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 σθ.
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 σθ=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.