Deep Water (NADW) compensation, leads to a reduction of the overturning strength by 10% at most. These results imply that the way in which the NADW renewal takes place has implications for both the strength and stability of the Atlantic overturning circulation,giv ing the discussion about the warm vs. cold water route for NADW compensation dynamical significance. Moreover, when the stabilizing effect of Agulhas Leakage on the overturning disappears, the destabilizing influence of the Bering Strait freshwater input becomes more effective. The system is then close to a regime where the northern and southern overturning circulations coexist as stable solutions. Perturbations in Bering Strait inflow may then easily lead to switches between the two circulation states. These results suggest that the absence of Agulhas Leakage during the last ice-age may have contributed to weakening the glacial overturning circulation in the Atlantic. It may have made the thermohaline circulation vulnerable to variability, caused either by regime switches, or by the excitation of oscillatory modes. The sudden restart of the Atlantic overturning circulation at the beginning of the Holocene may well have been stimulated by the coincident reopening of the Agulhas gap. The presence of Agulhas Leakage may contribute to the relative stability of Holocene climate. Present-day climate may thus be more stable than previously thought

Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy

The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, that extend well into the intermediate layers. It is shown that the models overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change. The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The nonlinear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic. Two additional experiments illustrate the adjustment of the overturning circulation upon sudden introduction of heat and salt sources in the South Atlantic. The North Atlantic overturning circulation responds within a few years after the sources are switched on. This is the time it takes for barotropic and baroclinic Kelvin waves to reach the northern North Atlantic. The advection of the anomalies takes 3 decades to reach the northern North Atlantic. The model results give support to the hypothesis that the re-opening of the Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological data, may have stimulated the coincident strengthening of the Atlantic overturning circulation

Impact of interbasin exchange on the Atlantic overturning circulation

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed ofth ermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic. The sensitivity ofth e OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60°N and 30°S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30°S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources ofp otential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the strati fication, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition oft he NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably. It is also shown that the circulation is much more sensitive to changes in the shape ofth e lateral buoyancy flux than to changes in the shape ofth e surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes ofhe at and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies

Coupled Oscillators with Chemotaxis

A simple coupled oscillator system with chemotaxis is introduced to study morphogenesis of cellular slime molds. The model successfuly explains the migration of pseudoplasmodium which has been experimentally predicted to be lead by cells with higher intrinsic frequencies. Results obtained predict that its velocity attains its maximum value in the interface region between total locking and partial locking and also suggest possible roles played by partial synchrony during multicellular development.Comment: 4 pages, 5 figures, latex using jpsj.sty and epsf.sty, to appear in J. Phys. Soc. Jpn. 67 (1998

Prediction of sequential antigenic regions in proteins

AbstractPrediction of antigenic regions in a protein will be helpful for a rational approach to the synthesis of peptides which may elicit antibodies reactive with the intact protein. Earlier methods are based on the assumption that antigenic regions are primarily hydrophilic regions at the surface of the protein molecule. The method presented here is based on the amino acid composition of known antigenic regions in 20 proteins which is compared with that of 314 proteins [(1978) Atlas of Protein Sequence and Structure, vol. 5, suppl. 3, 363-373]. Antigenicity values were derived from the differences between the two data sets. The method was applied to bovine ribonuclease, the B-subunit of cholera toxin and herpes simplex virus type 1 glycoprotein D. There was a good correlation between the predicted regions and previously determined antigenic regions

Can the salt-advection feedback be detected in internal variability of the atlantic meridional overturning circulation?

Evidence for the assumptions of the salt-advection feedback in box models is sought by studying the Atlantic meridional overturning circulation (AMOC) internal variability in the long preindustrial control runs of two Earth system models. The first assumption is that AMOC strength is proportional to the meridional density difference between the North Atlantic and the Southern Oceans. The model simulations support this assumption, with the caveat that nearly all the long time-scale variability occurs in the North Atlantic density. The second assumption is that the freshwater transport variability by the overturning at the Atlantic southern boundary is controlled by the strength of AMOC. Only one of the models shows some evidence that AMOC variability at 45°N leads variability in the overturning freshwater transport at the southern boundary by about 30 years, but the other model shows no such coherence. In contrast, in both models this freshwater transport variability is dominated by local salinity variations. The third assumption is that changes in the overturning freshwater transport at the Atlantic southern boundary perturb the north-south density difference, and thus feed back on AMOC strength in the north. No evidence for this assumption is found in either model at any time scale, although this does not rule out that the salt-advection feedback may be excited by a strong enough freshwater perturbation

OLED Encapsulation by Room Temperature Plasma-Assisted ALD Al2O3 Films

Organic light emitting diodes (OLEDs, both small molecule and polymer LEDs) require excellent gas and moisture permeation barrier layers to increase their lifetime. The quality of the barrier layer is ultimately controlled by the presence of defects in the layer. Although a barrier layer may be intrinsically excellent (water vapor transmission rate, WVTR = 10-6 g·m-2·day-1) the protected device may fail in the presence of defects that lead to preferential diffusion pathways for H2O (e.g., defects caused by particles from the environment and/or production process). The state-of-the-art barrier coatings are micrometer-thick multi-layer structure, in which organic interlayers are alternated with inorganic barrier layers with the purpose of decoupling the above-mentioned defects. Recently, atomic layer deposition (ALD) has been successfully tested for the deposition of very thin (<50 nm) single layer permeation barriers on pristine polymer substrates [1,2], showing the potential of this highly uniform and conformal deposition technique in the field of moisture permeation barriers. In this contribution the encapsulation of OLEDs by plasma-assisted ALD of thin (20-40 nm) Al2O3 layers is addressed. The layers are synthesized at room temperature by sequentially exposing the substrate to Al(CH3)3 vapor and a remote inductively coupled O2 plasma in Oxford Instruments FlexALTM and OpALTM reactors. The intrinsic quality of the deposited ALD layers was determined by monitoring the oxidation of a Ca film encapsulated by the Al2O3 film: WVTR values as low as 2·10-6 g·m-2·day-1 have been measured. The potential of ALD layers in encapsulating OLEDs, and therefore in successfully covering the defects present on the device, has been investigated by means of electroluminescence measurements of polymer-LEDs (effective emitting area of 5.8 cm2). The black spot density and area growth were followed as a function of the time under standard conditions of 20°C and 50% relative humidity. Within a 500 h test ALD-encapsulated OLEDs show approximately half the black spot density compared to devices encapsulated by plasma deposited a-SiNx:H (300 nm thick). The black spot density is further reduced by combining the a-SiNx:H and ALD Al2O3 layers. These results point towards a very promising application of ALD Al2O3 layers in the field of OLED encapsulation and will be interpreted in terms of possible mechanisms related to film growth in multi-layer structures

Human Fronto-Tectal and Fronto-Striatal-Tectal Pathways Activate Differently During Anti-Saccades

Almost all cortical areas in the vertebrate brain take part in recurrent connections through the subcortical basal ganglia (BG) nuclei, through parallel inhibitory and excitatory loops. It has been suggested that these circuits can modulate our reactions to external events such that appropriate reactions are chosen from many available options, thereby imposing volitional control over behavior. The saccade system is an excellent model system to study cortico-BG interactions. In this study two possible pathways were investigated that might regulate automaticity of eye movements in the human brain; the cortico-tectal pathway, running directly between the frontal eye fields (FEF) and superior colliculus (SC) and the cortico-striatal pathway from the FEF to the SC involving the caudate nucleus (CN) in the BG. In an event-related functional magnetic resonance imaging (fMRI) paradigm participants made pro- and anti-saccades. A diffusion tensor imaging (DTI) scan was made for reconstruction of white matter tracts between the FEF, CN and SC. DTI fiber tracts were used to divide both the left and right FEF into two sub-areas, projecting to either ipsilateral SC or CN. For each of these FEF zones an event-related fMRI timecourse was extracted. In general activity in the FEF was larger for anti-saccades. This increase in activity was lateralized with respect to anti-saccade direction in FEF zones connected to the SC but not for zones only connected to the CN. These findings suggest that activity along the contralateral FEF–SC projection is responsible for directly generating anti-saccades, whereas the pathway through the BG might merely have a gating function withholding or allowing a pro-saccade