On the role of spiciness in Pacific Decadal Variability

Decadal modulations of the tropical Pacific impact the weather and climate worldwide and modulate the rate of change of the global warming trend. However, the mechanisms driving these long-term changes, especially the role of subsurface ocean dynamics, remain debated. By connecting the extratropical and tropical Pacific, the upper-ocean circulation may act as a low-pass filter of stochastic wind forcing, providing a source of memory on decadal time scales. Here, we investigate the role of spiciness (i.e., density compensated temperature and salinity) anomalies as one possible driving mechanism of Tropical Pacific Decadal Variability (TPDV). Based on 100 realizations of the Community Earth System Model Version 2 - Large Ensemble (CESM2-LE), we construct a Linear Inverse Model (LIM), which highlights the coupling at decadal time scales between the subtropics and the equatorial Pacific by propagating spiciness anomalies and suggests a link to TPDV. The eigenmodes of the LIM (i.e., the Principal Oscillation Patterns) reveal distinct spiciness pathways with decadal time scales, accompanied by corresponding decadal SST signals in the tropics. Spiciness signals originating in the Southern Hemisphere indicate the strongest response of the equatorial Pacific with warm and salty equatorial spiciness anomalies corresponding to a positive equatorial SST anomaly. However, the exact contribution of the spiciness mechanism needs to be further quantified, as well as the contribution of other pycnocline processes linked to extratropical atmospheric forcing.