Figure 5: (a) Time derivative of tracer concentration as a function of distance and time on the trajectory shown in Fig. 6. The vertical axis of this figure shows the years after the injection of the tracer at the sea surface and the horizontal axis is the along-the-trajectory distance. It is so arranged that it takes two years for a water parcel on the trajectory to move from one letter to the next. The straight lines in the flgure thus indicate the paths that a water parcel follows. Contour intervals are 0.004℃ year-1. (b) Same as Fig. 4b, but at the 5th year after the inject of the tracer from the sea surface. The dashed-dotted and long-dashed lines show the advection and time derivative of tracer concentration, respectively.
The rapicl decay near the western boundary at the equator (Fig. 2a; Fig. 3) is due to the Southern Hemisphere water, with almost zero tracer value in our experiments. It, is transported into the equatorial region through both the interior and the western boundary route (Fig. 2b).This western boundary current joins the EUC around 150｡?, diluting the Northern Hemisphere anomalies. The mixing with the Southern Hemisphere water that enters the equator through the interior route further contributes to the gradual eastward decrease of the negative tracers east of 160｡? (Fig. 3).
To examine a dependence on the magnitude of the coefficient of the diffusion, we perform a sensitivity experiment with a isopycnal diffusivity reduced to a half, 1.0×107cm2s-1. The tracer concentration on the trajectory followed from the same point, as in the standard case (Fig. 4, dash-dotted line) shows that the first rapid decay (from A to D) associated with the mixing with positive tracers to the east weakens. Meanwhile, the replenishment, around point F is also reduced. These two effects of the weak isopycnal mixing compensate each other and make the tracer concentration at the equatorial region almost the same as the standard case.
