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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

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

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 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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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 Drijfhout and Leo R. M. Maas

Abstract

The generation and propagation of internal tides has been studied with an isopycnic three-dimensional ocean model. The response of a uniformly stratified sea in a channel, which is forced by a barotropic tide on its open boundary, is considered. The tide progresses into the channel and forces internal tides over a continental slope at the other end. The channel has a length of 1200 km and a width of 191.25 km. The bottom profile has been varied. In a series of four experiments it is shown how the cross-channel geometry affects the propagation and trapping of internal tides, and the penetration scale of wave energy, away from the continental slope, is discussed. In particular it is found that a cross-channel bottom slope constrains the penetration of the internal tidal energy. Most internal waves refract toward a cross-channel plane where they are trapped. The exception is formed by edge waves that carry part of the energy away from the continental slope. In the case of rotation near the continental slope, the Poincaré waves that arise in the absence of a cross-channel slope no longer bear the characteristics of the wave attractor predicted by 2D theory, but are almost completely arrested, while the right-bound Kelvin wave preserves the 2D attractor in the cross-channel plane, which is present in the nonrotating case. The reflected, barotropic right-bound Kelvin wave acts as a secondary internal wave generator along the cross-channel slope.

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Yann Friocourt, Sybren Drijfhout, and Bruno Blanke

Abstract

The dynamics of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula are investigated in two different models, one analytical and one numerical. Investigated here is the hypothesis that the steady-state slope current system is driven by the large-scale meridional density gradients. An analysis of the observed density fields evidences a four-layer structure with meridional gradients of alternate signs, which is also found in the numerical model. The linear analytical model of the continental margin shows that such a density structure is enough to obtain a steady-state four-layer slope current system comparable to the observed annual mean circulation. The slope currents result from a balance between bottom friction and meridional density gradients. The numerical simulation with an ocean general circulation model forced only by the large-scale density gradients at the lateral boundaries presents a four-layer slope current system similar to the circulation obtained in the analytical model. The study confirms that the large-scale meridional density gradients are the main driving mechanism for the steady-state slope current system; the large seasonality of these currents, however, requires a more extended model, which is discussed in a companion paper (Part II).

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Yann Friocourt, Bruno Blanke, Sybren Drijfhout, and Sabrina Speich

Abstract

The seasonality of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula is investigated in a joint analysis of an analytical model and numerical simulations with various forcings. A distinction is made between local winds and basin-scale winds, in which the effect of the latter is indirectly apparent through the basin-scale density gradients. The slope currents are mainly forced by the large-scale structure of the density field. The analysis indicates significant differences in the behavior of the uppermost slope current and of the deeper currents. At all depths, seasonal variations in the large-scale density structure of the ocean alter the strength of the slope currents but are not able to cause robust, long-lasting reversals. Reversals of the uppermost slope current appear to be caused by changes in the alongshore component of the local wind stress, provided that the opposing forcing from the density structure is weak enough. However, the deeper slope currents are not very much affected by the wind stress, so that flow reversals can be explained neither by the wind nor by seasonal changes in the density structure. A numerical simulation suggests that the reversals of the deeper slope currents are at least partly forced by seasonal changes in the flow upstream of the slope current system. The authors demonstrate that the larger part of these seasonal changes is associated with annual baroclinic Rossby waves caused by the seasonal cycle of the large-scale wind stress over the whole basin.

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Barry A. Klinger, Sybren Drijfhout, Jochem Marotzke, and Jeffery R. Scott
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