AL/PDO Forces a Decadal Subsurface Spiciness Propagating Mode in the
North Pacific
Abstract
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.