Aleutian low/PDO forces a decadal subsurface spiciness propagating mode in the North Pacific

Abstract

Analysis of observational data reveals the existence of a decadal subsurface spiciness mode that involves ocean–atmosphere coupling in the North Pacific. Specifically, the Aleutian Low, 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 spiciness anomaly can be observed at 14° N after 7 years of propagation while the downstream negative anomaly can be tracked to 10° N after 3 years from its appearance. In addition, a negative spiciness anomaly appears in the midlatitude, followed by the formation of the positive spiciness anomaly 2 years later. It follows a similar pathway toward the tropics. Further analysis demonstrates a strong connection between equatorial sea surface temperature variability and extratropical spiciness anomalies. These processes, in turn, potentially lead to a decadal climate oscillation in the North Pacific involving extratropical-tropical interaction. The dominant physical processes responsible for subsurface spiciness variability differ significantly between the eastern midlatitude and subtropical North Pacific. In the midlatitude, isopycnal spiciness variability exhibits similar characteristics to temperature variations at around 50–120 m depth, mainly generated through subduction and reemergence processes. Conversely, interior subtropical spiciness variability follows the evolution of salinity anomalies at around 120–250 m and is mainly formed via subduction and spice injection. Furthermore, anomalous advection across mean spiciness gradients strengthens subsurface signals from the midlatitude to the subtropics.