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Henk A. Dijkstra
,
Wilbert Weijer
, and
J. David Neelin

Abstract

Different equilibria of oceanic thermohaline circulation may exist under the same forcing conditions. One of the reasons for the existence of these multiple equilibria is a feedback between the overturning circulation and the advective transport of salt and heat. In an equatorially symmetric configuration, the multiple equilibria arise through symmetry-breaking pitchfork bifurcations when the strength of the freshwater forcing is increased. Here, continuation methods are used to track the fate of the different equilibria under equatorially asymmetric conditions in a three-dimensional, low-resolution ocean general circulation model in an Atlantic-like basin coupled to an energy-balance atmosphere model. The effect of the continental geometry, the presence of the Antarctic Circumpolar Current (ACC), and asymmetric air–sea interaction on the preference of equilibria are considered. Although all asymmetry-inducing mechanisms favor northern Atlantic sinking states, the open Southern Ocean and ACC are shown to be substantial contributors. The origin of the hysteresis behavior between strong and weak overturning states is clarified in terms of the overall bifurcation picture. The disappearance of a class of southern sinking equilibria because of the combined effects of all asymmetry mechanisms leads to a substantial regime with a unique steady state. The relationship between the hysteresis regime and the unique-state regime provides a larger context for quantitative determination of the relevance of each to climate.

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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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Wilbert Weijer
,
Frédéric Vivier
,
Sarah T. Gille
, and
Henk A. Dijkstra

Abstract

Observations of the sea surface height in the Argentine Basin indicate that strong variability occurs on a time scale of 20−30 days. The aim of this study is to determine the physical processes responsible for this variability. First, results are presented from two statistical techniques applied to a decade of altimetric data. A complex empirical orthogonal function (CEOF) analysis identifies the recently discovered dipole mode as the dominant mode of variability. A principal oscillation pattern (POP) analysis confirms the existence of this mode, which has a period of 25 days. The second CEOF displays a propagating pattern in the northern Argentine Basin, plus a rotating dipole in the southwest corner. The POP analysis identifies both patterns as individual modes, with periods of 30 and 20 days, respectively. Second, the barotropic normal modes of the Argentine Basin are studied, using a shallow-water model capturing the full bathymetry of the basin. Coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability. This analysis implies that the 20-day mode detected by recent bottom-pressure measurements is a true barotropic mode. However, the 25-day variability, as found in altimeter data, cannot be directly attributed to the excitation of a free Rossby basin mode. This study indicates that the results of several apparently conflicting observations of the flow variability in the Argentine Basin can be reconciled by assuming that multiple basin modes are involved.

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Wilbert Weijer
,
Frédéric Vivier
,
Sarah T. Gille
, and
Henk A. Dijkstra

Abstract

In this paper the spectrum of barotropic basin modes of the Argentine Basin is shown to be connected to the classical Rossby basin modes of a flat-bottom (constant depth), rectangular basin. First, the spectrum of basin modes is calculated for the Argentine Basin, by performing a normal-mode analysis of the barotropic shallow-water equations. Then a homotopy transformation is performed that gradually morphs the full-bathymetry geometry through a flat-bottom configuration into a rectangular basin. Following the eigenmodes through this transition establishes a connection between most of the basin modes and the classical Rossby basin modes of a rectangular geometry. In particular, the 20-day mode of the Argentine Basin is identified with the lowest-order mode of classical theory. Sensitivity studies show that the decay rate of each mode is controlled by bottom friction, but that it is insensitive to lateral friction; lateral friction strongly impacts the oscillation frequency. In addition, the modes are found to be only slightly sensitive to the presence of a background flow.

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Janine J. Nauw
,
Henk A. Dijkstra
, and
Eric P. Chassignet

Abstract

The effect of the parameterization of lateral friction on the separation of western boundary currents is addressed in an idealized context. The study is motivated by a puzzling issue that arises from the nonlinear theory of the wind-driven double-gyre circulation in shallow-water models. Subtle changes in the representation of the lateral friction in these models have a substantial effect on both steady-state and transient flows. The aim of this paper is to explain how lateral friction introduces a north–south asymmetry in the steady double-gyre flows and why the degree of this asymmetry depends on the type of frictional parameterization. A more conceptual model of a zonal jet in a channel turns out to be very useful to determine the dynamical processes behind the asymmetries. It is also shown that the north–south asymmetries have an impact on the low-frequency variability of the time-dependent flows. This is caused by changes in stability behavior of the steady-state flows.

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Selma E. Huisman
,
Matthijs den Toom
,
Henk A. Dijkstra
, and
Sybren Drijfhout

Abstract

Recent model results have suggested that there may be a scalar indicator Σ monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity Σ is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of Σ is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to Σ. The sign of Σ precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.

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Leela M. Frankcombe
,
Henk A. Dijkstra
, and
Anna von der Heydt

Abstract

In this paper it is proposed that the stochastic excitation of a multidecadal internal ocean mode is at the origin of the multidecadal sea surface temperature variability in the North Atlantic. The excitation processes of the spatial sea surface temperature pattern associated with this multidecadal mode within an idealized three-dimensional model are studied by adding noise to the surface heat flux forcing. In the regime where the internal mode is damped, the amplitude of its sea surface temperature pattern depends on the type of noise forcing applied. While the mode is weakly excited by white noise, only the introduction of spatial and temporal coherence in the forcing, with characteristics of the North Atlantic Oscillation in particular, causes the amplitude of the variability to increase to levels comparable to those observed. Within this idealized model the physical mechanism of the excitation can be determined: the presence of the noise rectifies the background state and consequently changes the growth factor of the internal mode.

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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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Werner Kramer
,
Henk A. Dijkstra
,
Stefano Pierini
, and
Peter Jan van Leeuwen

Abstract

In this paper, sequential importance sampling is used to assess the impact of observations on an ensemble prediction for the decadal path transitions of the Kuroshio Extension. This particle-filtering approach gives access to the probability density of the state vector, which allows the predictive power—an entropy-based measure—of the ensemble prediction to be determined. The proposed setup makes use of an ensemble that, at each time, samples the climatological probability distribution. Then, in a postprocessing step, the impact of different sets of observations is measured by the increase in predictive power of the ensemble over the climatological signal during one year. The method is applied in an identical-twin experiment for the Kuroshio Extension using a reduced-gravity shallow-water model. This study investigates the impact of assimilating velocity observations from different locations during the elongated and the contracted meandering states of the Kuroshio Extension. Optimal observation locations correspond to regions with strong potential vorticity gradients. For the elongated state the optimal location is in the first meander of the Kuroshio Extension. During the contracted state it is located south of Japan, where the Kuroshio separates from the coast.

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Matthijs den Toom
,
Henk A. Dijkstra
,
Andrea A. Cimatoribus
, and
Sybren S. Drijfhout

Abstract

The impact of atmospheric feedbacks on the multiple equilibria (ME) regime of the Atlantic meridional overturning circulation (MOC) is investigated using a fully implicit hybrid coupled model (HCM). The HCM consists of a global ocean model coupled to an empirical atmosphere model that is based on linear regressions of the heat, net evaporative, and momentum fluxes generated by a fully coupled climate model onto local as well as Northern Hemisphere averaged sea surface temperatures. Using numerical continuation techniques, bifurcation diagrams are constructed for the HCM with the strength of an anomalous freshwater flux as the bifurcation parameter, which allows for an efficient first-order estimation of the effect of interactive surface fluxes on the MOC stability. The different components of the atmospheric fluxes are first considered individually and then combined. Heat feedbacks act to destabilize the present-day state of the MOC and to stabilize the collapsed state, thus leaving the size of the ME regime almost unaffected. In contrast, interactive freshwater fluxes cause a destabilization of both the present-day and collapsed states of the MOC. Wind feedbacks are found to have a minor impact. The joint effect of the three interactive fluxes is to narrow the range of ME. The shift of the saddle-node bifurcation that terminates the present-day state of the ocean is further investigated by adjoint sensitivity analysis of the overturning rate to surface fluxes. It is found that heat feedbacks primarily affect the MOC stability when they change the heat fluxes over the North Atlantic subpolar gyre, whereas interactive freshwater fluxes have an effect everywhere in the Atlantic basin.

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