Figure 1: Winter time (Jan.-Feb.-Mar.) SST difference between 1977-87 and 1965-75, calcu-lated based on Comprehensive Ocean. Atmosphere Data Set (COADS). Contour intervals are 0.2℃ with negative value dashed. The subtropical―Tropical exchange window at the bottom of the mixed layer in the model is shaded.
There is a problem with assumption of the theory that mid-latitude SST anomalies are advected into the equator intact. It arises from the relative positioning of the SST anornaly with the so-called exchange window from which the subducted water can reach the equatorial region (Liu 1994). A Iarge portion of the major oval-shaped decadal SST anomaly pattern is not, in the exchange window while the SST anomalies of the opposite sign occupy about, a half of the exchange window (Fig. 1). Here arise two questions: Does the equatorial temperature anomaly have the same sign as the major oval-shape SST anomaly? How large is it relative to the North Pacific SST anomaly? The answer to the latter questions gives a measure of the efficiency of the oceanic pipeline between the North Pacific and the Equator.
As a first step and as done in the delayed action theory, the equatorward spreading from the North Pacific of a passive tracer analogous to the interdecadal temperature anomaly is examined, and the tracer is advected by ocean currents but has no effect, on dynamics. Thus, here we neglect the complex nonlinear advection, making our problem simple and enables us to focus the advective-diffusive processes. Our simplified experiment is a necessary first, step toward comprehensive treatment of temperature as an active tracer.
The present study addresses these questions by means of a numerical simulation with an OGCM of the Pacific. We will inject the ”temperature anomaly” passive tracer at the sea surface with the observed decadal/interdecadal SST anomaly distribution and see how the tracers are spread three-dimensionally into the Pacific.
