The simulated tracer distribution on the isopycnal surface (Fig. 2a) resembles the southwestward migrating anomaly fields of the isothermal depth (Schneider et al. 1998). These similarities suggest that, the processes of the equatorward spreading of the passive tracers examined in this study are important in understanding the North Pacific temperature anomalies.
The tracer corresponding to the North Pacific interdecadal SST anomaly reaches the equator, and as hypothesized in the delayed action theory, the tracer has the same sign at the equator as the SST anomalies in the major oval-shaped pattern occupying most of the North Pacific but with a much reduced magnitude. Our simulation reveals that, the majority of the SST anomalies do not reach the Tropics and isopycnal mixing is an important process in the water exchange problem between the North Pacific and the equator.
Here we summarize our results, by following a water parcel on the trajectory shown in Fig. 4:
a) The subducted water with negative tracers is subjected to strong mixing with positive tracers to the east;
b) In the southwestern part of the subtropical gyre, the water is replenished with negative tracers leaked from the recirculation region to the north;
c) The water particle enters the equator through the western boundary region; and
d) The negative tracers are substantially diluted by the intrusion of the Southern Hemisphere
thermocline water and the remnants are carried eastward by the EUC along the equator.
Conclusion a) spotlights the importance of SST distribution: the positive SST anomaly region near the eastern boundary, which has been somewhat overlooked in literature, affects the magnitude of the equatorial tracers. As shown in Fig. 5, the leaking of the recirculating subtropical tracers onto an equatorward pathway provides a short cut to the equatorial region. As a result, the shorter time scale of the equatorward spreading of the North Pacific tracers than the pure advection time scale is established. The simulated fields agree with the obsertations. suggesting that, the passive tracer distribution determined by advective-diffusive processes as shown in this study is relevant to the thermocline variability in the real North Pacific.
This study has focused on the problem of how North Paciflc interdecadal SST anomalies would spread as a passive tracer into the thermocline and to the equator.
