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Fig. 4.77 A north-south vertical section of tritium (TU) vs. oq along the eastern GEOSECS track at 125° W (Fine et al, 1987).

Tritium (TU)

Date of station collection decay-corrected 74-01-01

Tritium (TU)

Date of station collection decay-corrected 74-01-01

Longitude along equator

Fig. 4.78 An east-west vertical section of tritium (TU) vs. oq along the equator (Fine et al., 1987).

Longitude along equator

Fig. 4.78 An east-west vertical section of tritium (TU) vs. oq along the equator (Fine et al., 1987).

blocks the local communication between the subtropics and tropics. The position of the ITCZ can be seen clearly from the thermal structure in the ocean (Fig. 4.69). The latitude band of the ITCZ is closely linked to the choking latitude and the communication window discussed below. In fact, the pathway and the blockage of the communication have been interpreted in terms of the high potential vorticity ridge in the eastern basin (e.g., Lu and McCreary, 1995; Johnson and McPhaden, 1999).

4.7.2 Interior communication window between subtropics and tropics

Communication window inferred from data and numerical models

The interior communication associated with the circulation in the subtropical-tropical regime can be explored using analytical and numerical models (e.g., Liu, 1994; McCreary and Lu, 1994). Analysis of tracer and hydrographic data leads to a better estimate of the communication rate (e.g., Wijffels, 1993; Johnson and McPhaden, 1999). It is estimated that this communication rate is about 5 Sv for the North Pacific and 16 Sv for the South Pacific. The communication rate in the Atlantic was estimated by Fratantoni et al. (2000) as 1.8 Sv for the North Atlantic and 2.1 Sv for the South Atlantic. The communication window can be clearly demonstrated through analysis of a numerical model for the oceans, and pathways for the Pacific obtained from a numerical model are shown in Figure 4.79.

Fig. 4.79 Pathways from the subtropics to the tropics (Liu and Philander, 2001); the thin arrows indicate the western boundary windows and the thick arrows indicate the interior communication windows, identified by tracking the particles that are subducted at 50 m depth in the NCEP model data (B. Huang and Liu, 1999); numbers in boxes are the times in years for the particles to reach 5° latitude.

Fig. 4.79 Pathways from the subtropics to the tropics (Liu and Philander, 2001); the thin arrows indicate the western boundary windows and the thick arrows indicate the interior communication windows, identified by tracking the particles that are subducted at 50 m depth in the NCEP model data (B. Huang and Liu, 1999); numbers in boxes are the times in years for the particles to reach 5° latitude.

A sketch of the communication window One outstanding feature in the subtropical-tropical ocean is the positive Ekman pumping areas within the ITCZ (or the South Pacific Convergence Zone in the South Pacific). The existence of a positive Ekman pumping rate gives rise to a small cyclonic gyre in the equatorial ocean, which is rather strong near the eastern boundary. In addition, there might be a second cyclonic gyre near the western boundary, as indicated by the heavy dashed lines in Figure 4.80.

Although the circulation involving the cyclonic gyres may seem complicated, it is, however, quite straightforward to deal with a model ocean including a patch of positive Ekman upwelling within the otherwise negative Ekman pumping. If the easterlies near the equator relax, the Ekman pumping rate will become more negative, and the western boundary of the eastern cyclonic gyre will extend far more westward; thus, the communication window will become narrower. If the western boundary of the eastern cyclonic gyre joins with the eastern boundary of the western cyclonic gyre, the interior communication window will be closed. Such a possibility can be explored, using wind stress datasets and results from an oceanic data assimilation system.

The situation can be illustrated in terms of a two-moving-layer model of the ventilated thermocline (Fig. 4.80). The lower layer outcrops in the subtropics and the upper layer covers the upper ocean in the tropics where the layer is stagnant in the shadow zone. (Within the cyclonic gyres, the lower layer is in motion because it is directly exposed to the Ekman upwelling.) However, the lower layer is in motion west of the shadow zone. In this two-layer model, the communication window is represented by potential flow in the

Fig. 4.80 Sketch of the communication window between the subtropics and tropics.

lower layer between the eastern cyclonic gyre and the western cyclonic gyre, depicted by the heavy dashed half ellipses in Figure 4.80.

The area covered by the shadow zone depends on the choice of layer thickness along the eastern boundary. As will be shown shortly, when the lower layer is very thick, the shadow zone is so large that its western boundary meets the western boundary north of the western cyclonic gyre, depicted by curve A. For such a case, there is no interior communication between the subtropics and tropics.

As the lower layer thickness is reduced, the boundary of the shadow zone moves toward the equator. Curve B indicates the critical case when the boundary of the shadow zone is tangential to the edge of the western cyclonic gyre. For smaller lower layer thickness, the boundary of the shadow zone is west of the western cyclonic gyre, depicted by curve C, and there is a communication window opening up between curve C and the western cyclonic gyre. The maximum width of the communication window is limited to the gap between the eastern and western cyclonic gyres.

The existence of the communication between the subtropics and tropics can be seen clearly using a ventilated thermocline model. The essential part of the model formulation is to simulate the effect of ITCZ by including a small positive Ekman pumping zone near the eastern boundary. We denote the bottom (and motionless) layer as layer 0, the lowest moving layer as layer 1, and the layers above have numbers increasing upward. The thickness and the depth of the i-th layer are denoted as h and Hi. The northernmost outcrop line is labeled as fi.

The Sverdrup relation for an «-layer model is where gi1 = gi/gl, and gi is the reduced gravity across the i-th interface. Using this relation, the Sverdrup function can be defined as and we (x, y) = -(x/f )y is the Ekman pumping rate. North of the outcrop line f1 there is one moving layer only, and the solution is

A 2l/2-layer model for an idealized ocean n n

South of the outcrop line f1 there are two moving layers, and the solution is

However, south of 1 there is also a shadow zone in the lower layer. Within the shadow zone, the lower layer is stagnant and the lower interface is at a constant level equal to the undisturbed depth of the lower interface set along the eastern boundary, H0. The western boundary of the shadow zone, SB, can be calculated by following the streamline in the lower layer starting from the eastern boundary at the outcrop latitude. However, it is more convenient to identify this boundary as the line where the lower interface depth H1, calculated from Eqn. (4.445), is equal to H0.

East of the shadow zone boundary SB there can exist two dynamical regions: a shadow zone S and a cyclonic gyre CG. The boundary between the shadow zone S and the cyclonic gyre CG is determined by the zero barotropic streamline f0 = 0. Within the shadow zone S, only the upper layer is in motion. Assuming that the reduced gravity across both the lower and upper interfaces is the same, the solution in the shadow zone is h2 = + H02, h1 = H0 - h2 (4.447)

Within the cyclonic gyre CG, layer 2 (the upper layer) vanishes, so layer 1 is the only moving layer, and the solution here is h1 = H1 = .^D^ + H^.

We apply a 21/2-layer model to study the circulation in the subtropical-tropical ocean. The lower layer thickness is 150 m along the eastern boundary. The model is forced by a simplified wind stress profile (Fig. 4.81a), and the corresponding Ekman pumping rate is shown in Figure 4.81b.

As a departure from the practice of using a simple wind stress pattern independent of the zonal coordinate, we will also include a small positive Ekman upwelling regime near the eastern boundary of the model basin (Fig. 4.82b).

As shown in Figure 4.82, including a small regime of positive Ekman upwelling near the eastern boundary in the forcing field produces a small cyclonic gyre nearby, which is a critical component of the subtropical-tropical circulation.

A 21/2-layer model for the North Pacific

To make our discussion more practical, we apply this 21/2-layer model to the North Pacific, forced by the Ekman pumping field calculated from the Hellerman and Rosenstein (1983) wind stress data. The disadvantage of the multi-layer model is that the solution is rather sensitive to the choice of the outcrop line, the reduced gravity across the interfaces, and the a t

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