On the connection between South Pacific subtropical spiciness anomalies and decadal equatorial variability in an ocean general circulation model

Simulations from a 49-year, realistically forced numerical model experiment indicate that decadal variability of temperature and salinity along the equator originates from subsurface spiciness anomalies in the South Pacific. Through western boundary and interior pathways in the thermocline, the subsurface anomalies in the South Pacific are first transferred westward and then northward, eventually appearing along the equator. The large spiciness anomalies in the South Pacific are formed in the eastern subtropics where large unstable salinity gradients are present in conjunction with weak stratification and strong mixing during winters. Our analysis shows that positive anomalies are generated in late winter by diapycnal mixing across isopyenal surfaces that are not exposed to the surface, i.e., through the injection process, in agreement with Yeager and Large (2004). In addition, we show that spiciness anomalies can also be created along isopycnals that outcrop to the surface through the subduction process, although this process alone is not enough to explain a significant part of the decadal variability along the equator based upon an active tracer experiment. Both the injection and subduction processes are responsible for forming positive subsurface anomalies in the eastern subtropical South Pacific, while negative anomalies there can be generated by subduction of negative surface anomalies and accumulation via isopycnal advection.