Analysis of observational data reveals the existence of a decadal spiciness mode that involves ocean-atmosphere coupling in the North Pacific. Specifically, the Aleutian Low (AL) which is the dominant atmospheric forcing of the Pacific Decadal Oscillation (PDO) drives a dipole pattern of positive and negative spiciness anomalies in the eastern midlatitude and subtropics, respectively. These anomalies then propagate equatorward along a deflected route defined by the mean acceleration potential. The positive anomaly can be observed at 14⁰N after 7 years of propagation while the downstream negative anomaly can be tracked to 10⁰N after 3 years of its appearance. In addition, a negative spiciness anomaly appears in the midlatitude, followed by the formation of the positive 2 years later. It takes a similar pathway toward the tropics. Further analysis suggests the potential impact of extratropical signals on tropical climate variability while the tropical surface signatures also feedback to the extratropical spiciness variability. These, in turn, potentially lead to a decadal climate oscillation in the North Pacific involving both atmospheric and oceanic bridges.

The dominant physical processes responsible for the subsurface spiciness variability are significantly different between the eastern midlatitude and subtropical North Pacific. In the midlatitude, isopycnal spiciness variability exhibits similar characteristics as the temperature variation at around 60-120m depth which is mainly produced via the subduction and reemergence mechanisms. In contrast, subtropical interior spiciness variability follows the evolution of salinity anomalies at around 120-240m. Both injection and anomalous advection across mean spiciness gradient likely dominate the subtropical isopycnal spiciness variability.