(a)

(b)

Figure 2:　(a) Tracer concentration on σ_θ=25.5 surface. Contour intervals are 0.25, but are reduced to 0.05 for small values between -0.25 to 0.25. Negative values are dashed and shaded. (b) Trajectories of water parcels starting at 30 °N, 35 °N, and 30 °S, on the isopycnal surface σ_θ=25.5. Shades are same as in (a). Characters A to H show the points of the water parcel moving along the trajectory with two years interval from 1st to 15th year.

Figure 3:　Tracer concentration (solid) and density (dashed line) on zonal sections at the equator at the 30th year. Negative tracer values are shaded. Contour intervals are 0.025 for tracer, and are 0.50 for σ_θ.

　

The positive tracer tongue starting from the North American coast around 25｡? is likely due to the downward diffusion from the overlying layer. This positive tongue also extends southwestward, but does not reach the equator.

The tracer is transported to the equator mainly through the western boundary layer where its concentration reaches a maximum at the equator (Fig. 3; see also Fig. 2a). This suggests that the western boundary route is more important for the transport of the tracers than the interior route. At the equator, the core of the negative tracer is centered on σ_θ=25.5 , and extends eastward along the equator, riding on the strong Equatorial Undercurrent, (EUC). On its way toward the east, the concentration of the tracer decreases rapidly from the western boundary. East of 160°E, the value of the equatorial subsurface tracer concentration is less than 10% of the maximum SST anomalies prescribed over the North Paciflc.

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