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Tomoko Inui and Zhengyu Liu

Abstract

An OGCM (MOM1) is used to examine the oceanic response to localized anomalous surface wind and buoyancy forcings. Wind stress and surface cooling anomalies are imposed at several different locations with respect to the positions of the mixed layer front and the LPVP (low potential vorticity path). Surface cooling locally creates sea surface temperature anomalies, which are subducted to the thermocline in remote places. The way in which wind anomalies affect the thermocline structure can be observed by using the following indicator. The LPVP is defined as a line that consists of water with minimum potential vorticity at each latitude. It is defined at each isopycnal surface and is affected through changes in the mixed layer depth or the position of the outcrop lines.

Sea surface height (SSH) anomalies created by localized anomalous wind stress forcing propagate westward at the same speed as the lower-thermocline depth anomalies, corresponding to the first baroclinic mode. When the forcing region is east of the LPVP, the depth of various isopycnal surfaces induces large variability in the region of the LPVP, caused either by propagation of the first baroclinic mode wave or variations in the mixed layer front position. These results imply that the subsurface temperature anomalies, associated with the change of isopycnal depths, are large in the vicinity of the LPVP, even if the wind stress anomaly is remote.

Previous studies suggest that propagation of subsurface temperature anomalies is forced primarily by surface cooling. In this work, the authors observe that temperature anomalies created by surface cooling primarily follow the subtropical circulation. However, it is shown that the subducted temperature anomalies may also be generated by remote wind-forcing effects, through their impact on the position of the LPVP.

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Atsushi Kubokawa and Tomoko Inui

Abstract

In the North Pacific, there is a shallow eastward current called the subtropical countercurrent, which flows across the central subtropical gyre. The present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model (GCM) with a simple geometry, driven by surface wind stress and surface buoyancy forcing.

In the ocean GCM, the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre. The mixed layer front, the narrow transition zone of the mixed layer depth, slants from the western central subtropical gyre to the northeast, and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line. Since the surface density is almost zonally uniform and the mixed layer front slants northeastward, the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region, while those on lighter isopycnals are formed in the western region. Subducted and advected southwestward, the low potential vorticity fluid in each isopycnal overlaps that on another isopycnal and makes a thick low potential vorticity pool in the western central subtropical gyre. It is found that the model subtropical countercurrent occurs along the southern edge of this pool.

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Tomoko Inui and Kimio Hanawa

Abstract

A three-dimensional ocean general circulation model is used to investigate the effects of an idealized, nonzonal wind stress curl on the subduction process in the subtropical gyre. Idealized zonal winds are used to force the model ocean. Analyses of potential vorticity, water particle trajectories, and tracer-injection experiments are conducted. Two characteristic features appear in response to the nonzonal distribution of the wind stress curl: an upwelling region occurs in the subtropical gyre, and a downwelling region extends northward in the northeastern corner of the subtropical gyre. In comparison to results obtained from a zonal wind stress curl, a pool region, where there is no signal from the surface of the subarctic region, shrinks, a ventilated region becomes deeper, and the subducted water has a wider range of potential density.

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Tomoko Inui, Kensuke Takeuchi, and Kimio Hanawa

Abstract

A three-dimensional ocean general circulation model, forced by idealized zonal winds, is used to investigate the effect of an abrupt intensification of westerly winds on the subduction process. Four experiments are carried out: 1) a control experiment with standard wind stress forcing, 2) an intensified winds experiment with wind stress that is larger in the region of the westerlies than the control, 3) an increased surface cooling experiment, and 4) an experiment with both intensified wind stress and surface cooling. Experiments 2 through 4, which contain surface anomalous forcing, are run from the steady state obtained in experiment 1, the control experiment. The results obtained for each of these runs are compared to the control experiment. A subarctic tracer injection experiment is also carried out to verify the differences in the subduction process of each of these experiments.

In the wind stress intensified experiment, an examination of the subsurface temperature field shows that negative temperature anomalies occupy the western portion of the southern part of the subtropical gyre, whereas in the surface cooling experiment, negative temperature anomalies occupy the eastern portion of the basin. The source of these negative temperature anomalies is not local since the forcing in the southern part of the subtropical gyre does not change from the control. A close analysis of the evolution of a subarctic surface tracer field indicates that the intensification of the wind stress increases the tracer concentrations, whereas surface cooling decreases the temperature in the region that contains the maximum tracer concentration.

In the standard case, the mixed layer is deep (shallow) in the northern (southern) part of the subtropical gyre. Between these two regions a mixed layer front, where the mixed layer depth changes drastically from north to south, exists. A water column with low potential vorticity that originates in the mixed layer penetrates into a subsurface layer from the point where an outcrop line and the mixed layer front intersect. This point is called the penetration point.

Intensified westerly winds bring about a deeper thermocline and shoaling subsurface isopycnals. These shoaling subsurface isopycnals are not predicted in classical models such as that of Luyten et al. The model experiment with intensified westerlies demonstrates that the penetration point shifts to the west. As a result, low potential vorticity water penetrates southwestward from the shifted penetration point and takes a more westward path. Therefore, the negative temperature anomalies appear in the southwestern part of the subtropical gyre. This study shows that the westward shift of the path of low potential vorticity water could cause the shoaling of subsurface isopycnal surfaces.

The intensification of the westerlies increases Ekman pumping and cools the ocean surface by enhancing sensible and latent heat flux. In the surface cooling experiment, the position of the outcrop lines moves southward significantly. This southward shift makes the subducted water colder and distributes it throughout the ventilated region of the southern part of the subtropical gyre.

The combined effect of wind intensification and surface cooling is approximately a linear combination of both experiments.

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Tomoko Inui, Alban Lazar, Paola Malanotte-Rizzoli, and Antonio Busalacchi

Abstract

A reduced-gravity, primitive equation, upper-ocean GCM is used to study subduction pathways in the Atlantic subtropical and tropical gyres. In order to compare the different responses in the pathways to strong and weak wind stress forcings, Hellerman and Rosenstein (HR) and da Silva (DSV) climatological annual-mean and monthly wind stress forcings are used to force the model. It is shown that subtropical–tropical communication is dependent on both the strength and structure of the wind forcing. A comparison between the two experiments shows two results for the North Atlantic: 1) the full communication window between the subtropical and tropical gyres is similar in width despite the difference in the intensity of the winds and 2) the interior exchange window width is substantially larger in the weak forcing experiment (DSV) than the strong forcing experiment (HR), accompanied by a larger transport as well. The South Atlantic exhibits a similar communication between the subtropics and Tropics in both cases. The annual-mean of the seasonally varying forcing also supports these results. A two-layer ventilated thermocline model is developed with a zonally varying, even though idealized, wind stress in the North Atlantic, which includes the upward Ekman pumping region absent from the classical ventilated thermocline model. The model shows that the communication window for subduction pathways is a function of the zonal gradient of the Ekman pumping velocity, not the Ekman pumping itself, at outcrop lines and at the boundary between the subtropical and tropical gyres. This solution is validated using three additional GCM experiments. It is shown that the communication windows are primarily explained by the ventilated thermocline model without considering the buoyancy effects. From the GCM experiments, the interior exchange window, which is a part of the communication window and cannot be explained by the ventilated thermocline model, is widened by two factors: 1) eliminating part of the positive Ekman pumping region in the eastern North Atlantic and 2) weakening the Ekman pumping over the whole region. The implications of these results suggest that changes in the wind forcing on the order of the difference in the wind products used here can have a significant effect on the attributes of the communication window and, hence, the thermocline structure at lower latitudes.

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