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Claes Rooth

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Lian Xie and Claes Rooth

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A shear stress at the air–sea interface is induced by the presence of surface currents. In the case of isotropic wind, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity. This causes a spindown effect on the surface vorticity field at a rate proportional to the characteristic wind speed. This effect of surface vorticity spindown or friction on propagating baroclinic Rossby waves is analyzed in a continuously stratified system with well-mixed boundary layers above and below it. In this system, both bottom friction and surface friction modulate the baroclinic Rossby waves in the stratified interior each time a wave group is reflected at the lower and the upper interfaces. The reflection coefficient and the corresponding spatial attenuation rate are calculated for various values of wave frequency, wind speed, and interior stratification. Both bottom friction and surface friction are important dissipation mechanisms for the baroclinic Rossby waves. Within a normal parameter range appropriate for deep oceans, the nondispersive, long Rossby waves are more effectively damped by surface friction than by bottom friction, whereas the dispersive, short Rossby waves are primarily damped by bottom friction.

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Claes Rooth and Walter Düing

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The vertical displacements associated with oscillations near the local inertial frequency f, in a stratified ocean, are found to be significant, even very close to f. For a frequency, ω=(1+ε)f, the ratio of rms vertical velocity to horizontal velocity is O(ε1/2). Observations near Hawaii, made with a recently developed pycnocline follower, show inertial oscillation events similar to those found by Webster in current meter records, and discussed theoretically by Crepon and by Pollard.

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Eric P. Chassignet, Rainer Bleck, and Claes G. H. Rooth

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It is shown that the main characteristic of the Persons western boundary current separation mechanism, namely a separation south of the zero wind stress curt line (ZWCL), is no longer present when diabatic effects are included through the addition of a mixed layer to the model structure. When diabatic processes are taken into account, the separation is associated with the outcropping of interior layers into the mixed layer where the horizontal density gradient is a maximum. The maintenance of this maximum results from a balance between potential vorticity conservation and diabatic processes. In spite of wide variations in either forcing functions or parameters, the flow patterns of the thermally forced experiments described herein remain very similar from one experiment to the other. With a separation latitude located at the ZWCL. It is only when salinity is allowed to vary significantly in space that one observes a significant move in the jet separation latitude.

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Rainer Bleck, Claes Rooth, Dingming Hu, and Linda T. Smith

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An isopycnic-coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented. Special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation. The model performance is illustrated by a 30-year spinup run with coarse horizontal resolution (2° mesh) in a domain with North Atlantic topography extending from 10° to 60°N latitude. The vertical structure encompasses 10 isopycnic layers in steps of 0.2 σ units, capped by a thermodynamically active mixed layer. From an initially isohaline state with isopycnals prescribed by zonally averaged climatology, the model is forced by seasonally varying wind stress, radiative and freshwater fluxes, and by a thermal relaxation process at the surface. After a mechanical spinup time of about 15 years, a quasi-stationary pattern of mean circulation and annual variability ensues, characterized by pronounced subtropical mode-water formation and a gradual growth in the salinity contrast between the subtropics and the subpolar region. The effect of the freshwater flux forcing on the ventilation of the thermocline is a key point of discussion. Finally, a low-viscosity experiment suggests that the thermohaline processes represented in the model are quite insensitive to dynamic noise development at the grid resolution limit.

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Tamay M. Özgökmen, Eric P. Chassignet, and Claes G. H. Rooth

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As the salty and dense Mediteranean overflow exits the Strait of Gibraltar and descends rapidly in the Gulf of Cadiz, it entrains the fresher overlying subtropical Atlantic Water. A minimal model is put forth in this study to show that the entrainment process associated with the Mediterranean outflow in the Gulf of Cadiz can impact the upper-ocean circulation in the subtropical North Atlantic Ocean and can be a fundamental factor in the establishment of the Azores Current. Two key simplifications are applied in the interest of producing an economical model that captures the dominant effects. The first is to recognize that in a vertically asymmetric two-layer system, a relatively shallow upper layer can be dynamically approximated as a single-layer reduced-gravity controlled barotropic system, and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity.

Two sets of computations are presented within the 1½-layer framework. A primitive-equation-based computation, which includes the divergent flow effects, is first compared with the equivalent quasigeostrophic formulation. The upper-ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the β effect until the flow encounters the western boundary. In the steady state, the circulation pattern consists of bidirectional zonal flows with a limited meridional extent: eastward to the south of the sink and westward to the north of the sink. The localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink’s strength. It is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component.

A set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation, driven both by wind forcing and “thermohaline” flow through the open southern and northern boundaries. When a sink near the eastern boundary is superimposed on the idealized midlatitude circulation, it is shown to alter significantly the upper-ocean flow and induce an eastward zonal current, which resembles the Azores Current in location and transport. This mechanism also generates a westward current to the north of the sink location, which could be associated with the Azores Countercurrent. An extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes, sink strength, sink distribution, model resolution, and wind forcing. The magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline.

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Shan Sun, Rainer Bleck, Claes Rooth, John Dukowicz, Eric Chassignet, and Peter Killworth

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Buoyancy anomalies caused by thermobaricity, that is, the modulation of seawater compressibility by potential temperature anomalies, underlie a long-standing argument against the use of potential-density-framed numerical models for realistic circulation studies. The authors show that this problem can be overcome by relaxing the strict correspondence between buoyancy and potential density in isopycnic-coordinate models. A parametric representation of the difference between the two variables is introduced in the form of a “virtual potential density,” which can be viewed as the potential density that would be computed from the in situ conditions using the compressibility coefficient for seawater of a fixed (but representative) salinity and potential temperature. This variable is used as a basis for effective dynamic height computations in the dynamic equations, while the traditionally defined potential density may be retained as model coordinate. The conservation properties of the latter assure that adiabatic transport processes in a compressibility-compliant model can still be represented as exactly two-dimensional. Consistent with its dynamic significance, the distribution of virtual potential density is found to determine both the local static stability and, to a lesser degree, the orientation of neutrally buoyant mixing surfaces. The paper closes with a brief discussion of the pros and cons of replacing potential density by virtual potential density as vertical model coordinate.

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