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As a result, the Antarctic Intermediate Water formation near the southern tip of South America cannot be accurately simulated using the Levitus dataset. On the other hand, the large disturbances appearing in the winter mixed layer depth may not be entirely due to the paucity of the dataset. As discussed by Gille , downstream of the East Pacific Rise the jets associated with the ACC are less intense than they are upstream of the mid-Pacific ridge. However, the detailed dynamics of the interaction between the mixed layer and the ACC is left for further study.

The annual mean subduction rates estimated from the analytical and the diagnostic models show that there is only a moderate amount of subtropical mode water being formed annually in the South Pacific, Fig. According to Roemmich and Cornuelle, the lifetime of the South Pacific STMW is short so that, in spite of being locally formed as a mode water, it is not traceable to locations far from its formation zone.

The lack of strong subtropical mode water formation in the South Pacific less than 3 Sv per 0.

Our analytical model also provides a three-dimensional view of the wind-driven gyre in the ocean interior. The Sverdrup mass flux is divided into three components, that is, the mass fluxes in the mixed layer, the ventilated thermocline, and the unventilated thermocline, Fig.

In previous studies, the term ventilation ratio was defined as the ratio beween the total mass flux in the ventilated thermocline and the total mass flux in the thermocline e. Because the wind-driven circulation is essentially a three-dimensional feature, it may not be possible to represent the structure with some simple numbers, such as the ventilation ratio. In fact, if one defines the ventilation ratio as the ratio of mass flux in the ventilated thermocline to total mass flux in the thermocline, such a ventilation ratio apparently increases toward the equatorial boundary Fig.

Mass flux in the unventilated thermocline declines toward the equator; meanwhile, the mass flux in the mixed layer remains almost constant. The small discrepancy between the barotropic Sverdrup flux and the sum of the baroclinic layers is due to the errors accumulated in the numerical calculation. In this study we have applied both a diagnostic model and a simple analytical model to an analysis of the wind-driven circulation in the subtropical South Pacific.

That these two approaches provided similar results is encouraging. For example, the total amount of ventilation and the contributions from vertical pumping and lateral induction in these two approaches are the same within the bounds of possible error. The annual-mean subduction rate in the South Pacific is estimated in this study from both the analytical and diagnostic models. There are several sources of error in calculating the subduction rate. They include errors in the Ekman pumping rate, uncertainties in the mixed-layer density and background stratification patterns, and the assumption of the reference level.

In the following, we evaluate some of the errors. Neglect of the seasonal cycle in the mixed layer introduces errors.


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According to the definition, the subduction rate calculated in the analytical model is a local property, whereas the rate calculated by the diagnostic model involves averaging over 1-yr trajectories. By estimating subduction locally, the analytical model underestimates the annual subduction rate in the southern part of the subtropical basin but overestimates it in the northern part of the basin because the definition neglects changes in the Ekman pumping velocity along the 1-yr trajectories. Nevertheless, results from the two models are fairly similar. The major difference between the two approaches actually stems from the smoothing of the forcing fields used in the analytical model.

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A major assumption of the Stommel demon is the shortness of the subduction period. Along the equatorial rim of the subtropical gyre, the base of the mixed layer is shallow and flat. The seasonal cycle is relatively weak and the subduction period may not be very short, so the subduction rate, especially the lateral induction term, calculated from the models may not be accurate.

Possible errors in the subduction rate can be roughly estimated as follows. The total amount of lateral induction is 3. Assume that the mixed layer depth maximum is increased to m and the minimum mixed layer depth and the velocity at the base of the mixed layer remain approximately the same. Then the total amount of lateral induction in this case would increase to The total amount of vertical pumping at the base of the mixed layer will be slightly reduced; thus the total amount of subduction would increase less than 0.

There are two difficult issues associated with the accurate estimation of the subduction rate in the South Pacific. One is that there are few observations made in the Southern Ocean, so climatologies do not truly represent the time-mean ocean. The large variations in the winter mixed layer depth Fig. However, it is also possible that such large disturbances are some kind of permanent feature associated with bottom topography; this issue is left for further study. The second issue is that both approaches used in this study assume the existence of a stagnant level somewhere above the ocean bottom.

The diagnostic calculation is based on a reference level of m. The ambiguity about the reference velocities may lead to the mass flux imbalance in the unventilated thermocline layer of our models. A comparison between the ventilation in the North Atlantic and the North Pacific has been carried out by Qiu and Huang The differences between these two oceans include: First, the thermohaline circulation in the North Atlantic is quite strong, while that in the North Pacific is very weak with the upper ocean dominated by a shallow halocline.

These differences give rise to different mixed layer properties and ventilation rates. The present study provides an interesting comparison between the wind-driven circulation in the South Pacific and the Northern Hemisphere oceans see Table 1. First, the mixed layer depths in most parts of the North and South Pacific are quite shallow and uniform.

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In the South Pacific, this is due to the lack of strong surface cooling Oberhuber , whereas in the North Pacific, it is due to the weak thermohaline circulation. As a result, contributions to the annual subduction rate due to lateral induction are quite small, about 9 Sv in the North Pacific and 3.

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Relative to the total subduction rate, the lateral induction contributes about one-third in the North Pacific and one-sixth in the South Pacific. In contrast, the lateral induction term contributes one-half of the subduction rate in the North Atlantic.

Second, the wind-driven circulation in the South Pacific extends relatively deep; maximum depth is about 2. In comparison, the wind-driven circulation in the North Pacific reaches only 1. The relatively deep wind-driven circulation in the South Pacific is associated with the deep late winter mixed layer along the southern edge of the subtropical gyre. Third, the mass flux in the unventilated thermocline is On the other hand, the total subduction rate in the South Pacific is only Fourth, the SE—NW tilted isopycnal outcropping lines in the eastern South Pacific give rise to an isopycnal slope reversal i.

The most serious problem in the South Pacific, particularly in the ACC region, is the lack of a reliable, large-scale climatological dataset. Results from both the diagnostic and analytical models provide a consistent picture for the subtropical mode water formation. On the other hand, neither approaches provides a reliable estimate for the formation and spreading of AAIW. In addition, the poor quality of the Levitus climatology in this area makes any diagnosis of water mass formation and movement vague.

The results discussed in this study are our first attempt to reveal the structure of the wind-driven circulation there. Many uncertainties remain, as do many scientific questions, especially the formation of subantarctic mode water SAMW and AAIW, which greatly contribute to the deep circulation in the South Pacific.

More observations are needed to improve our understanding of the Southern Hemisphere ocean. Discussions with Drs. McCartney, Talley, Toole, and Wijffels were very helpful in digesting the results from this study. September mixed layer depth a and density b of the South Pacific Ocean. Calculated from Levitus monthly climatology. Lagrangian trajectories over a 1-yr period. The starting points are indicated by crosses where the particles are released from the base of the mixed layer in September. Subduction rate maps from the diagnostic model: a Ekman pumping rate, b vertical pumping rate at the base of the mixed layer, c lateral induction rate at the base of the mixed layer, and d subduction rate.

The transport values for the Ekman layer are based on the HR climatological wind data. The transport values through the lateral boundaries of the thermocline layers are estimated from the geostrophic flows with a reference at m. Smoothed mixed layer depth a and density b fields used for the analytical model. Here potential thickness is defined as the inverse of the potential vorticity. Sea surface topography in cm determined from the analytical model. The shaded areas indicate the shadow zone.

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Subduction rate map calculated from the analytical model, in meters per year. Vertical pumping rate a , lateral induction rate b , and subduction rate c for each 0. Solid lines are from the data analysis and dashed lines from the analytical model. Thin lines in c indicate the net subduction rate solid thin line and obduction rate dashed thin line. Mass flux partition among the ventilated thermocline Vent , the unventilated thermocline Unvent , and the mixed layer Mix from the analytical model.

The sum of these three components Model is indicated by the heavy dashed line on the top, compared with the barotropic mass flux calculated by integrating the Ekman pumping rate Data indicated by the heavy solid line. Table 1.

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Next Article. Previous Article. Rui Xin Huang x. Search for articles by this author. The diagnostic calculation. The analytical model. Forcing the model with Levitus data. The structure of the wind-driven gyre. Acknowledgments Discussions with Drs. View larger version 64K Fig.