where we is the Ekman pumping velocity calculated from wind stress. The total meridional volume flux involved in the wind-driven gyre across a latitudinal section of the basin interior is the sum of these two fluxes, i.e., MSv = ME + MG. The return flow into the western boundary is MSv. The bifurcation of the western boundary current is a nonlinear phenomenon; however, here we assume that the bifurcation latitude of the western boundary current is the latitude where MSv vanishes; this can be determined from the wind stress distribution in the basin. For the climatological mean wind stress, this gives the separation latitudes at 14.5° N for the North Pacific and 15° S for the South Pacific: values which are close to the bifurcation points estimated from a numerical model (e.g., B. Huang and Liu, 1999).
For a given amount of wind stress, the depth of the main thermocline can be calculated using a simple reduced-gravity model. The corresponding Bernoulli function can be calculated according to the definition: B = g'H, where g' is the reduced gravity, chosen as g' = 2 cm/s2 for the Pacific, and H is the depth of the main thermocline.
This simple approach is, however, unsatisfactory because wind stress is slightly stronger in the Northern Hemisphere and the corresponding Bernoulli head in the Northern Hemisphere is larger than that in the Southern Hemisphere. As a result, this simple approach would lead to a conclusion that water from the Northern Hemisphere western boundary regime should invade the Southern Hemisphere. However, observations indicate that most water forming the Equatorial Undercurrent does come from the south, as discussed above. Thus, in order to have the water from the Southern Hemisphere overshoot the equator, another mechanism is needed. A strong contender is the Indonesian Throughflow. With the through-flow, the western boundary current system changes and the source of the undercurrent also changes accordingly, as shown in Figure 4.73b.
Australia can be treated as a big island, so the simple island rule can apply and yield a circulation around it that is a simple function of wind stress, as discussed in Chapter 2. The island rule has been discussed in many papers, and the most essential physical mechanisms associated with this rule have been discussed thoroughly by Godfrey (1989, 1993).
Although volume flux associated with the throughflow predicted from the island rule can be large, on the order of 17 Sv, other physical processes reduce the flux to about 10 Sv. In our simple purely inertial model, water from the Southern Hemisphere could not penetrate into the Northern Hemisphere or feed the throughflow without the potential vorticity changing sign. The western boundary current from the Southern Hemisphere has to go through the undercurrent first. Over the eastward trajectory this volume flux upwells into the Ekman layer and then moves poleward in the form of Ekman transport. Thus, volume flux from the Southern Hemisphere becomes part of the Sverdrup volume flux in the ocean interior of the Northern Hemisphere. Within the simple ideal-fluid formulation of our model, the only option is to assume that the throughflow is fed from the western boundary current from the Northern Hemisphere; hence, we will simply assume that, indeed, about 10 Sv of water leaves the basin and forms the Indonesian Throughflow.
In the Northern Hemisphere, therefore, the separation latitude is not affected by the throughflow; volume flux in the western boundary current is zero at the separation latitude. However, the effective western boundary current that feeds the undercurrent actually comes from the ocean interior. To find the Bernoulli head of this western boundary layer, one has to search eastward, starting from the western boundary at the separation latitude, for a place where the equatorward flux, integrated from the eastern boundary, satisfies fhive wbc = fwbc + Af (4.440)
where fwbc = -MSv is the Sverdrupian flux at the outer edge of the western boundary; and Af = 10 Sv is the volumetric contribution to the throughflow from the wind-driven circulation.
In the Southern Hemisphere, the separation latitude is now determined by the constraint that the total volume flux in the western boundary layer fSffU'ctive wbc = -Msv + Af (4.441)
vanishes. Without the throughflow, the separation latitude in the Southern Hemisphere is near 15° S; with the throughflow, however, the separation latitude is pushed southward to 17° S.
4.7 Communication between subtropics and tropics 4.7.1 Introduction
The subtropical and tropical cells The linkage between the subtropics and tropics constitutes a major component of the global oceanic circulation and climate system. The circulation in the subtropical-equatorial ocean involves complicated dynamical processes and pathways. To simplify the picture, this system can be described in terms of two cells (McCreary and Lu, 1994). The division into two cells is conceptual only because the circulation is essentially three-dimensional. There is no clear boundary between these two cells, but we will use the so-called choking latitude, which will be defined shortly, as the boundary separating these two cells (Fig. 4.76a). The tropical and subtropical cells are closely related to the following processes:
• In the subtropical basin interior, the horizontal convergence of Ekman flux leads to Ekman pumping that drives the anticyclonic gyre in the subtropical basin and gives rise to subduction of subtropical
Tropical œN | Subtropical cell
Tropical œN | Subtropical cell
Via the western boundary current ^^_____
or the interior communication window - - - -Equat0riai thermoCline a Meridional view of the two cells b Three-dimensional view of the pathways
Fig. 4.76 a Meridional view of the tropical and subtropical cells; b sketch of the pathways from the subtropics to the equator.
water masses. These water masses are transported through the equatorward geostrophic flows in the subtropical thermocline, where there are three pathways for them.
• First, water masses subducted in the subtropical basin can reach the tropics through the interior communication window and the subsequent equatorward flow on isopycnal surfaces. Gu and Philander (1997) proposed that such a linkage plays a vital role in climate variability on decadal time scales (Fig. 4.76b).
• Second, water masses subducted in the subtropical basin can reach the tropics as follows. These water masses move westward and reach the western boundary where the flow bifurcates, and part of the water flows in the form of the equatorward western boundary current and eventually the eastward Equatorial Undercurrent.
• Third, at the bifurcation point of the western boundary, some of the subducted water is returned to mid latitudes via the poleward western boundary current. This part of the subducted water is not counted as the subtropical cell; on the other hand, water masses moving through the first and second pathways constitute the subtropical cell.
• In the equatorial band, the subsurface water mass joins the eastward Equatorial Undercurrent, and eventually it is lifted to the surface via the equatorial upwelling driven by the easterlies along the equator. It is important to emphasize that the upward motion during this phase is mostly adiabatic, without much diapycnal mixing (Bryden and Brady, 1985). As discussed in the previous section, the undercurrent can be described as an inertial boundary current in an ideal-fluid model.
• The off-equator Ekmanflux brings water back to mid latitudes. Off equator the local convergence of Ekman flux gives rise to Ekman pumping, which pushes water down into the thermocline, and thus completes the loop.
As discussed below, not all subtropical water pumped down and going through the base of the mixed layer can reach the equator. There is a choking latitude where an interior communication window exists between the subtropics and tropics; only part of the subtropical water subducted into the subtropical thermocline can pass through this window and thus contribute to the subtropical cell. The remaining part of the subducted subtropical water turns westward and reaches the western boundary, where the current bifurcates. Only the equatorward branch of this western boundary current can contribute to the subtropical cell; on the other hand, its poleward branch is unable to reach the equatorial band and makes no contribution to the subtropical cell.
On the equatorward side of the choking latitude, the equatorward transport continues to increase owing to the equatorward geostrophic flow induced by the local Ekman pumping. This additional transport of water also goes through the two pathways similar to those of the subtropical cell, and it eventually returns poleward in the form of the off-equator Ekman transport, thus completing the cell. Accordingly, this additional transport should be classified as part of the tropical cell, as depicted in Figure 4.76a.
The mechanism described above applies to both the Pacific and Atlantic Basins, where easterlies prevail in the equatorial band; however, in the Indian Basin, westerlies prevail in the equatorial band, and the dynamics of equatorial circulation is different from that discussed in this section.
Communication window identified from tritium data in the tropical Pacific
The linkage between the subtropical and tropical oceans can be identified from tracer distribution in these regimes. In fact, such links were first discovered through tracer studies. Tracer observations indicated that in the upper layer, tritium in the North Equatorial Current has decreased since 1974, while tritium south of the North Equatorial Current increased from 1965 through 1979. Thus, the flow in the subtropical cell communicating between mid-latitude and equatorial oceans is characterized with a time scale of about 10 years. Since tritium has a half-life of 12.4 years, it can be used as a good tracer for climate change on a decadal time scale.
The interior communication window was identified through tracer studies in the 1970s and 1980s. Fine and her colleagues (Fine etal., 1981,1987; McPhaden and Fine, 1988)analyzed the tritium data and found a local tritium maximum around 140° W along the equator (Figs. 4.77 and 4.78), which they rightly attributed to the ventilation of the subtropical water via subduction. Since this local maximum of tritium is not linked to any high concentration of tritium in the western part of the equatorial band, it is readily seen that the existence of such a local maximum in tritium cannot be attributed to the Equatorial Undercurrent.
In this section, we discuss the volume flux through the interior communication window, using a simple index that is based on wind stress data only. This index can be used to illustrate the asymmetric nature of the interior communication between the tropics and the subtropics. In addition, the index can be used to infer the decadal variability of the interior communication.
The ITCZ and communication between subtropical and tropical oceans
One of the major features in wind stress near the equator is the existence of the Inter-Tropical Convergence Zone (ITCZ) in the Northern Hemisphere. The existence of a ridge-like feature in the meridional section indicates that the Ekman pumping rate is positive near the ITCZ. Such a ridge in the density section can be interpreted as a potential vorticity barrier which
GEOSECS along 125°W Stations 1973-1974
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