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Henk A. Dijkstra and Wilhelmus P. M. de Ruijter

Abstract

From previous model studies, it has become clear that several physical mechanisms may be at work in the retroflection of the Agulhas Current. Here, a systematic study of steady barotropic flows connecting the Indian Ocean and South Atlantic Ocean in several idealized setups is performed. By solving directly for the steady circulation with continuation methods, the connection between different retroflection regimes can be monitored as external conditions, such as the wind forcing or bottom topography, as well as parameters, such as the lateral friction and layer depth, are changed. To distinguish the different steady retroflecting flows, an objective measure of the degree of retroflection, a retroflection index R, is introduced. By monitoring R along a branch of steady solutions, using the horizontal friction as control parameter, several steady retroflecting regimes are found. At large friction there exist stable steady states with viscously dominated retroflection. When friction is decreased, inertial effects become more dominant, and eventually unstable steady states with strong retroflection characteristics exist. Within this framework, different results from earlier studies can be reconciled.

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Wilbert Weijer, Wilhelmus P. M. De Ruijter, and Henk A. Dijkstra

Abstract

The role played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, Drake Passage, and 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 (northern sinking pole to pole circulation, NPP), 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), leaving the NPP circulation as a unique and stable solution. A large, factor 3 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, contributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model, shutting off the 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, giving the discussion about the warm versus cold water route for NADW compensation dynamical significance.

Moreover, when the stabilizing effect of the 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 the Agulhas Leakage during the last ice age may have contributed to weakening of 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.

Presence of the Agulhas Leakage may contribute to the relative stability of Holocene climate. Present-day climate may thus be more stable than previously thought.

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Hsien Wang Ou and Wilhelmus P. M. De Ruijter

Abstract

A two-layer model is used to examine the separation of an inertial boundary current from a curved coastline and its subsequent path as a free jet. To isolate the inertial effect, the boundary current is confined to the upper layer and insulated from the ocean interior by a free streamline. The separation occurs when the interface outcrops and forms a free streamline. Besides the constraint imposed by the coastal boundary, the primary dimensionless parameter that regulates the separation point and the subsequent current path is the scaled volume flux of the current (Q). Increasing Q caused the current to separate at a lower latitude. The separation also occurs where the coastline has a large positive curvature (i.e., convex outward). After the separation, the current can either mender or loop back on itself depending on the flow direction at the separation point. Application of the model to the Agulhas Current can reproduce the retroflection feature (i.e., a current turning back on itself) with roughly the correct dimensions, suggesting that the inertial and beta effect play a dominant role in the phenomenon.

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Wilbert Weijer, Wilhelmus P. M. de Ruijter, Henk A. Dijkstra, and Peter Jan van Leeuwen

Abstract

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a dataset 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 of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic.

The sensitivity of the 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 of potential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the stratification, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition of the 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 of the lateral buoyancy flux than to changes in the shape of the surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes of heat 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.

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Dewi Le Bars, Wilhelmus P. M. De Ruijter, and Henk A. Dijkstra

Abstract

An analysis of the Indian Ocean circulation and the Agulhas Current retroflection is carried out using a primitive equation model with simplified coastline and flat bottom. Four configurations with 0.25° and 0.1° horizontal resolution and in barotropic and baroclinic cases are considered. The wind stress is taken as control parameter to increase the inertia of the currents. The volume transport of the Indonesian Throughflow, Mozambique Channel, and Agulhas Current are found to increase linearly with the wind stress strength, and three nonlinear retroflection regimes are found. A viscous and an inertial regime had already been documented, but a new turbulent regime appears at large wind stress amplitude. In this turbulent regime, the volume of Agulhas leakage reaches a plateau because of strong mesoscale variability and, in contrast to the other regimes, does not depend on the wind stress magnitude. The physical mechanism causing the plateau is shown to be associated with the cross-jet exchange of Indian Ocean water and water from the Antarctic Circumpolar Current. In the turbulent regime, the permeability of the Agulhas Return Current to material transport increases and the Indian Ocean water available for the Agulhas leakage decreases.

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Wilhelmus P. M. de Ruijter, Peter Jan van Leeuwen, and Johann R. E. Lutjeharms

Abstract

Solitary meanders of the Agulhas Current, so-called Natal pulses, may play an important role in the overall dynamics of this current system. Several hypotheses concerning the triggering of these pulses are tested using sea surface height and temperature data from satellites. The data show the formation of pulses in the Natal Bight area at irregular intervals ranging from 50 to 240 days. Moving downstream at speeds between 10 and 20 km day−1 they sometimes reach sizes of up to 300 km. They seem to play a role in the shedding of Agulhas rings that penetrate the South Atlantic. The intermittent formation of these solitary meanders is argued to be most probably related to barotropic instability of the strongly baroclinic Agulhas Current in the Natal Bight. The vorticity structure of the observed basic flow is argued to be stable anywhere along its path. However, a proper perturbation of the jet in the Natal Bight area will allow barotropic instability, because the bottom slope there is considerably less steep than elsewhere along the South African east coast. Using satellite altimetry these perturbations seem to be related to the intermittent presence of offshore anticyclonic anomalies, both upstream and eastward of the Natal Bight.

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Caroline A. Katsman, Paul C. F. Van der Vaart, Henk A. Dijkstra, and Wilhelmus P. M. de Ruijter

Abstract

Ocean rings, when isolated from major ocean currents, can have life spans on the order of years. This study focuses on the stability of such isolated ocean rings. Assuming axisymmetric basic-state profiles, the linear stability of a wide variety of rings is analyzed by examining the properties of the modes to which they become unstable and the associated energy conversions. Earlier studies have indicated that corotating rings, with a large barotropic component, are far less unstable than counterrotating ones. This sharp contrast between co- and counterrotating rings appears to be a consequence of the choice for a radial profile of the azimuthal velocity that decays only gradually on the ring's outer flank. For more realistic velocity profiles, co- and counterrotating rings have similar growth rates. Nearly compensated rings, that is, those with a weak flow in the deepest layer, are found to be the least unstable ones. In this paper, the problem for warm-core rings with a Gaussian profile is first revisited in a two-layer setup. A systematic survey of the sensitivity of the results for this standard case with respect to various ring parameters, such as the stratification, ring width, and, in particular, the radial profile of the azimuthal velocity, is presented. Besides exponential profiles, as used in earlier studies, the stability of rings with a core in solid-body rotation is also examined. Subsequently, more realistic cases are considered by discussing the stability of ocean rings designed as fits to an observed cold-core Gulf Stream ring and a warm-core Agulhas ring. Minimal growth rates for the latter rings are very large: the calculated e-folding timescales are about one week.

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