This paper evaluates the rapidly evolving understanding of naturally occurring tropical Pacific variations at timescales of 7–70 years, termed tropical Pacific decadal variability (TPDV). This variability involves basin-scale sea surface temperature (SST), sea-level pressure, and ocean heat content anomalies that are thought to be considered as sources of skill for decadal Earth system predictions. Several mechanisms are proposed to explain TPDV, which can originate through oceanic processes, atmospheric processes, or as an El Niño/Southern Oscillation (ENSO) residual.
We synthesize current knowledge of mechanisms, their characteristics, and their contributions to TPDV. This assessment provides a new understanding of the origins of TPDV, as well as the processes that produce TPDV. Such understanding is essential for improving decadal Earth system predictions.
Mechanisms that produce TPDV include oceanic processes involving off-equatorial Rossby waves, which mediate oceanic adjustment and contribute to variations in equatorial thermocline depth and SST; variations in the strength of the shallow upper-ocean overturning circulation, which exhibit a large anti-correlation with equatorial Pacific SST at interannual and decadal timescales; and the propagation of salinity-compensated temperature (spiciness) anomalies from the subtropics to the equatorial thermocline. Atmospheric processes include midlatitude internal variability leading to tropical and subtropical wind anomalies, which result in equatorial SST anomalies and feedbacks that enhance persistence; and atmospheric teleconnections from Atlantic and Indian Ocean SST variability, which induce winds conducive to decadal anomalies of the opposite sign in the Pacific.