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Sybren S. Drijfhout

Abstract

The response of the tropical atmosphere to a collapse of the thermohaline circulation (THC) is investigated by comparing two 5-member ensemble runs with a coupled climate model (CCM), the difference being that in one ensemble a hosing experiment was performed. An extension of the Held–Hou–Lindzen model for the Hadley circulation is developed to interpret the results. The forcing associated with a THC collapse is qualitatively similar to, but smaller in amplitude than, the solstitial shift from boreal summer to winter. This forcing results from reduced ocean heat transport creating an anomalous cross-equatorial SST gradient. The small amplitude of the forcing makes it possible to arrive at analytical expressions using standard perturbation theory. The theory predicts the latitudinal shift between the Northern Hemisphere (NH) and Southern Hemisphere (SH) Hadley cells, and the relative strength of the anomalous cross-equatorial Hadley cell compared to the solstitial cell. The poleward extent of the Hadley cells is controlled by other physics. In the NH the Hadley cell contracts, while zonal velocities increase and the subtropical jet shifts equatorward, whereas in the SH cell the opposite occurs. This behavior can be explained by assuming that the poleward extent of the Hadley cell is determined by baroclinic instability: it scales with the inverse of the isentropic slopes. Both theory and CCM results indicate that a THC collapse and changes in tropical circulation do not act in competition, as a possible explanation for abrupt climate change; they act in concert.

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Sybren S. Drijfhout

Abstract

The role of mesoscale eddies in the poleward heat transport in the ocean is investigated; in particular, the compensation of poleward eddy heat transport by an eddy-induced mean meridional circulation is examined.

A multilayer isopycnic primitive equation model of an idealized western North Atlantic is presented to test whether compensation also occurs within an isopycnic model and when the poleward eddy heat transport becomes comparable to the mean transport.

Also, in this model configuration compensation of the poleward eddy heat transport arises. It is brought about by a westward eddy heat transport in the midlatitude jet, which results in a pressure drop across the basin and consequently in a modified mean meridional overturning.

This compensation is discussed within the framework of wave–mean flow interaction. It is demonstrated that compensation results from eddy–mean flow interaction when the diabatic forcing is sufficiently weak; rings are recaptured before their SST anomaly is modified significantly. When the time scale of SST anomalies is smaller than the lifetime of mesoscale rings, it is hypothesized that the non–heat transport character of eddies breaks down.

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Sybren S. Drijfhout

Abstract

Various ocean circulation models have been compared with respect to their performance in the genesis of rings and the subsequent heat transport. Emphasis has been placed on the role of the spurious diapycnal fluxes of heat and momentum in Cartesian models, arising when the horizontal dissipation mixes through sloping isopycnals.

Quasigeostrophic, isopycnal coordinate, and Cartesian primitive equation models in a two-layer periodic channel domain have been used to simulate the process of eddy detachment from an eastward-flowing jet. This jet is modeled after the Gulf Stream east of Cape Hatteras. On this jet a small sinusoidal disturbance is super-imposed, which, through the release of available potential energy, grows until it ultimately has developed into ringlike eddies.

Simulations with the Cartesian primitive equation model appear to suffer from spurious diapycnal mixing of both heat and momentum. This retards the process of Rossby wave breaking and prolongs the growth of the meander, thus causing a doubled heat transport at 10-km resolution, compared to a 5-km resolution experiment. The isopycnic model does not show this degree of overshoot in heat transport. In general, the Cartesian model is much more sensitive to both resolution and closure formulation than the isopycnic model.

The quasigeostrophic model does not simulate the small-scale processes of Rossby wave steepening and breaking correctly. However, as a consequence the diapycnal mixing of heat and momentum hardly affects these processes. For this reason, the quasigeostrophic model does not show an overshoot in heat transport.

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Sybren S. Drijfhout
and
Fred H. Walsteijn

Abstract

The midlatitude meridional heat transport in the ocean can be partitioned into a transport by the mean flow and an eddy transport. This heat transport has been studied in several ocean-only models. Surprisingly, it was found that eddy-resolving and coarse-resolution models have similar total heat transport. This is caused by a compensation mechanism in which poleward eddy transport is counterbalanced by an eddy-induced meridional circulation. Recently it was shown that this compensation depends on details of the atmospheric forcing and, in fact, only occurs for weak thermal coupling, where thermal coupling is defined as the rate of change of the surface heat flux with respect to the sea surface temperature (SST). The thermal coupling varies with the spatial scale of the SST anomaly. To study the actual strength of this coupling on the eddy length scale the authors have coupled an isopycnic ocean model (with embedded mixed layer) to an atmospheric anomaly model. By comparing coarse-resolution and eddy-resolving simulations it is found that 1) the thermal coupling is strong on the eddy length scale and 2) the aforementioned compensation does not occur. Consequences for the temperature boundary condition in ocean-only models, the Gent and McWilliams eddy parameterization, and climate modeling are discussed.

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Sybren S. Drijfhout
and
Wilco Hazeleger

Abstract

Parameterizations of the eddy-induced velocity that advects tracers in addition to the Eulerian mean flow are traditionally expressed as a downgradient Fickian diffusion of either isopycnal layer thickness or large-scale potential vorticity (PV). There is an ongoing debate on which of the two closures is better and how the spatial dependence of the eddy diffusivity should look like. To increase the physical reasoning on which these closures are based, the authors present a systematic assessment of eddy fluxes of thickness and PV and their relation to mean-flow gradients in an isopycnic eddy-resolving model of an idealized double-gyre circulation in a flat bottom, closed basin. The simulated flow features strong nonlinearities, such as tight inertial recirculations, a meandering midlatitude jet, pools of homogenized PV, and regions of weak flow where β/h dominates the PV gradient. It is found that the zonally averaged eddy flux of thickness scales better with the zonally averaged meridional thickness gradient than the eddy flux of PV with the PV gradient. The reason for this is that the two-scale approximation, which is often invoked to derive a balance between the downgradient eddy flux of PV and enstrophy dissipation, does not hold. It is obscured by advection of perturbation enstrophy, which is multisigned and weakly related to mean-flow gradients. On the other hand, forcing by vertical motions, which enters the balance between the downgradient eddy flux of thickness and dissipation in most cases, acts to dissipate thickness variance. It is dominated by the conversion from potential to kinetic energy and the subsequent downgradient transport of thickness. Also, advection of perturbation thickness variance tends to be more single-signed than advection of perturbation enstrophy, forcing the eddy flux of thickness to be more often down the mean gradient. As a result, in the present configuration a downgradient diffusive closure for thickness seems more appropriate to simulate the divergent eddy fluxes than a downgradient diffusive closure for PV, especially in dynamically active regions where the eddy fluxes are large and in regions of nearly uniform PV.

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Sybren S. Drijfhout
and
Alberto C. Naveira Garabato

Abstract

The three-dimensional structure of the meridional overturning circulation (MOC) in the deep Indian Ocean is investigated with an eddy-permitting ocean model. The amplitude of the modeled deep Indian Ocean MOC is 5.6 Sv (1 Sv ≡ 106 m3 s−1), a broadly realistic but somewhat weak overturning. Although the model parameterization of diapycnal mixing is inaccurate, the model’s short spinup allows the effective diapycnal velocity (the sum of model drift and the explicitly modeled diapycnal velocity) to resemble the true, real-ocean diapycnal velocity. For this reason, the model is able to recover the broad zonal asymmetry in the turbulent buoyancy flux that is suggested by observations. The model features a substantial deep, depth-reversing zonal circulation of nearly 50% of the MOC. The existence of this circulation, brought about by the zonally asymmetric distribution of diapycnal mixing, implies a much slower ventilation of the deep Indian Ocean (by a factor of 5–6) than would be in place without zonal interbasin exchanges. It is concluded that the zonal asymmetry in the distribution of diapycnal mixing must have a major impact on the deep Indian Ocean’s capacity to store and transform climatically significant physical and biogeochemical tracers.

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Renske Gelderloos
,
Caroline A. Katsman
, and
Sybren S. Drijfhout

Abstract

Restratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the Labrador Sea. In the subsurface, it is primarily governed by lateral buoyancy fluxes during early spring. The roles of three different eddy types in this process are assessed using an idealized model of the Labrador Sea that simulates the restratification season. The first eddy type, warm-core Irminger rings, is shed from the boundary current along the west coast of Greenland. All along the coastline, the boundary current forms boundary current eddies. The third type, convective eddies, arises directly around the convection area. In the model, the latter two eddy types are together responsible for replenishing 30% of the winter heat loss within 6 months. Irminger rings add another 45% to this number. The authors’ results thus confirm that the presence of Irminger rings is essential for a realistic amount of restratification in this area. The model results are compared to observations using theoretical estimates of restratification time scales derived for the three eddy types. The time scales are also used to explain contradicting conclusions in previous studies on their respective roles.

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Achim Stössel
,
Seong-Joong Kim
, and
Sybren S. Drijfhout

Abstract

Most of the Southern Ocean (SO) is marginally stably stratified and thus prone to enhanced convection and possibly bottom-water formation whenever the upper ocean is cooled or made more saline by ice formation. Sea ice modifies the heat and freshwater fluxes, which in turn constitute a critical surface condition in this sensitive region of intense vertical exchange. The authors investigate the effect of SO sea ice in modifying these fluxes in a global, coarse-resolution, primitive-equation ocean general circulation model, which has been coupled to a comprehensive dynamic–thermodynamic sea ice model. Specifically, the long-term impact of a series of modifications in the formulation of the sea ice model and its forcing on quantities such as the overturning circulation, the deep ocean water-mass characteristics, the sea ice thickness, the strength of convection, as well as the strength of the major volume transports are investigated. The results indicate that the rate of Antarctic bottom-water formation is strongly coupled to the local sea ice processes in the SO, which in turn vary sensitively depending on their model formulation and their forcing from the atmosphere. The largest impacts arise from the effect of brine release due to sea ice formation and that of employing more variable winds over SO sea ice.

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Caroline A. Katsman
,
Sybren S. Drijfhout
, and
Henk A. Dijkstra

Abstract

Recent modeling and observational studies have indicated that the interaction of the Gulf Stream and the deep western boundary current (DWBC) in the North Atlantic may induce low-frequency (decadal timescale) variability. To understand the origin of this low-frequency variability, a line of studies is continued here addressing the stability and variability of the wind-driven circulation using techniques of dynamical systems theory. In an idealized quasigeostrophic 2-layer model setup, stationary solutions of the coupled wind-driven gyres/DWBC system are computed, using the lateral friction as control parameter. Simultaneously, their stability is assessed. When a DWBC is absent, only oscillatory instabilities with intermonthly timescales are found. However, when the strength of the DWBC is increased, the coupled 2-layer flow becomes susceptible to instabilities with interannual timescales. By computing transient flows at relatively low friction, it is found that the existence of these interannual modes induces low-frequency variability in the coupled Gulf Stream/DWBC system with a preferred interannual timescale.

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Yanli Jia
,
Andrew C. Coward
,
Beverly A. de Cuevas
,
David J. Webb
, and
Sybren S. Drijfhout

Abstract

The behavior of the Mediterranean Water in the North Atlantic Ocean sector of a global ocean general circulation model is explored, starting from its entry point at the Strait of Gibraltar. The analysis focuses primarily on one experiment in which explicit watermass exchange between the Mediterranean Sea and the Atlantic at the Strait of Gibraltar is permitted. The model produces an exchange rate of approximately 1 Sv (Sv ≡ 106 m3 s−1). This is comparable to estimates derived from field measurements. The density of the Mediterranean outflow, however, is lower than observed, mainly because of its high temperature (more than 2°C higher than in reality). The lower density of the outflow and the model’s inadequate representation of the entrainment mixing in the outflow region cause the Mediterranean Water to settle in a depth range ∼800–1000 m in the North Atlantic, about 200 m shallower than observed. Here an interesting current system forms in response to the intrusion of the Mediterranean Water, involving three main pathways. In the first, the Mediterranean Water heads roughly westward across the basin and joins the deep western boundary current. In the second, the water travels northward along the eastern boundary reaching as far as Iceland, where it turns westward to participate in the deep circulation of the subpolar gyre. In the third, the water initially moves westward to the central Atlantic just north of 30°N before turning northwestward to reach an upwelling region at the Grand Banks off Newfoundland. At this location, the saline Mediterranean Water is drawn upward to the ocean upper layer and entrained into the North Atlantic Current system flowing to the northeastern basin; part of the current system enters the Nordic seas.

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