Subduction of decadal North Pacific thermal anomalies in an ocean GCM

A new version of the GFDL Modular Ocean Model (MOM 3) is used to simulate interannual and interdecadal variability over the entire Pacific basin from January 1948 to April 2000, forced by NCEP reanalysis atmosphere-ocean fluxes. The MOM 3 with high resolution and sophisticated physics realistically reproduces the subduction of temperature and salinity anomalies associated with decadal climate variability in the North Pacific. Two major decadal subduction events (one warm and salty in the 1970s, and another cold and fresh in the 1980s) are demonstrated in this paper, In the 1970s temperature anomaly signal propagates along the subduction pathway into the western boundary at 18 degrees N. The time needed to propagate from the midlatitude outcrop sites into the western boundary is about 5 years, shorter than that suggested by previous advection-based analyses. It is further shown that the simulated temperature and salinity variations coincide in such a way that cold (warm) anomalies tend to be accompanied by fresh (salty) anomalies along the subduction pathway, indicating that they are largely compensating in terms of density on the decadal time scales. The implication of such compensated thermal anomalies subducted from the midlatitudes are discussed with respect to the tropical variability and ocean data assimilation issues.