Numerical simulation of the equatorward spreading of a passive tracer corresponding to North Pacific interdecadal temperature anomalies
Masami Nonaka*1, Shang-Ping Xie*2 and Kensuke Takeuchi*1
*1 Institute of Low Temperature Science, Hokkaido University
*2 Graduate School of Environmental Earth Science, Hokkaido University
A simulation is conducted with a realistic ocean general circulation model to investigate the three dimensional spreading of a passive tracer prescribed at the sea surface with the same distribution as the interdecadal sea surface temperature (SST) anomalies observed in the North Pacific.
The tracers reaching the equator have the same sign as the major oval-shaped SST anomaly pattern in the North Pacific, as hypothesized in the "delayed action theory" of the North Pacific decadal / interdecadal variations, but with a much reduced magnitude less than 10% of the mid-latitude SST anomaly. The mixing with the water containing tracers of an opposite sign off the west coast of North America, and with the Southern Hemisphere thermocline water both contribute to the reduced equatorial amplitude, suggesting the importance of the sea surface distribution of the tracer. On the way to the equator in the southwestern part of the subtropical gyre, the subducted water is replenished by tracers leaking from the recirculation region to the north by isopycnal mixing. The replenishment provides a short cut that reduces the journey to the equator to less than 10 years, two-thirds of a pure advection time scale.
The simulated passive tracer field in the subsurface layers agrees with the observed interdecadal temperature anomalies, suggesting the relevance of the subduction-ventilation process shown here to the thermocline variability in the real North Pacific.